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Equipment Discussions >> Refractors

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Nippon
scholastic sledgehammer
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Reged: 10/22/09

Loc: Central Florida
Are achromats enjoying a come back?
      #5514165 - 11/11/12 09:52 AM

There seems to be a resurgence in interest in the achromatic scopes. If so what are some of the reasons do you think. I have a Vixen A 105 M. I bought it because I wanted a 4" scope with good optical figure for doubles and planets. The color correction is good and I'm pleased with it. But part of my reason was just the classic appeal of a good 4" inch achro.

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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Are achromats enjoying a come back? new [Re: Nippon]
      #5514200 - 11/11/12 10:25 AM


"There seems to be a resurgence in interest in the achromatic scopes."

Nah, it's just that the folks who normally haunt the Classics forum have finally discovered that there are other forums on CN.

I "get" achromats when a good one can be had at a given aperture for materially less money than a like-apertured ED doublet or triplet. Indeed when I bought my Antares 105/1500 it was just $650 and the competing 4" ED doublets from Synta were $900. What I don't "get" though is paying more for an achromat of a given aperture than you could pay for a decent quality RD triplet or doublet. Other than indulging one's curiosity, I feel there's little logic in such a move.

Regards,

Jim


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astroneil
Pooh-Bah
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Reged: 07/28/09

Loc: res publica caledoniae
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5514248 - 11/11/12 10:54 AM

Nippon,

They are indeed!

It's great isn't it?

Let me tell you why, in my opinion.

Apochromats are transitional technology.

There are innovations on the way that will allow flat lenses with perfect optical correction to be made cheaply. It will be scalable too.

See here for details:

http://www.seas.harvard.edu/news-events/press-releases/flat-lens-offers-perfe...



Apochromats will be phased out eventually.....like all other types of transitional technology.

Then, Nostalgic Man, suffering from a kind of poverty of the imagination, will seek out achromats above all other telescopic relics from the past. For these were the first kinds of refractors that were corrected well enough to give both an accurate and picturesque simulacrum of the heavenly bodies, as history so nobly attests to.

Achromats have a future (because they are the true gems in the box); apochromats don't.

Enjoy your Vixen achromat; one day it will fetch a whole lot more than a TEC, Tak or AP.

My two cents.

Regards,

Neil.

Edited by astroneil (11/11/12 09:24 PM)


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sg6
professor emeritus


Reged: 02/14/10

Loc: Norfolk, UK.
Re: Are achromats enjoying a come back? new [Re: Nippon]
      #5514291 - 11/11/12 11:24 AM

Possibly people are realising that a decent achro is a decent scope. If designed reasonably they can produce some good viewing at a reasonable cost.

Maybe the imaging side carried over too much into the visual side, as in imaging you cannot put up with CA. Whereas in visual some people simply don't worry about CA and on dim objects the CA may well not be noticable. Why pay 3-5x for something that is of no concern or relevance.

You will have to accept a longer scope and I would say there is a practical limit to length, guess about 1.2 metre focal length.

The available glass types are now more varied, perhaps manufacturers do not use an ED glass, but they might use one that is closer to ED then what was around 15-20 years ago. Still achro but better then previously available.


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dlapoint
scholastic sledgehammer


Reged: 08/18/03

Loc: Moncton NB Canada
Re: Are achromats enjoying a come back? new [Re: sg6]
      #5514343 - 11/11/12 11:54 AM

Refractors are illogical any way you slice it. So picking an achromat over an apo for dso observing is no more crazy than picking a refractor over a dob for said purpose.
If you want a 6 inch refractor and don't want to sell one of your kids to buy it, the achromat is the only game in town. You use your scopes for their intended purpose. Play to the strengths of the design and you will be happy. I just picked a 6" achromat and I'm having fun exploring many classic dso's that my other scopes just can't show off, like this scope. So yes I think they are, and should make a come back.


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Nippon
scholastic sledgehammer
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Reged: 10/22/09

Loc: Central Florida
Re: Are achromats enjoying a come back? new [Re: dlapoint]
      #5514433 - 11/11/12 12:51 PM

Well I like my Vixen achro well enough but fetch a higher price some day than a TAK or TEC??? I really doubt that. Now if you have a TAK or TEC that you want to swap to get my Vixen I'd be glad to help you out.

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astroneil
Pooh-Bah
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Reged: 07/28/09

Loc: res publica caledoniae
Re: Are achromats enjoying a come back? new [Re: Nippon]
      #5514504 - 11/11/12 01:47 PM

Quote:

Now if you have a TAK or TEC that you want to swap to get my Vixen I'd be glad to help you out.




No Sir, I don't own one of these kinds of instruments any more.

I gave the last one I had away. God knows where it is now.

Here's that story, with no regrets.

http://www.cloudynights.com/item.php?item_id=2765

I do however have a very nice 6" f/8 ED doublet on loan right now for an up-and-coming magazine review.

Kind Regards,

Neil.


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Astrojensen
Post Laureate
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Are achromats enjoying a come back? new [Re: dlapoint]
      #5514543 - 11/11/12 02:07 PM

Quote:

Refractors are illogical any way you slice it.




I don't think so. They do have their niches, which they serve well. One thing they do better than anything else in my experience, is wide-field deep-sky observing. They also cool down quickly and so if you have only a limited amount of time for lunar-planetary observing, a refractor is the way to go. They are also superior for solar observing. Double star observing is also one of their traditional niches.


Clear skies!
Thomas, Denmark


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5514576 - 11/11/12 02:29 PM

There are times when I miss my second telescope, a Parks 80mm f/11 achromat/equatorial. I gave it to a relative after acquiring the fluorite, along with a couple of oculars I considered expendable. With it, I once observed Venus from about five in the morning when it was still dark until the sun approached high noon, the planet's surface at that point appearing as sand, just visible, peppered, grainy, the mount motorised only in right ascension and positioned partly on the grass, partly on the walk at our home in midtown Memphis. Earlier, just before dawn, an unsuspecting fellow, known to be challenged, was walking down the sidewalk. As he approached, he suddenly caught sight of me, crouched down, peering through my refractor. He jumped back a bit, veered off the sidewalk, off the curb into the street, then back onto the path and resumed his walk. I'll never forget his startled expression, sustained for several moments as he passed, nor the face of Venus alight that morning in 1992.

That being related, I'm wanting another achromat, with said Antares f/15 appearing more and more desirable, especially with its Japanese doublet, if I'm not mistaken, and costing less than that of the Skylight produced in London.

Alan


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robboski2004
member


Reged: 01/14/08

Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5514619 - 11/11/12 03:05 PM


Apochromats are transitional technology.

120 Years and counting !

Interesting comment Neil.

Regards.
Ian.


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5514637 - 11/11/12 03:16 PM

Technological advancement is absolute and does not grant exceptions. Just as the achromat replaced Galileans and Keplerians, the aprochromat has just as surely and irreversibly replaced the achromat. One day, something else will replace the apochromat, but by then the achromat will be as well represented in use as the Galileans and Keplerians are today. Know where a guy can buy a quality Keplerian these days?

There's nothing wrong with make-believe, of course. It can be fun. Who doesn't like dressing in Elizabethan garb, hefting a tankard in one hand and turkey leg in the other, and talking funny? But that doesn't signal a return to relevance of Elizabethan culture, fashion or speech mannerisms. It's just recreational "pretending".

Just as the achromat rendered the Keplerian and other singlets largely irrelevant, so too the apochromat has rendered the achromat largely irrelevant (in a commercial sense), and indeed something new will eventually relegate the apochromat to oblivion as well. But that's tomorrow. Today, the relevance of refractors is all about apochromats. Far more energy is devoted to making better aporchromats than any other refractor design presently.

In thirty years, the pretentious re-enactors will be using A-Ps, TECs and Taks, and find all kinds of justifications for clinging to those designs, other than technical superiority. Today, though, the refractor anachronists cling to achromats. But don't feel bad. You should hear what those Keplerian fanboys are saying about both achromats and apochromats!

- Jim


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stevew
Now I've done it


Reged: 03/03/06

Loc: British Columbia Canada
Re: Are achromats enjoying a come back? new [Re: Nippon]
      #5514652 - 11/11/12 03:26 PM Attachment (58 downloads)

Quote:

There seems to be a resurgence in interest in the achromatic scopes.




I didn't realize that they had left.


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7331Peg
Sirius Observer
*****

Reged: 09/01/08

Loc: North coast of Oregon
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5514655 - 11/11/12 03:32 PM

Quote:



. . . That being related, I'm wanting another achromat, with said Antares f/15 appearing more and more desirable, especially with its Japanese doublet, if I'm not mistaken, and costing less than that of the Skylight produced in London.
Alan




If you can swing it, Alan, go for the Skylight instead. I've had an Antares f/15 (actually f/14.3) and now have the Skylight f/13. The Antares did a pretty good job, but the views through the Skylight are much more crisp, and it will handle a whole lot more magnification than the Antares before the image starts to break down. The construction of the Skylight is much, much better, and the focuser is a huge improvement over the one provided on the Antares scope. I hesitated at the cost, but it was well worth it.


I never would have believed it if I hadn't seen it for myself. A well-figured achromatic lens is really something special. If you've ever looked through a Zeiss Telementor or the 80/1200 Zeiss, you'll understand what I'm referring to.

And you can dispense with the Elizabethan garb -- they only wear that in some strange area of California located north of San Francisco.


John


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Astrojensen
Post Laureate
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5514671 - 11/11/12 03:42 PM

Quote:

Just as the achromat replaced Galileans and Keplerians,




The achromat hasn't replaced the galileian and keplerian telescopes, but improved them. The optical layout of the modern apochromatic telescope, with a positive eyepiece, is still, technically, called a keplerian telescope. Likewise, you can buy small achromatic galileian binoculars for use in theaters and operas or even as ultra-specialized 2.8x40 glasses for observing the milky way with 28° TFOV. There's a tremendous difference in their level of sophistication, but all modern telescopes and binoculars are based on the same basic optical principles Galileo and Kepler found.

Quote:

Know where a guy can buy a quality Keplerian these days?




Almost everywhere. Even shopping malls. OK, you said "quality", so maybe not shopping malls...

Quote:

the apochromat has rendered the achromat largely irrelevant (in a commercial sense)




No, not yet. Achromats still outnumber apochromats by a huge number. Think binoculars, spottingscopes and rifle scopes. Rarely are these apochromats and they outsell amateur telescopes by huge amounts.


Clear skies!
Thomas, Denmark


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robboski2004
member


Reged: 01/14/08

Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5514675 - 11/11/12 03:46 PM

Quote:

Technological advancement is absolute and does not grant exceptions. Just as the achromat replaced Galileans and Keplerians, the aprochromat has just as surely and irreversibly replaced the achromat.

Jim,

At a guess ?? more achromats have been produced in the last 20 years, than the preceeding 300 ! So how they have been replaced with apos is an interesting assumption.

I know everyone is looking for the latest big thing !
Just like diffractive optics were going to make all other lens types obsolete 40 years ago , now we have this development.

Suggest we don't hold our breath !!

Ian.

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simpleisbetter
Carpal Tunnel
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Reged: 04/18/11

Re: Are achromats enjoying a come back? new [Re: robboski2004]
      #5514746 - 11/11/12 04:32 PM

To me it has to do with the economy. When I bought my AT-102ED it was affordable, the AT-111 was not, nor are the 106's or other triplets. When I bought my C6R OTA it's because I CANNOT afford or even think about a 6" in an ED doublet, let alone triplet. And being a visual observer only, CA isn't that big a deal. Today, if I were looking at a 4", the only choice for me would be an achromat since the ED's have all but disappeared and I can't afford the 4" triplets, especially for visual use grab-and-go.

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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: 7331Peg]
      #5514793 - 11/11/12 04:58 PM

Hi John,

I, too, enjoy observing binaries and will take your advice in regard to the Skylight over the Antares. Thanks.

Incidentally, how often does the weather allow for clear nights there in the Northwest?

Cheers,

Alan


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Abb
super member


Reged: 11/19/10

Loc: Windsor Ontario Canada
Re: Are achromats enjoying a come back? new [Re: simpleisbetter]
      #5514805 - 11/11/12 05:08 PM

Well, I'd be very surprised if (e.g.) one could see (meaning "visually") the $1300+ difference between an ES 127 Achro and Apo if you had them side by side looking at (e.g) The Orion Nebula. No doubt the Apo would have drastically reduced CA if viewing the moon. How about the $???? difference between ES's 152 mm Achro and Apo.

I'll stick with my 2 Achro's for now....until I win a multi-million dollar lottery


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John Huntley
scholastic sledgehammer
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Reged: 07/16/06

Loc: South West England
Re: Are achromats enjoying a come back? new [Re: Abb]
      #5514972 - 11/11/12 07:29 PM

There is a resurgence of interest in achromats, thanks in part to Neil and his interesting writings, and companies like Skylight in the UK who are prepared to put them together very nicely, albeit for quite a few £'s.

Over the past 24 months I've been able to directly compare good 4" and 5" achromats with my Vixen 102mm F/6.5 and Skywatcher 120mm F/7.5 ED doublets and I've concluded that I won't be moving back (and that's how I see it) to achromats.

In short my ED's did everything the achromats did a little better and are much easier to mount steadily to boot.

My ED's are both older models so both cost me less than scopes such as the Skylight achromats and Antares 105mm F/15's.

It's good to find these things out for yourself though, if you can, and let the views you get be your guide


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rocketsteve
sage
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Reged: 04/17/11

Loc: Southeast Louisiana
Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5515051 - 11/11/12 08:37 PM

Quote:

Quote:

Refractors are illogical any way you slice it.




I don't think so. They do have their niches, which they serve well. One thing they do better than anything else in my experience, is wide-field deep-sky observing. They also cool down quickly and so if you have only a limited amount of time for lunar-planetary observing, a refractor is the way to go. They are also superior for solar observing. Double star observing is also one of their traditional niches.


Clear skies!
Thomas, Denmark




It's almost like you read my mind.


I use my Sky-Watcher 80mm achro for solar, lunar and planetary viewing. I love the tack-sharp images that it produces and the focuser is butter-smooth.


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5515150 - 11/11/12 10:03 PM

Back in 2003, fate blessed me, two-folded, with my first apochromat: a Takahashi FS-102 with a superb doublet, the front element being of calcium fluorite, a crystal rather than glass, and a rear element of low-dispersion glass(ED?). One way in which it was fortunate, it was on sale at the time for $1899, down from $2200 or so. Then, later, in 2006 or '07 with the beginning of the rise in cost of calcium fluorite blanks, it was replaced by the current TSA-102, a triplet(crown, ED, flint or crown?). No matter, for the FS-102 is decidedly superior given it's a tad brighter, with better contrast and sharpness, however in exchange it does exhibit slightly more spurious colour on brighter objects, though just slightly. I'll take the before-mentioned advantages instead.

The current four-inch triplet is described within advertisements as a "New Design", perhaps a ploy designed to mask Takahashi's necessary exodus from true fluorite, however Takahashi still produces fluorite elements, but only in 60mm and 90mm sizes, and then skips to this leviathan...THE FET...

http://www.takahashi-europe.com/en/FET-300.php

...all the scope you'd ever need, and then some.

Imagine all the used four-inch Takahashi fluorite refractors on sale in this ad or that, right now, with perhaps many of the sellers patently oblivious given the above, unless they're selling to acquire a larger one, whether it be an FS-152 or the astronomically-priced TOA-150, the latter a non-fluorite but optically excellent nonetheless in comparison to the former, though not nearly as aesthetic in that it is an evolution beyond pomp and circumstance.

Get them while they last, before someone else gets a clue...

On the other hand, there's just something about a plain and simple, good, old-fashioned crown and flint achromat that beckons. I can't explain it, as it's ineffable, or perhaps it's the child seemingly lost in all of us, regardless of technological advancements and monetary considerations; Elizabethan ruffs even, which I would sport in a heartbeat, but only on Halloween or in a past life.

Alan


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photiost
professor emeritus
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Reged: 12/14/06

Loc: Montreal, Canada
Re: Are achromats enjoying a come back? new [Re: rocketsteve]
      #5515170 - 11/11/12 10:13 PM

When I was 12 years old my first telescope was a 60mm Towa refractor.

I now own a few refractors the largest being 150mm

Yes with low cost Objectives coming out of China and recently some of the better ones from Japan, USA ..etc can be purchsed at "reasonable" prices I believe they are making a comeback.
-


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: photiost]
      #5515207 - 11/11/12 10:30 PM

My first telescope was a 60mm Sears "Discoverer", when I was 7 or 8, and the first view was of Saturn through the recently-restored 20mm Kellner pictured within my avatar. Saturn appeared very sharp, though in a weird, fluorescent shade of green I'll never forget.

We've come a long way.


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rocketsteve
sage
*****

Reged: 04/17/11

Loc: Southeast Louisiana
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5515251 - 11/11/12 10:53 PM

My father bought my mother a brand new Meade Model 285 Refractor for her birthday when I was in my mid-teens, and paid WAY too much money for it. Looking through the 25mm M.A. EP produced decent lunar views, but when I placed the cheap, cheesy 9mm EP into the diagonal, for a high-power view, the moon looked like it had a severe case of the chicken pox. Needless to say, I was hooked, but it would be another decade before I would have the disposable income or time to pursue this hobby.

Refractors and EPs sure have come along way since the late 70s...


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: rocketsteve]
      #5515417 - 11/12/12 01:24 AM

Interesting and enlightening read....

I am sure the achromats will stay around as cheap alternatives for a long time to come, in my view they never left the amateur scene!
They will still be sold as entry level telescopes or for aficionados that are ready to spend more on hand figured achros with long focal lengths.

But let's be honest the market place has changed with the relatively cheap and always improving ED's out of China. Better color correction with shorter easier to mount tubes are hard to pass - especially when available on the used market.

I personally do not plan to go back to my first and second scope which were 70mm and 100mm Achros by now I am spoiled by more capable APO's!

best regards
Chris


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7331Peg
Sirius Observer
*****

Reged: 09/01/08

Loc: North coast of Oregon
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5515453 - 11/12/12 02:28 AM

Quote:

Hi John,

I, too, enjoy observing binaries and will take your advice in regard to the Skylight over the Antares. Thanks.

Incidentally, how often does the weather allow for clear nights there in the Northwest?

Cheers,

Alan




The observing weather in the northwest -- well to put it politely, from about the middle of November of last year until now (geez, already a year), it's been fickle and uncooperative. Prior to that, I could almost always find at least a few hours of clear skies a couple of times a week.

I'm on the Oregon coast, so I get more cloudy nights than say the Portland area or Seattle/Tacoma, but even taking that into consideration, the periods of clear skies were frequent enough that I didn't suffer from photon deprivation.

I keep thinking this streak will end soon, but then I've been thinking that for about the last eleven months, too. Can't last forever . . . . . . can it?

If you're seriously interested in the Skylight 100mm, I did a review of it that was published in the March/April 2012 issue of Astronomy Technology Today. If you don't have access to the magazine, the un-edited version is available here.

After almost a year's experience with it, I'm still convinced it is truly a remarkable 100mm refractor.

John


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5515562 - 11/12/12 07:27 AM

It's always a price question.
An excellent 100mm apo will cost more than a respectable 150mm achromat well designed and well corrected, free of aberration and injurements.
The 150mm in spite of the 100mm apo colorless will do always more even on planets.
Apart the dimensions fact, this should be a matter of interrest and selection for an observer who wants something in his hands. For lot of reasons.
Make Venus and Mercure in daylight, Mars with a simple light yellow filer, jupiter as well, saturn as well and Uranus too. Visually.
You will see the big differences between the 2.
Now try to find this kind of acurate doublet: old TMB achromat, DGO, Istar, old Clavé, etc...
The vixen 102mm achromat, the antares serie 102mm will do the same job visually than the 100mm apo even prestigious.
Think apo above 130mm to get a step sensitive for the 150mm, relative, but above a great step, for bankrupcy also.
Reason why people goes to 8-10" with perfect optics and this out perform the 130 apo, any, just equals the 150 apo for the 8" newtonian in terms on contrasts, resolution remains for the bigger.
All these scopes can be moved on average mounts.
If you think ccd go to apo or newtonian and any compact costless, even achromats well filtered.
Stanislas-Jean


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Jeff B
Carpal Tunnel
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Reged: 12/30/06

Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5515753 - 11/12/12 10:47 AM

Well Barry's backlog at D&G has gone from 1 to 2 years so I guess that says something about achro-popularity.

Funny, thinking about it, over the last 50 years I've been in the hobby, I've never been without a 3 inch or larger achromat.

Jeff


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Zamboni
scholastic sledgehammer
*****

Reged: 01/03/05

Loc: Colorado Springs
Re: Are achromats enjoying a come back? new [Re: Jeff B]
      #5515944 - 11/12/12 12:41 PM

Achros started their comeback when the first truly surprising Synta refractor hit the market as the C102 HD. Even though Chinese APOs are getting cheaper and better, there will always be a market for achromats when there are scopes like the Omni 102 around. Even taking into account CA, the optics are impressive by any standard and it's a well built "complete" system for under 500.

Achros will always be around as a good "stepping stone" for people getting into refractors.


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: Jeff B]
      #5515953 - 11/12/12 12:45 PM

I started with the 3" unitron and now owns a 102 vixen and a 150mm istar. Results are excellent. I was on a 100ED and a 110ED that were not on the level in comparison.
Also a 200 newtonian from astrola and the 250 from orion uk that were excellent but also big.
For the compacts there was only the OMC 250 that was excellent and some DK difficult to hold in perfect alignment.
Get look in prestigious apo for comparison and found even with stronger contrasts the data collected are not more in quantity and not finer.
The long achromat get better images because in elevation with regards to the ground proximity. That makes mostly better results over a long time use.
Stanislas-Jean


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rwiederrich
Goldfinger
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5515977 - 11/12/12 01:05 PM

Quote:

Interesting and enlightening read....

I am sure the achromats will stay around as cheap alternatives for a long time to come, in my view they never left the amateur scene!

Chris




Actually Chris, Achromats enjoyed many years as professional instruments. Currently, refractors of any kind are not used by the professional..but by amateurs... APO's included.

Reflecting or compound designs are scopes of the professional these days.

Rob


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rwiederrich
Goldfinger
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Re: Are achromats enjoying a come back? new [Re: Jeff B]
      #5515992 - 11/12/12 01:16 PM

Quote:

Well Barry's backlog at D&G has gone from 1 to 2 years so I guess that says something about achro-popularity.

Funny, thinking about it, over the last 50 years I've been in the hobby, I've never been without a 3 inch or larger achromat.

Jeff




Comeback comes with a financial restriction as its cause sometimes. Less free cash around in personal accounts causes this kind of influx. Folks getting to enjoy a quality achro because of economy..where they may never have had the chance in a better economy.

I know for a fact most folks who look through my 6"f/15 or the 10"f/15 never in their wildest dreams thought the views could have been so magnificent. Cheap short achro's have falsified their impressions. A good quality long focus achromatic refractor can change that impression quickly....

Rob


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: Abb]
      #5515995 - 11/12/12 01:18 PM

"Well, I'd be very surprised if (e.g.) one could see (meaning "visually") the $1300+ difference between an ES 127 Achro and Apo if you had them side by side looking at (e.g) The Orion Nebula."

That's a very good point. The answer, however, depends on the quality of the two scopes and the magnification you're operating at in each. If the quality of each were indeed equal and the baffling of each identical, I think you'd be right. If, however, one had better optics and/or better baffling than the other, and you were operating a a non-trivial magnification (100x+), I suspect the better quality scope might show you more. Specifically it would likely show you Trapezium E and F more of the time than the poorer quality/poorer baffled scope.

There is no guarantee of quality with any mass-produced refractor, apochromat or achromat. However, the economics favor getting a better quality apochromat than achromat. Why? Apochromats sell for considerably more than like aperture achromats. This typically provides the manufacturer with larger per unit margins. Since optical quality is directly linked to time spent figuring and testing the optics, and time is money, the apochromat provides a larger budget to use in producing the optics. Of course, there's no guarantee that the apochromat maker doesn't instead put that extra margin in its pocket, but in the case of the achromat, that margin doesn't exist and there's little chance that the maker will spend more making the optic than they sell it for.

That extra margin in the case of the apochromat also typically translates into more and better OTA features and accessories, so in that regard too, you might notice a difference (better focuser for example).

There's absolutely nothing wrong with achromats. They can be lovely scopes and more often than not offer great value (capability per dollar). That said, their days as serious commercial astronomical instruments are numbered, IMO. Trickle down is a product of progress and innovation. Eventually it will become as cheap or cheaper to make apochromats as achromats. When that happens, even the price/value advantage will vanish I suspect.

In other words, soon you won't need to win the lottery - you'll be buying 5" ED triplets for the same price you pay today for a 5" achromatic doublet.

Regards,

Jim


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5516125 - 11/12/12 02:34 PM

If we follow your post that means for the same amount we will get an apo of same 5" aperture.
This could be but with regards to the optical acuracy this will involve something just average with such cn apo.
This was my edventure with the 100 and 110ED, colorless but just average strehl, washed on small features that achros showed conspisciously.
The better example was Venus featureless into that apos.
Chromatic aberration is not the only link for serious results, visually.
Stanislas-Jean


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LivingNDixie
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5516195 - 11/12/12 03:20 PM

I have been in this hobby for a long time. A long time ago if you wanted a refractor it was either Astro Physics, Tak or TV. Orion also sold some rebadged Vixen stuff. Those were the choices then, Astro Tech didn't exist and SV was just starting with their 80mm scope that got some interesting reviews.

Then one day the 80mm Apo from Orion showed up and changed everything.

Today you can buy a nice refractor for a fraction of what they cost a decade ago. So I would say yes Achros are having a comeback and Apos are in a golden age!


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AstroSteve
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Re: Are achromats enjoying a come back? new [Re: rocketsteve]
      #5516264 - 11/12/12 04:02 PM

I have owned some nice WO ED refractors and they served me well. I own a AT72ED and love it but for visual I just love my Orion ST120 on a Versa-Go II alt-az setup. simple easy to pick up from the garage to the driveway or backyard. With some good EP it's just a delight to own and the contrast on DSO is just wonderful. The Plaides is my favorite this time of the year to view.

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KaStern
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Re: Are achromats enjoying a come back? new [Re: Nippon]
      #5516289 - 11/12/12 04:22 PM

Hi folks,

Quote:

Are achromats enjoying a come back?




no. Achromats are outdated.
You can build an ED-refractor for a small additional money.
Wat do you need?
Tube: Same cost
Lens-cell: Same cost
Focusser: Same cost
Polish and figure: Same cost
glass: Additional cost.

Cheap chinese ED glass will be more expensive than typical low-cost Flint and BK7,
but it will not cost much more. Add 100 USD to the cost of an 5" achromat.
If both are 5" f/10 the ED will easily outperform the achromat because the ED has much significant colour fidelity.

Cheers, Karsten


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jrbarnett
Eyepiece Hooligan
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Loc: Petaluma, CA
Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5516299 - 11/12/12 04:33 PM

Actually that really doesn't follow. Capital investment in optical fabrication and design is being made in designs that employ low dispersion glass types. "ED glass" is the new "fact" that the industry is assimilating. It is becoming plentiful and relatively inexpensive due to the large scale investment in its fabrication.

This availability of new materials, in turn, pulls investment in equipment optimized to mass-produce optics employing this new material. No one is making large scale equipment investment in computerized grinding and polishing machines for purposes of making C-F achromats. The ED 80 that essentially crippled the small achromat market that had been thriving before its arrival, was a creature of massive investment by Synta in the finest Zeiss optical fabrication equipment. These machines can turn out 1/5 to 1/7 wave ED doublet optics in an endless stream in apertures from 80mm to 120mm. With virtually no investment in modernizing the toolings for achromats (due to low demand) such optics require more hand work and therefore (aside from the cost of materials) are actually more costly to make than ED optics produced with a high degree of automation.

Typically hand-figured achromats from the 50s, 60s and 70s, when tested with an interferometer, on average weren't as well figured as mass-produced ED optics made by robots at the end of the last century. There are exceptions, of course, but I've seen many a 4" Unitron with suspect optical quality. The handful of Clark refractors I've seen bench test data for came in right around 1/4 wave.

Not only is fabrication more consistent and reliable today, but also testing is more accurate and precise too.

Regards,

Jim


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Mark Costello
Carpal Tunnel
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Re: Are achromats enjoying a come back? new [Re: KaStern]
      #5516376 - 11/12/12 05:32 PM

Just what fraction of owners build their own refractors??

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mikey cee
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Re: Are achromats enjoying a come back? new [Re: Mark Costello]
      #5516525 - 11/12/12 06:57 PM

I'm emphatically stating that I've not never had one to come back to! Mike

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edwincjones
Close Enough
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Re: Are achromats enjoying a come back? new [Re: dlapoint]
      #5516542 - 11/12/12 07:10 PM

Quote:

Refractors are illogical any way you slice it. ............




so is spending one's nights under a cold, dark sky looking at faint fuzzies,
but we all do it

edj


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Smack
super member


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Re: Are achromats enjoying a come back? new [Re: edwincjones]
      #5516637 - 11/12/12 08:02 PM

Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.


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Sky Muse
sage


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Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: 7331Peg]
      #5516782 - 11/12/12 09:21 PM

Hi John,

I first saw the Skylight advertised recently. It's priced right, now that it's on sale, but I'd need a mount, an equatorial, perhaps the Vixen GPD2. I'm certain the Losmandy GM8 would accommodate it. Perfectly matched they'd be, almost wholly black in addition. I can see it now. I could craft a pier using a pine 4" x 4", or a custom-cut 6" x 6" even, and paint it in weatherproof black, or better yet, polyurethaned black walnut...just imagine. Although the Vixen would ideally match the Takahashi, too...

Here in the extreme northwest corner of anachronistic Mississippi, fifteen miles south of Memphis, it's cloudy about half the time with most nights being clear or partly cloudy. I observe from a three-acre homestead about a mile or so from the edge of a one-hundred-foot high ridge, a southerly continuation of the Memphis bluff overlooking the sprawling north Mississippi Delta, with its agricultural endeavors and...the casinos. I have, to the north, Memphis with its light dome, and to the west, Tunica, the third largest gambling complex in the nation, and with its own "halo". I've lived here since 1996, though when the land was purchased in 1991 my cousin brought me down from midtown Memphis to see the night sky here for the first time, as I had asked. The sky stellarly shone and glittered from just north of the zenith all the way to the southern horizon, and to the east and west. With the ever-encroaching light dome of the casinos, the sky to the west and partly to the south has brightened somewhat...

To put it mildly, I wish they'd all pack up and float away.

...so much for wishing.

Instead, here's to wishing for clear skies 24/7/365 for all!

Cheers,

Alan


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galaxyman
Vendor - Have a Stellar Birthday
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Re: Are achromats enjoying a come back? new [Re: Mark Costello]
      #5517079 - 11/12/12 11:55 PM

Quote:

Just what fraction of owners build their own refractors??




Hi Mark

Yeah, I can't imagine myself building an 8" f/9 APO in the near or very distant future to replace my 8" f/9 achro.


Karl
E.O.H.

Chesmont Astronomical Society - www.chesmontastro.org
Galaxy Log - http://www.youtube.com/user/GalaxyLog4565?feature=mhee
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB/APM 8" f/9 Refractor”The Beast”. One great DEEP SKY achro
ES 6" f/6.5 achro. Good one
Celestron Omni XLT 102 refractor.
Celestron 10x60mm Binos


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5517204 - 11/13/12 02:03 AM

Yes mass production indeed.
But the figure of PTV5 7 you said I think is the top best of the serie with few units.
This doesnot explain the featureless views on venus with apo because this was not at a level of PTV4. I think they are lot of units like this.
The question of refractor doesnot resume to color aberration.
For capturing a detail of 1" size on Mars it can appear to be 0.3 contrast level in apo, .2 in the refractor achromat and almost nothing (say .15 or less) in my apo example. When approaching the optical aperture limits, my apo give nothing, the achromat still something accessible say .05-0.1 and the actual apo 0.06-0.12 only.
I think we get a lot in an apo will convince immediateley anybody by the contrast provided, but when pushing a little more this is not so evident. In both cases we get the same data but at a slight different contrast.
To-day, I think the achromats are fabricated not individually but by batch of one or few hundred of objectives depending on the aperure (until 6") on the same polishing machine.
PTV4 for refracting optics is good, for reflecting optics just on the minimum.
In europe we can get a zygo for 100 (for a general control) and still have the roddier to get similar data, but after the buy.
The come-back is here but confidential for achromats and visual observers.
Stanislas-Jean


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7331Peg
Sirius Observer
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5517205 - 11/13/12 02:05 AM

I can vouch for the Losmandy G8 working with the Skylight f/13 100mm, Alan -- it's an ideal combination. No experience with the Vixen GPD2, though.

I do have an old non-driven CG5 -- it has the capacity to handle the weight and length of the Skylight, but I got a lot vibration at the eyepiece end when focusing. Gave up on that combination and went back to the Losmandy.

That scope also works well on the T-Rex and the DM-6, too.


John


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5517238 - 11/13/12 03:31 AM

Quote:

Yes mass production indeed.
But the figure of PTV5 7 you said I think is the top best of the serie with few units.
This doesnot explain the featureless views on venus with apo because this was not at a level of PTV4. I think they are lot of units like this.
The question of refractor doesnot resume to color aberration.
For capturing a detail of 1" size on Mars it can appear to be 0.3 contrast level in apo, .2 in the refractor achromat and almost nothing (say .15 or less) in my apo example. When approaching the optical aperture limits, my apo give nothing, the achromat still something accessible say .05-0.1 and the actual apo 0.06-0.12 only.
I think we get a lot in an apo will convince immediateley anybody by the contrast provided, but when pushing a little more this is not so evident. In both cases we get the same data but at a slight different contrast.
To-day, I think the achromats are fabricated not individually but by batch of one or few hundred of objectives depending on the aperure (until 6") on the same polishing machine.
PTV4 for refracting optics is good, for reflecting optics just on the minimum.
In europe we can get a zygo for 100 (for a general control) and still have the roddier to get similar data, but after the buy.
The come-back is here but confidential for achromats and visual observers.
Stanislas-Jean




Stanislas-Jean

If both the APO and Achro are produced to the same standards and give a similar Strehl and PV error I think it is save to say that the APO will deliver the better visual image.
If you compare a badly made "APO" with loads of spherical aberration, with a premium hand corrected Achro, you might get to the conclusions in your posting quoted above.

Or do you want to tell us that any mass produced Achro will have a high Strehl?
Why should the production quality automatically be better on the Achro than it is on the APO?
I agree that a well made long focus Achro has its place but I am seriously having trouble following your logic.
We should make valid comparisons... it comes down to production quality not to the telescope system.

best regards
Chris


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5517266 - 11/13/12 04:17 AM

You see it's also a matter of numbers.
Speaking always apo-achro without characterisation of the acuracy in hands we will never progress.
An apo even acurate is not the perfection: in blue channel still where the strehl may be less than some published, in other words it remains always an amount of spherochromatism.
If well designed and fabricated for reducing this a lot it will cost a lot, the price of a 500mm newtonian with excellent optics. These apo are not mass produced.
Those mass produced are affected by limited acuracy, always a matter of cdc, and for reminding on commercial acuracy only that is PTV2 to 4 for the bests with 2 scratch dig assumed not guaranted.
Now achromats, not the classic synta models but something other that is designed for specific jobs (planets, sun, stars), what I see is the fact that for 6" for example even we have better pretty images in a tak, astroph, that are, an excellent achromat get the SAME data visually with less contrast at the eyepiece. This is my words.
Now for ccd this is an other debate.
I have here still the 4" vixen, excellent, the 6" istar of PTV 7 with zygo test.
Stanislas-Jean


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5517289 - 11/13/12 05:22 AM

Well in this case you agree with me that it comes down to the quality of production... this is exactly my opinion. It all comes down to the manufacturing process and QC of the manufacturer no matter if it is an Achro or APO.

The difference between the two systems is that even if the Achro is perfectly figured it will still show CA on planets that will reduce your ability to recognize finest details, unless you use colour filters...

In any case I stand by my opinion that Achro's with a diameter above 6" inch are an interesting option due to the cost per Inch of aperture.

best regards
Chris


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5517319 - 11/13/12 06:28 AM

Quote:


See here for details:




Flat lenses are dispersive, they depend on resonances on a nano-scale for their properties and therefore by their inherent nature, their properties are frequency dependent.

"The array of nanoantennas, dubbed a “metasurface,” can be tuned for specific wavelengths of light by simply changing the size, angle, and spacing of the antennas."

One also has to look at the transmission properties of these "meta-materials/surfaces." Because of their very nature, they are dissipative.

Meta-Materials

I was actually involved in what were the first demonstrations of meta-materials, I can only say, if the apochromatic refractor is a transitional design, it's going to be a long transition.

Jon Isaacs


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: LivingNDixie]
      #5517341 - 11/13/12 06:56 AM

Quote:

I have been in this hobby for a long time. A long time ago if you wanted a refractor it was either Astro Physics, Tak or TV. Orion also sold some rebadged Vixen stuff. Those were the choices then, Astro Tech didn't exist and SV was just starting with their 80mm scope that got some interesting reviews.

Then one day the 80mm Apo from Orion showed up and changed everything.

Today you can buy a nice refractor for a fraction of what they cost a decade ago. So I would say yes Achros are having a comeback and Apos are in a golden age!






It is true. Achromats are making a comeback but apochromats are in their golden age... Never before so many, nor so many good ones.

I think the development of the affordable apo/ED scope has lead to improved achromats, particularly in terms of mechanical quality, the improvements in manufacturing trickle down from the apo/EDs to the achromats. A scope like the ES AR-102 did not exist 10 year ago.

I think there is a place for a decent quality achromat, the great virtue of the apo is that it can do about everything, high power, widefield all in one scope. A 4 inch F/6 achromat might do a pretty good job at low magnifications, a 4 inch F/15 achromat might do a pretty good job at high magnifications, but a 4 inch F/6.5 apochromat can do both jobs and do them very well.

But the place for the achromat... just like any scope, doing what it does best. If the role of the scope is narrow, say low power, widefield, then an achromat may serve the purpose...

Jon


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astroneil
Pooh-Bah
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5517349 - 11/13/12 07:00 AM

Then you'll have to tell that to the entire photographic industry.

That's where I got the original lead on the story.

Toodlepip,

Neil.


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5517368 - 11/13/12 07:33 AM

I donot follow you on one point the respectable achromat will show the same amount of features and also near the aperture limit ability. Apo are not superior on that point, may be less if the design involves many lenses, 3, 4,5.
This is also when filters are in use when some seeing is present. Some apo on that view when seeing is present have some color dispersion amplified by the seeing level, well perceptible visually at high power.
6" is effectively a good to excellent compromise for a respectable achromat (the spherochromatism being reaching PTV3.4 at F15, the light filtering will do the difference easily, except in deep blue channel where the apo respectable is better).
Stanislas-Jean


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5517392 - 11/13/12 07:59 AM

Quote:


Achromats are making a comeback but apochromats are in their golden age... Never before so many, nor so many good ones.





I think this describes the current situation in a short and clear sentence.

best regards
Chris


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5517409 - 11/13/12 08:17 AM

Quote:

Then you'll have to tell that to the entire photographic industry.

That's where I got the original lead on the story.

Toodlepip,

Neil.




The entire scientific community is aware of it... it makes for a great press conference but take a look at the data and you will see strong frequency dependency as well as dissipation, the way they achieve these responses is by taking advantage of resonances which, by their very nature are frequency dependent.

Jon

Edited by Jon Isaacs (11/13/12 08:18 AM)


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Are achromats enjoying a come back? new [Re: dlapoint]
      #5517522 - 11/13/12 09:15 AM

Quote:

Refractors are illogical any way you slice it. So picking an achromat over an apo for dso observing is no more crazy than picking a refractor over a dob for said purpose.
If you want a 6 inch refractor and don't want to sell one of your kids to buy it, the achromat is the only game in town. You use your scopes for their intended purpose. Play to the strengths of the design and you will be happy. I just picked a 6" achromat and I'm having fun exploring many classic dso's that my other scopes just can't show off, like this scope. So yes I think they are, and should make a come back.




I disagree that refractors and apos are an illogical choice. While one can live without an apo (the same can be said for any telescope, as they are luxury items), they perform better as photo-visual instruments than achros. Although I have larger (and high quality) telescopes at my disposal (e.g., 10" Mak-Cass, Mewlon 300, Obsession 25"), I tend to default to an apo (175 mm).

If one can't afford an apo, so be it. That doesn't make others' choice to purchase one illogical.


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: 7331Peg]
      #5517538 - 11/13/12 09:24 AM

...the GM-8, or even the G-11 then, if I decide to mount the 8" Newtonian in addition.

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Abb
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Loc: Windsor Ontario Canada
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5517609 - 11/13/12 10:15 AM

I've never had the chance to look through an Apo but I'll say this, the difference between a good lens and a cheap lens of equal (or almost equal) size can make a noticeable visual difference. Fortunately, this I've seen/experienced through my refractors.

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philjay
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5517611 - 11/13/12 10:16 AM

I used a 4" F15 Skylight on an HEQ5 to good effect on visual, it was on my homemade Ash extra high tripod but it gave a respectable performance with reasonable damping times. I reckon the HEQ5 is about the lower limit for these scopes, I wouldn't go lower.


I am also of the opinion that the Achro is making a comeback, hey did fade back from the scene or awhile but with the quality achros that are now on the market more and more folks are considering them seriously.

Its always a pleaseure when using my 4" F15 at star parties or dark site meets to hear Apo and reflector owners alike saying " now thats what a scope should look like". Also its even more pleasurable to show them views of Jupiter and completely ruin their preconceived ideas and opinions that achros have loads false colour


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drollere
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5517691 - 11/13/12 11:13 AM

Quote:

Technological advancement is absolute and does not grant exceptions. ... Just as the achromat rendered the Keplerian and other singlets largely irrelevant, so too the apochromat has rendered the achromat largely irrelevant (in a commercial sense), and indeed something new will eventually relegate the apochromat to oblivion as well. But that's tomorrow. Today, the relevance of refractors is all about apochromats. Far more energy is devoted to making better aporchromats than any other refractor design presently.



jim, you're very entertaining when you go in for absurd argument.

is it not true that technology marches along an irreversible path of progress: speaking of anachronisms, that's an amusingly 19th century view of the matter. it's more accurate to say that culture evolves a wider variety of technics, which continue to innovate by mutual influence. it's true that only a small niche uses a bow and arrow for hunting nowadays, but nobody would equate the bow and arrow of today with the weapons of the 16th century; we still make paper and print and use books, but in radically improved ways consistent with our whole range of technology.

the point you dismiss with your pantomime is that, for visual purposes at a reasonable price point, achromats are a fully satisfactory and even superior choice to apochromats. that hardly makes consumers who prefer them clowns of the renaissance faire. perhaps it makes them people who believe any dollar difference should have a proportional visual delta.

aside from price, achromats benefit from improvements in glass manufacture, economies in lens fabrication, and robust lens assemblies. they do not require fragile fluorite or expensive ED glass.

anyway ... equipment choice is a personal preference. "objective facts" and parodic argument can be marshaled on both sides. separate from that, achromats form a long and honorable tradition, and i'd wager that the best achromats today are the best achromats ever. like books compared to computers, they're outdated technology when compared to apochromats, but apochromats are unlikely to dislodge them. even 19th century myths are able to endure -- probably because they are quaintly entertaining.


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: drollere]
      #5517770 - 11/13/12 11:51 AM

Quote:


jim, you're very entertaining when you go in for absurd argument.




Hopefully... Aren't we all...

In addition to Bruce's comments, it's worth noting that the differences between the single lens "aerial telescopes" and the achromats that replaced them were huge, focal lengths of 200 feet and more were necessary. By comparison, the differences between a decent achromat and a decent apo are small.

Jon


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: dlapoint]
      #5517913 - 11/13/12 01:07 PM

Refractors are most logical, in that...

1. Galaxies are known to refract light via gravitational lensing.
2. The Earth's atmosphere acts as a lens, albeit cataracted.
3. A refractor's objective and an ocular are comprised of lenses; and a prism, if integrated, refracts light in addition.
4. The human eye, its lens, also refracts, with its cones and rods conveying the images of said galaxies to the brain.

And now for a little fun...

A gent bought a scope in Nantucket;
A refractor instead of a light bucket.
Refracted it did rather than reflecting his id,
And all the world vied to look through it!

Cheers,

Alan


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Andy Taylor
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Re: Are achromats enjoying a come back? new [Re: Mark Costello]
      #5517939 - 11/13/12 01:23 PM Attachment (30 downloads)

Quote:

Just what fraction of owners build their own refractors??




Meee.....

Carton 100mm F13.

Go on - you know you want to look through it....


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Mark Costello
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Re: Are achromats enjoying a come back? new [Re: Abb]
      #5517942 - 11/13/12 01:24 PM

Quote:

I've never had the chance to look through an Apo but I'll say this, the difference between a good lens and a cheap lens of equal (or almost equal) size can make a noticeable visual difference. Fortunately, this I've seen/experienced through my refractors.




"anyway ... equipment choice is a personal preference. "objective facts" and parodic argument can be marshaled on both sides. separate from that, achromats form a long and honorable tradition, and i'd wager that the best achromats today are the best achromats ever. like books compared to computers, they're outdated technology when compared to apochromats, but apochromats are unlikely to dislodge them. even 19th century myths are able to endure -- probably because they are quaintly entertaining."

Well said and thanks, Abb and Bruce. Sometimes when I pick up a book while people pick up Kindles, I have the same thoughts that Samuel T. Cogley expressed on the StarTrek episode "Court Marshall."

As to achros, I had no idea they were making a comeback because I had no idea that they went away anywhere. As to the cost of an apo coming down to match the cost of an achro, well, we've had "budget" 80mm apos for nine years now and they still start at about twice the price of short tube achros and 80mm F11 achros. So I'm guessing I won't live to see a 5" apo sell for the price of something like my AR127 or a 120mmF8.3 achro. As to making one, don't forget the hidden costs of acquiring the skill to make a refractor tube assembly and acquiring the tools and machines with which to do it....


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Mark Costello
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Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5517963 - 11/13/12 01:32 PM

Hi Andy. Just for clear communications, I wasn't implying that no one built their own refractors, only that it might be a small fraction of those who own them. My post was in response to someone who basically was saying that one could build an apo for the cost of an achro (assuming that the builder would buy the lens or the complete lens cell). My problem with that is that I don't have the skills to build an OTA and would have to spend time and money learning the skills and more visibly, I'd have to spend money setting up a shop with which I could make a refractor OTA....

Lest I forget, that refractor is a beautiful job.


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: Mark Costello]
      #5518019 - 11/13/12 02:11 PM

Quote:

As to achros, I had no idea they were making a comeback because I had no idea that they went away anywhere. As to the cost of an apo coming down to match the cost of an achro, well, we've had "budget" 80mm apos for nine years now and they still start at about twice the price of short tube achros and 80mm F11 achros. So I'm guessing I won't live to see a 5" apo sell for the price of something like my AR127 or a 120mmF8.3 achro.




Mark:

I think it is unlikely that apos and ED scopes will ever get down to the price of an achromat though there are instances where this might be true... The ED glass is more expensive and more difficult to work.

But if you look at what was available 10-15 years ago in terms of a quality achromat, apo's like the ED-80 and the ED-100 probably represent comparable prices...

One example might be the StellarVue 102D which sold for about $700 8-10 years ago and the Lunt 102ED which sells for about $700 today. Another example might be the SV AT-1010 which sold for something like $400 or even more before the arrival of the ED-80, on sale, at times the ED-80 has been on sale at $400.

I think the quality of affordable telescopes is improving year by year. But probably the most important thing about a telescope is that some one enjoys it. It really doesn't matter if it's an apo, an achromat, a SCT or a Newtonian, what is important is that someone gets out there under the stars and just has a good time with it.

Jon


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DarkStar1984
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Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5518030 - 11/13/12 02:18 PM

Andy,

Beautiful scope. I just purchased a classic original 4" Carton 100mm F13 and mount. I can't wait for it to arrive Cloud magnet you say...


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7331Peg
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Re: Are achromats enjoying a come back? new [Re: drollere]
      #5518057 - 11/13/12 02:33 PM

" . . . for visual purposes at a reasonable price point, achromats are a fully satisfactory and even superior choice to apochromats. that hardly makes consumers who prefer them clowns of the renaissance faire. perhaps it makes them people who believe any dollar difference should have a proportional visual delta."

No one will ever say it any better than that -- extremely well put, Bruce.

"Proportional visual delta" -- now why didn't I think of that!


John


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Yu Gu
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5518155 - 11/13/12 03:33 PM

I would really appreciate a source for the numbers you mentioned.

Quote:

Actually that really doesn't follow. Capital investment in optical fabrication and design is being made in designs that employ low dispersion glass types. "ED glass" is the new "fact" that the industry is assimilating. It is becoming plentiful and relatively inexpensive due to the large scale investment in its fabrication.

This availability of new materials, in turn, pulls investment in equipment optimized to mass-produce optics employing this new material. No one is making large scale equipment investment in computerized grinding and polishing machines for purposes of making C-F achromats. The ED 80 that essentially crippled the small achromat market that had been thriving before its arrival, was a creature of massive investment by Synta in the finest Zeiss optical fabrication equipment. These machines can turn out 1/5 to 1/7 wave ED doublet optics in an endless stream in apertures from 80mm to 120mm. With virtually no investment in modernizing the toolings for achromats (due to low demand) such optics require more hand work and therefore (aside from the cost of materials) are actually more costly to make than ED optics produced with a high degree of automation.

Typically hand-figured achromats from the 50s, 60s and 70s, when tested with an interferometer, on average weren't as well figured as mass-produced ED optics made by robots at the end of the last century. There are exceptions, of course, but I've seen many a 4" Unitron with suspect optical quality. The handful of Clark refractors I've seen bench test data for came in right around 1/4 wave.

Not only is fabrication more consistent and reliable today, but also testing is more accurate and precise too.

Regards,

Jim




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Bonco
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Re: Are achromats enjoying a come back? new [Re: Yu Gu]
      #5518223 - 11/13/12 04:16 PM

Simply stated, achro's ARE making a comeback. Why? Because there are many models available at reasonable prices and excellent quality. Mid sized APO's on the other hand (4-6 inch) have truely astronomical prices. My humble 3 element AP 6 inch f/8 was offered in the late 80's for around $1500. I've seen that scope sell recently for over $5000. If I were in the market today for a 6 inch f/8, I'd be looking at achro's.
Bill


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Andy Taylor
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Re: Are achromats enjoying a come back? new [Re: DarkStar1984]
      #5518270 - 11/13/12 04:46 PM

Quote:

Andy,

Beautiful scope. I just purchased a classic original 4" Carton 100mm F13 and mount. I can't wait for it to arrive Cloud magnet you say...




Bet it wasn't cheap...

Probably the best objective ever made and around 40 years old too...

I've tried and recommend a prism diagonal. Mine now has almost zero false colour now using one. A stunning planet killer. Try the Double Cluster as a first target - it'll make your toes curl.

Who needs an APO?

PS: Never looked thru an Apo so what do I know...


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greju
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Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5518319 - 11/13/12 05:01 PM

"Probably the best objective ever made..." I cringe whenever I see a statement like this. Could be describing a very good item but immediatly gives me a bad feeling about it. YMMV

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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: 7331Peg]
      #5518324 - 11/13/12 05:05 PM

Quote:

" . . . perhaps it makes them people who believe any dollar difference should have a proportional visual delta."

No one will ever say it any better than that -- extremely well put, Bruce.

"Proportional visual delta" -- now why didn't I think of that!


John




Well, humm....

If you go with the concept of a proportional delta, that is double the cost, double the performance, naked eye is your best bet followed by last year's Walmart $40 Powerseeker 70 Achromat.

If one is looking to buy quality rather than quantity/aperture, then proportional delta is best figured in relation to perfection... a scope that is 95% perfect costs twice as much as a scope that is 90% perfect.

I have two 4 inch refractors, an Orion AstroView 100mm F/6 and a TeleVue NP-101. Both were purchased used, the achromat cost about $100, the NP-101 cost $2000. In terms of proportional increase in performance versus cost, clearly the 100mm achromat is a reasonable enough performer that one could not justify the NP-101 based achieving 2000% better views.

On the other hand, if one is looks at it from the other end, how close to perfect the scopes are, certainly the NP-101 about as close to perfection as I could want, the Astroview has some issues.

So in the long run... cost is relative, what it's worth to each of us, it varies. A good telescope is a lifetime investment, it doesn't wear out... I drive inexpensive cars, spend a little on some decent telescopes.

Works for me.

Jon


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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5518506 - 11/13/12 06:54 PM

One always sees apo's and ED's marketed to the imaging crowd like no one actually looks through the telescope anymore. I have no interest in all the imaging craze, I am purely a visual observer and that is where the advantages of the the apo versus achro debate start to crumble. The visual difference between a well made achro and an apo are not that great, I used to own an Orion 80ED and ST80. Could see stuff pretty much the same through both except a slight advantage on planets for the ED80. I suspect a slower 80MM scope such as the Vixen A80Mf could easily hold its own against an ED80.

It all depends on if you demand perfection and if you can afford to own that perfection. When I am able to get back into the hobby in the next few months with a new scope I am quite sure I will be sitting behind the eyepiece of a long achro, the Celestron Omni 102 is looking real good right now. A whole setup for less the $500.


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hottr6
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Re: Are achromats enjoying a come back? new [Re: 7331Peg]
      #5518807 - 11/13/12 10:19 PM

Quote:

" . . . for visual purposes at a reasonable price point, achromats are a fully satisfactory and even superior choice to apochromats. that hardly makes consumers who prefer them clowns of the renaissance faire. perhaps it makes them people who believe any dollar difference should have a proportional visual delta."

No one will ever say it any better than that -- extremely well put, Bruce.



Some of us clowns prefer our achros double-barreled!


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Pinbout
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5518969 - 11/14/12 12:55 AM

Quote:

I suspect a slower 80MM scope such as the Vixen A80Mf could easily hold its own against an ED80.








mine tests 1/6~ on double pass test against a flat.



the ca is only noticable on bright defocused stars.



but that stock diag. has to go.


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chboss
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5519015 - 11/14/12 02:29 AM Attachment (19 downloads)

Quote:

One always sees apo's and ED's marketed to the imaging crowd like no one actually looks through the telescope anymore. I have no interest in all the imaging craze, I am purely a visual observer and that is where the advantages of the the apo versus achro debate start to crumble. The visual difference between a well made achro and an apo are not that great, I used to own an Orion 80ED and ST80. Could see stuff pretty much the same through both except a slight advantage on planets for the ED80. I suspect a slower 80MM scope such as the Vixen A80Mf could easily hold its own against an ED80.




Well that is definitely a point where I have to disagree.
In 1997 I still owned my Japanese made Asahi-Pentax 4" f/11 Achro which gave me a lot of viewing pleasure and showed a very good start test pattern. But when I first had the chance to take a look through a Tak FS-102 (f/8) I recognized there was no contest. Mainly looking at Jupiter and Saturn the difference was so striking that I acted and bought myself an FS-102.
I still own and use this instrument to this day and must say that it was one of the best long term investments I have made in astronomy.

Prices may be higher but if you look at long term satisfaction, even for visual use the decision should be clear.
The whole picture changes of course if we are talking about 6" and larger refractors where the price difference is becoming astronomical.

best regards
Chris


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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5519030 - 11/14/12 03:14 AM Attachment (36 downloads)

Think the comparison is based on the contrasts of the images got from each.
Debate is more on the data collected in each. The comparison will go to no significant difference.
Here is the 6" achromat owned here. Nothing commercial except the doublet in cell from istar. Tube from a 6" newtonian made of resin, counter-cell made of resin, crayford support made of pieces of plywood and aluminium sheet plate, light hood from an alu steel plate. Every thing is internally floculed with a mat velours. Weight 9kgs and costs less than 800usd. Results are strictly similar to an apo on Venus, Mars and Saturn. For Uranus get better.
Easy to perform a 6" cheap and with results. The problem is not to get pleasing views but to get access to data. We have here.
Stanislas-Jean


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chboss
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5519085 - 11/14/12 05:32 AM

Nice work Stanislas-Jean, beautiful scope!
As stated above the equation looks different at 6" and above.

best regards
Chris


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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5519138 - 11/14/12 07:38 AM

Frankly this is on paper.
Because, if you consider for example people who are observing a lot from a balconee (10m and above the ground) they get higher images than the same scope on same area on the ground (very sensitive in a 16" reflector).
The power of the great refarctor is helped by the fact that the doublet or triplets for some are high with regards to the ground.
I did a statistic over 2010 when using on Mars the 6" refractor versus the intes 615 and newtonian 150mm.
With the 1m elevation seeing difference mostly a step in Danjon scale was win in the refractor (means 3/5 in the reflectors was mostly 4/5 in the refractor). By this fact the global efficiency was better.
Just to add that for each case of scope there was NO internal air currents and PTV levels the same ranging PTV7 to 9.
I think the considerations must be global not on paper.
For the anecdote, I reported a lot also on planets with 7"
compacts, a mewlon (sold) and an intes715. Reports with the intes were oftenly discarded because an intes, but with the mewlon so considered! Results were quite the same.
For refractors situation is similar. Stunning.
Stanislas-Jean


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5519153 - 11/14/12 08:11 AM

Quote:

The visual difference between a well made achro and an apo are not that great, I used to own an Orion 80ED and ST80. Could see stuff pretty much the same through both except a slight advantage on planets for the ED80. I suspect a slower 80MM scope such as the Vixen A80Mf could easily hold its own against an ED80.




Over the years, I have had several ST-80's, currently own the Ioptron version, have owned a ED-80, own an 80mm William Optics FD, owned a Vixen A80MF as well as two Mizar-Meade 80mm F/11's and a Meade Towa. To my eye this is what I see:

- The difference between an ST-80 and an ED-80 is significant when viewing the planets or splitting double stars, it's particularly noticeable on faint companions, delta Cygni comes to mind, a relatively easy split with an ED-80, tried but never done it with an ST-80. The longer 80mm F/11s are much better at this than the ST-80 but still not as capable as the ED-80s type scopes.

The thing that one comes back to time and time again is that an apo is a do it all scope in a compact package. An 80mm F/15 might be comparable at high magnifications to the William Optics FD but it's 4 feet long. An ST-80 might be nearly as capable of the widefield views as the 80mm F/7 with it's 2 inch focuser but it runs out of gas as magnification is increased. What the apo offers is the ability to provide the sharp, clear high magnification views as well as the big, wide low power views all in a compact package...

That said, the Omni 102 should be very enjoyable. In my experience with the Celestron 102 F/10 refractor, they provide quite reasonable planetary views and are capable of something like a 2.6 degree TFoV with the right eyepiece...

Jon


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Mark Harry
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5519154 - 11/14/12 08:11 AM

Read your reply; not sure of your source of information either.
******
The technology of CNC optics existed more than 10 years ago, and Zeiss-Jena wasn't in the business of mfring machines to do it at the time I was working there.
I also know something of designing achros, and 2 element apos---and the tolerances of the apos are far tighter on average than just a plain achro. I made a 70m F/25~ achro with absolutely no testing other than to use a spherometer, and it was diffraction limited for RGB wavelengths (essentially, no color)
I used to finish up reference elements for Zygo when the CNCs were having issues of hitting the tolerances needed. (done by HAND) Took about 15 minutes to finish one that typically sold for $10k---4" diameter. (ouch!)

My 2 cents, no offense. But it's based on first-hand knowledge/experience.
M.


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Andy Taylor
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Re: Are achromats enjoying a come back? new [Re: greju]
      #5519237 - 11/14/12 09:19 AM

Quote:

"Probably the best objective ever made..." I cringe whenever I see a statement like this. Could be describing a very good item but immediatly gives me a bad feeling about it. YMMV




Sorry - I meant achro objective. You can stop cringing now.


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Abb
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5519259 - 11/14/12 09:43 AM

Well, until the prices of APO's do come down to the current prices of Achro's (if I should live that long!), I'd sooner spend the $$ difference on good lenses. Besides, what good is an Apo if you haven't any $$ left over for some good lenses

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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: Abb]
      #5519278 - 11/14/12 09:58 AM

The problem is roughly
we expense 10 000€ for an AP 6" in france against
800usd for the given posted achomat 6" for same results on planets visually.
The diff is not few.
The problem again is not to get pretty images but to access to the same data, strictly the same data, that we have.
Now for 10 000€ with being a little astucious I can access to a 20" with PTV8. An other world under hands.
Stanislas-Jean


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: Abb]
      #5519407 - 11/14/12 11:34 AM

Hi Abb,

Used apochromats that spring up on the market from time to time are being sold for reasons ranging from the desire to fund a larger apo, a faster apo, a smaller apo even, an achromat, another format entirely such as those employing mirrors..., loss of interest altogether or, regrettably, arising from the need to meet daily expenditures. Given the latter, the economy is such that this is a good time to act if you're curious and seriously interested in owning an apochromat. Just don't buy the first one you see. A week of research prior to the big decision will help to ensure that you'll find one with which you'll be satisfied and tailored to your observing interests, for example...

http://www.astromart.com/classifieds/details.asp?classified_id=454629

Note the seller's reason for selling. Unfortunately, it's already sold, but there will be others.

One should always exchange mirrors for lenses, but never the other way around.

Cheers,

Alan


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Astrojensen
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5519660 - 11/14/12 03:05 PM

Quote:

One should always exchange mirrors for lenses, but never the other way around.




Why not? Observing habits and interests can change. My cheap 12" Meade Lightbridge is doing a fantastic job of showing deep-sky objects like no refractor I've ever looked through has ever shown them, even 7" apos. And it cost less than a 3.5" FT focuser...

Refractors are fantastic, but so are reflectors.


Clear skies!
Thomas, Denmark


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Mark Costello
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5519818 - 11/14/12 05:12 PM

"I think the quality of affordable telescopes is improving year by year. But probably the most important thing about a telescope is that some one enjoys it. It really doesn't matter if it's an apo, an achromat, a SCT or a Newtonian, what is important is that someone gets out there under the stars and just has a good time with it."


Hello there Jon. I thought about this a bit when I trotted out yesterday for 90 minutes with my 5" achro. The occasion was what might be my "farewell for the year" tour of Cygnus as it's starting to get under the trees in my back yard now. I'm going to write about last night's session in the Deep Sky Forum here and/or on AM. But I'll tell you that one thing that was NOT on my mind was "I wish I had an apo" or anything like that. There may have been one or two occasions when I was thinking "I wonder what that might look like in a big cat or Dob?"


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5519850 - 11/14/12 05:35 PM

I own two Newtonians, but why would I ever want to exchange a direct view for a reflection? That would be like exchanging a fine sapphire for common blue topaz.

In other words, purchase the largest Newtonian you can find, but never sell a fine apochromat to fund it.

And yes, refractors are indeed fantastic, as I just observed the Sun with my new solar filter, my FS-102 and the .965" 20mm Kellner depicted within my avatar, and for the first time, the Kellner being the first ocular I ever observed with, as a child. I was pleased to find it every bit as good as expected.

Cheers,

Alan


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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5519980 - 11/14/12 06:59 PM

No doubt the Tak apo put up a wonderful visual experience, it's a Takahashi. But noticed I mentioned affording perfection. The Takahasi OTA alone will run one over $3000 (and therefore should be optical perfection) while something such as the Omni 102 cost only about $500 and comes with a mount. Yes, I suspect the Omni 102 may not be an example of optical perfection, but it will have to do with my current budget.

Would I like to own a Tak apo some day? Heck yeah! But not happening right now. And I do not think I will feel that I am missing out when I turn that achro to the sky sometime in the future, having been without scope for over a year now.


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Astrojensen
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5519982 - 11/14/12 07:00 PM

Quote:

but why would I ever want to exchange a direct view for a reflection?




I simply don't understand this. I'm afraid you've lost me. If I see, in the eyepiece, a splendid view of NGC 891 (or whatever), I couldn't care less whether the photons are collected by a mirror or a lens. It doesn't make a scrap of difference to me.


Clear skies!
Thomas, Denmark


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jrbarnett
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Reged: 02/28/06

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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5520010 - 11/14/12 07:16 PM

"I simply don't understand this. I'm afraid you've lost me. If I see, in the eyepiece, a splendid view of NGC 891 (or whatever), I couldn't care less whether the photons are collected by a mirror or a lens. It doesn't make a scrap of difference to me."

By extension, "I simply don't understand this. I'm afraid you've lost me. If I see, on the computer screen [or on NASA's HST web page, etc.], a splendid view of NGC 891 (or whatever), I couldn't care less whether the photons are collected by a mirror or a lens. It doesn't make a scrap of difference to me."

And yet, it does make a huge difference to many observers. It's all about "being there". "Being there" with mirror-free refraction, if you subscribe to Feynman's quantum model for light at least, is closer to touching your target than "being there" using mirrors, which in turn is closer to "being there" using CCD cameras or looking at images by others of a target. It's all relative.

For me, I prefer drinking in that ancient light over increasingly removed facsimiles. I have no objection to you treating approximations as being equivalent to the real thing, however. Whatever collimates you mirrors.

- Jim


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: drollere]
      #5520050 - 11/14/12 07:49 PM

"...and i'd wager that the best achromats today are the best achromats ever."

Perhaps, or perhaps not. But that may not say much, really. What matters is how good are they relative to other refractor designs. Saying that the common cold is preferable to pneumonia doesn't really make either that attractive if health is a third alternative.

Apochromats fetch a higher price than like apertured achromats, and provide the manufacturer with larger per-unit margins. Because there is extra money in each scope (which could be used to improve optics quality should the manufacturer elect not to pocket all of it) producing better quality optics is a economically viable possibility for the apochromat seller. Not so for the achromat seller.

What would be truly interesting would be to determine how *bad* in terms of optical quality many beloved achromats of yesteryear actually were. I have a little data on the topic, but would love to have more.

- Jim

Edited by jrbarnett (11/14/12 08:20 PM)


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: Mark Harry]
      #5520089 - 11/14/12 08:18 PM

Sorry, Mark.

Please substitute "Satisloh" for "Zeiss" in my comment. My bad.

Let's start with this:

http://www.astromart.com/forums/viewpost.asp?forum_post_id=458245&pol...

"They are simply polished on high speed automatic polishing machines (made in Germany), coated and assembled. It is only when you get to 5"-6" sizes that any sort of figuring is necessary, and then only if you wish to have zero or very low chromatic aberration. If you allow some color error in the final design, you can conceivably aslo get away with almost no figuring. All you need to do is vary the spacing between front and rear to null out the lens."

"The typical optical shop in China these days is highly automated with the most modern CNC optical machines, almost all of which are made in Germany - and they are not cheap, but very effective. According to several trade magazines that I get here, China has invested almost 3 times as much money in modern automated machinery than the US has over the last 5 years. While a lot of US companies toil on with outdated machinery, some dating back to the second world war, China has forged ahead, in this case far ahead of us, with the most modern equipment that you can buy."

Then let's look at interferometric test reports (from a single source, using consistent testing methods) for recent Chinese-made achromats and apochromats. Not exhaustive, but here are two examples of each:

Recent Chinese (PROC and Taiwan) achromats:

DKD 130/1000 achromat (PROC): http://www.astro-foren.de/showthread.php?11121-China-FH-ein-brauchbarer-Achro...

Meade 90/1000 achromat (Taiwan): http://www.astro-foren.de/showthread.php?10898-Meade-Explorer-90-1000-Model-3...

Recent Chinese (PROC and Taiwan) apochromats (one doublet and one triplet):

Celestron (Synta; China) 80/600 ED doublet: http://www.astro-foren.de/showthread.php?8888-Celestron-80-ED&p=34104#pos...

107mm f/6.5 triplet (China): http://www.astro-foren.de/showthread.php?10801-SUPER-APO-APM-107-f-6-5&p=...

Not "cherry picked", but rather grabbed quickly (the first four I opened). Feel free to peruse the other data on the site. I think these are as likely as not representative.

It would appear to me that "China, Inc." is using its expensive German automated CNC grinders and polishers to make better quality apochromats than achromats. If so, the logical explanation is that there's little money in achros compared to apos, so it doesn't pay to put extra effort into the achro. You're better off putting your machine and human resources on apos.

No offense taken, and none intended. No personal knowledge on my end, either. Just a big memory, Google and reason.

Regards,

Jim

Edited by jrbarnett (11/14/12 08:22 PM)


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chboss
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Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5520173 - 11/14/12 09:18 PM

Thanks Jim

I was tempted to do a similar comparision using Wolfgang Rohrs data.
But after less than favorable comments in the past, I let that idea go.

I would love to see more comparison data with recent production Achro's both mass made like Synta, Meade, Celestron and boutique type such as D&G or Istar.
It seems that far more users have their expensive APO's tested by Wolfgang so there is more data available than for Achro's.

This data would give real proof of how good the current offerings in the Achro sector are and allow a fair comparison of price and performance.

best regards
Chris


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jrbarnett
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Reged: 02/28/06

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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5520312 - 11/14/12 10:56 PM

"It seems that far more users have their expensive APO's tested by Wolfgang so there is more data available than for Achro's."

I noticed that too. What I really need for an article I am working on are such reports for vintage optics; 1800s and early 1900s achromatic doublets, and 1950s and 1960s parabolic mirrors. I'm actually tempted to buy some truely vintage scopes and have the optics tested by OMI.

Regards,

Jim


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5520318 - 11/14/12 11:09 PM

I saw the core of Andromeda a few nights ago through my refractor in conjunction with the first-light test of my new 32mm, two-inch, 70-degree ocular(my first two-inch). It was a splendid view as well, except for the fact that I don't own a two-inch prismatic diagonal, the diagonal containing a mirrored flat instead. I attempted a straight-through observance, but, the focuser chose to exhibit its then heretofore unbeknownst lack of back-focus travel. I even tried to insert the ocular barrel in just enough for the compression spring to make contact. Imagine my horror.

I purchased the flat in lieu of a prism due to price considerations, as there were no prismatic diagonals of that size on sale for $99.00, at the time, and the William Optics, after careful research, seemed to be one of the best, for a flat. Incidentally, I purchased it only in future anticipation of a modest collection of two-inch oculars, with my now having acquired my first.

Had I had said extension that night, the view would've been better than splendid; aethereal more like it, but with the flat integrated...

<Holst's "Jupiter" playing loudly in the background>

Bill: "Frank, I came over here to talk to you about me and Marge!"
Frank: "I'm listening!"
Bill: "I can't talk to you while you're looking in the mirror, and I'm having to raise my voice!"
Frank: "I'm! I'm...cleaning it!"

However, admittedly, had it not been for the flat, the two-inch ocular would've remained in the drawer.

Cheers,

Alan


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5520391 - 11/15/12 12:10 AM

Jason,

Yes, this one...

http://www.chuckhawks.com/celestron_XLT102.htm

With Celestron, and with the optics coming out of China being of surprisingly very good quality nowadays, I've considered it myself, and as a replacement for the Parks 80mm f/11 achromat I had before the Tak. Back in the early 90's, I got a Meade 90mm refractor/alt-az setup. The optics were bad, very bad, and were made in Taiwan at the time. After I returned it, I got the Parks, made in Japan, and it was much better.

I think the Celestron Omni XLT on the CG-4 mount would make a great replacement. It comes with a 25mm ocular, and would need, say, a 10mm in addition at least to cover a wide range of viewing. The 10mm plossl that came with my Orion StarBlast 6 reflector is also surprisingly good. A lot of people have rated their plossls favorably.

Regards,

Alan


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5520431 - 11/15/12 12:56 AM

"Observing habits and interests can change."

An observing habit would be more along the lines of whether one utilises a diagonal or not, sitting rather than standing at an eyepiece, utilising a Newtonian in Dobsonian fashion other than being equatorially-mounted, or employing averted vision in one's detections versus not seeing it at all.

Additionally, a change in observing interests would consist of viewing planets one night, double-stars the following night, then open and globular clusters the next...

...but never preferring a reflection over direct observation.

Only because I have to, will I observe and have observed via a mirrored arrangement, and all the while wondering, "If only I might see this through a great refractor."

Cheers,

Alan


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5520443 - 11/15/12 01:10 AM

Quote:


I noticed that too. What I really need for an article I am working on are such reports for vintage optics; 1800s and early 1900s achromatic doublets, and 1950s and 1960s parabolic mirrors. I'm actually tempted to buy some truely vintage scopes and have the optics tested by OMI.




Jim, try to write Wolfgang an e-mail and ask him what kind of telescopes you are interested in. He might have some additional data of older telescopes. I am sure that he will be interested in your project. I am keen to see how vintage instruments measure up on an interferometer.

best regards
Chris


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5520481 - 11/15/12 01:59 AM

Jim, you may contact also Airylab in France
http://www.airylab.com/
ask Frederic for your enquiries.
There are few observers and semi-professionnals who are acting in the astronomy fields. They could report about the instruments existing in the country (refractors and reflectors still in activities. They did test investigations on (the Nice 760mm refractor it seems for one).
Stanislas-Jean


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chboss
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Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5520495 - 11/15/12 02:45 AM

Jim,

Here is an interesting report of a 1906 Zeiss 300mm telescope that was restored and the optics evaluated. Source W. Rohrs list of optic test reports:
http://rohr.aiax.de/Optikpruefung[1].pdf

This telescope is used for outreach by the Urania observatory in Zürich, Switzerland:
http://www.urania-sternwarte.ch/portraet.html

best regards
Chris


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Astrojensen
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Loc: Bornholm, Denmark
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5520560 - 11/15/12 04:34 AM

Quote:

By extension, "I simply don't understand this. I'm afraid you've lost me. If I see, on the computer screen [or on NASA's HST web page, etc.], a splendid view of NGC 891 (or whatever), I couldn't care less whether the photons are collected by a mirror or a lens. It doesn't make a scrap of difference to me."




OK, now I'm complete lost. What does astrophotography have to do with it all? Are you saying that looking at the stars visually with a reflector is basically the same as looking at a photography, as in, not looking at it at all?

When I am looking at the stars with a reflector or a refractor, I am looking at a live view. My eye registers photons that has just ended their long journey. If I am looking at a photography of something, I have let a camera do that final detection of the photon. That is different than visual. The choice of telescope makes no difference. The method of detection does. The difference between reflection and refraction in this regard is a philosophical discussion at best.

Why does refractor afficionados so often have to come up with the weirdest, most convoluted reasons and justifications to why they use refractors? Can't they just say that they love refractors? Love doesn't need reason, logic or justification. It just is. I love refractors. If I didn't, I wouldn't own ten of them, would I? But more than love refractors, I love telescopes, because they allow me to observe the universe in far greater detail than with my eye alone.

Or is love just a overused, boring word these days, its meaning diluted and its lustre pale from its constant overuse in the modern pop culture?


Clear skies!
Thomas, Denmark


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5520572 - 11/15/12 05:08 AM

Quote:

Quote:

but why would I ever want to exchange a direct view for a reflection?




I simply don't understand this. I'm afraid you've lost me. If I see, in the eyepiece, a splendid view of NGC 891 (or whatever), I couldn't care less whether the photons are collected by a mirror or a lens. It doesn't make a scrap of difference to me.


Clear skies!
Thomas, Denmark




Thomas:

It's just a bit of dogma, light that travels through glass is no more direct light than light that is reflected by a metallic coating. When you think of the tortured path that light travels through a refractor, the light is split apart, each color of spectrum traveling a different route and never really quite being put back together correctly, it's surprising they actually work at all.

You have years of making amazing observations with small long focal length refractors like your Zeiss Telementors. You developed your skills with small telescopes and now combined with the dark skies of a small island in the Baltic sea, you are now using those skills with a large reflector. How wonderful can that be?

Young, great eyes, dark skies, what more could one want? They say youth is wasted on the young but sometimes there are those who are wise beyond their years, how lucky you are.

My suggestion: Just let them have their fun, let them have their illusions.

Jon Isaacs


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Astrojensen
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5520585 - 11/15/12 05:35 AM

Thanks Jon.


Clear skies!
Thomas, Denmark


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Binojunky
Carpal Tunnel
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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5520613 - 11/15/12 06:14 AM

One could have the best of both worlds, the wonderful world of a fine Mak, get your lens and a mirror thrown in for free, for me anyway I have no preference how light is collected and brought to my eye,all designs have strengths and weaknesses, to put the argument into perspective if like friend of mine you have to get rid of your scopes because of impending blindness due to eye disease then the argument of which is the better way to collect the light is silly and irelevent, be thankfull you have sight, nuff said,DA.

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Astrojensen
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Re: Are achromats enjoying a come back? new [Re: Binojunky]
      #5520624 - 11/15/12 06:30 AM

Quote:

to put the argument into perspective if like friend of mine you have to get rid of your scopes because of impending blindness due to eye disease then the argument of which is the better way to collect the light is silly and irelevent, be thankfull you have sight,




Well said.


Clear skies!
Thomas, Denmark


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Mark Harry
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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5520699 - 11/15/12 07:53 AM

I got a question. Why do more APO owners get their scopes tested, as referred to on the last page?
********
Can't they use the scopes by themselves and assess the quality, or is this an angst about test results/numbers???


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covey
member


Reged: 07/02/11

Re: Are achromats enjoying a come back? new [Re: Mark Harry]
      #5520715 - 11/15/12 08:13 AM

Because it's part of the enjoyment of the hobby....and they enjoy doing it.

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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: covey]
      #5521057 - 11/15/12 11:46 AM

"Are you saying that looking at the stars visually with a reflector is basically the same as looking at a photography, as in, not looking at it at all?"

Not quite, just a further regression within the set...

1. Viewing through a refractor is exactly like standing in front of someone. They see you, and you see them, and a conversation ensues, or whatever.

2. Viewing through a mirrored arrangement also amounts to conversing with someone, though not standing before them face to face, instead seeing only their reflection via a mirror, as illustrated within a previous posting.

3. A photograph is even further removed from the living experience, the image being all the more impersonal, thus with #2 and #3 being patent regressions of #1.

And then, there's television...

<Holst's "Mars" playing loudly in the background>
Frank turns from the mirror, takes his Polaroid and snaps a photo.
Bill: "That does it! I'm leaving! Goodnight Frank!"
Frank: "Goodnight Bill!"
<door slams>
Frank walks over to the stereo, turns the volume down and bumps the needle to 'Saturn', thinking, "Wait 'til Marge sees this, his sad expression! She's sure to take him back!"

Cheers,

Alan


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521143 - 11/15/12 12:32 PM

Quote:

1. Viewing through a refractor is exactly like standing in front of someone. They see you, and you see them, and a conversation ensues, or whatever.




Not really...

Looking through a refractor is like looking through a fish bowl and for most us we are facing the ground, not the person or the sky.

With a Newtonian you look through the Sky End of the scope, with a refractor, the dirt end...



Jon


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Sky Muse
sage


Reged: 10/26/12

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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521159 - 11/15/12 12:40 PM

"...to put the argument into perspective if like friend of mine you have to get rid of your scopes because of impending blindness due to eye disease then the argument of which is the better way to collect the light is silly and irelevent, be thankfull you have sight..."

My father, 77, has been plagued all his adult life by cataracts. He now has only one "good" eye, but it nonetheless becomes filled with hundreds of floaters when disturbed. My older brother inherited this unfortunate condition as well, his having had more than one operation. Thankfully, I inherited my eyes from my mother, and escaped it, just barely, and never a day goes by without my reflecting upon it.

Happily, the other night, I was able to show my father Andromeda, and as I explained its particulars, its distance, the fact that you'd have to hop upon a beam of light and travel for two million years before reaching its perimeter. I asked, "Can you see it?" He replied, "Yes", and described it somewhat. Of course, I will never know how well in fact.

However, that, too, is beside the point.

Cheers,

Alan


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Sky Muse
sage


Reged: 10/26/12

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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5521166 - 11/15/12 12:44 PM



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Sky Muse
sage


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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521179 - 11/15/12 12:54 PM

The lens of an eye is just that, a lens, a "fish bowl", and just as a refractor's objective.

Sir, you dare question the design of the Maker?

Cheers,

Alan


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Astrojensen
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521309 - 11/15/12 02:14 PM

Quote:

never a day goes by without my reflecting upon it.




Did you just write that?


Clear skies!
Thomas, Denmark


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Astrojensen
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521320 - 11/15/12 02:19 PM

Quote:

The lens of an eye is just that, a lens, a "fish bowl", and just as a refractor's objective.

Sir, you dare question the design of the Maker?




There are actually at least one species of fish and, I believe, one of squid, who use reflectors in their eyes for collecting light. Their eyes strongly resemble Schmidt or maksutov cameras, have an extremely fast f/ratio and are very large, compared to the animal. These animals live deep in the sea, where there is basically no light, save for faint bioluminescence.


Clear skies!
Thomas, Denmark


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5521332 - 11/15/12 02:24 PM

You caught that, eh? So did I when I wrote it, and then left it.

Cheers,

Alan


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csrlice12
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Loc: Denver, CO
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521382 - 11/15/12 02:59 PM

"One should always exchange mirrors for lenses, but never the other way around."

Like anything else, use the proper tool for the job. Wanna look at Jupiter, Globular Clusters?--get a refractor

Want to see faint nebulas and galaxies--Get a Dob/Newt

Want to see everything--get both.

Neither a reflector nor a refractor can do it all. Neither is "better" then the other, just different. They both have their advantages and disadvantages.


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Sky Muse
sage


Reged: 10/26/12

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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5521388 - 11/15/12 03:01 PM

Then, there you have, in the animal kingdom, rare, living, extant inspirations for Newton's, Schmidt's and Maksutov's designs, though no doubt unbeknownst to said gentlemen at the time. I always suspected some organic reason or other for the origin of those optical designs. However, I have, as points of reference, the eyes of the vast remainder of the animal kingdom, including human beings.

Incidentally, we are not fish or squids, but human beings.

Cheers,

Alan


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csrlice12
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521389 - 11/15/12 03:03 PM

Quote:

Jason,

Yes, this one...

http://www.chuckhawks.com/celestron_XLT102.htm

With Celestron, and with the optics coming out of China being of surprisingly very good quality nowadays, I've considered it myself, and as a replacement for the Parks 80mm f/11 achromat I had before the Tak. Back in the early 90's, I got a Meade 90mm refractor/alt-az setup. The optics were bad, very bad, and were made in Taiwan at the time. After I returned it, I got the Parks, made in Japan, and it was much better.

I think the Celestron Omni XLT on the CG-4 mount would make a great replacement. It comes with a 25mm ocular, and would need, say, a 10mm in addition at least to cover a wide range of viewing. The 10mm plossl that came with my Orion StarBlast 6 reflector is also surprisingly good. A lot of people have rated their plossls favorably.

Regards,

Alan




Getting the 102XLT w/CG4 mount for Xmas. It will make a great combo with my 10XTi. Of course, now i gotta learn how to use an EQ mount--and for some reason, I look forward to it.....


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csrlice12
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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5521406 - 11/15/12 03:14 PM

Quote:

Quote:

The lens of an eye is just that, a lens, a "fish bowl", and just as a refractor's objective.

Sir, you dare question the design of the Maker?




There are actually at least one species of fish and, I believe, one of squid, who use reflectors in their eyes for collecting light. Their eyes strongly resemble Schmidt or maksutov cameras, have an extremely fast f/ratio and are very large, compared to the animal. These animals live deep in the sea, where there is basically no light, save for faint bioluminescence.


Clear skies!
Thomas, Denmark




Danged bottom-feeders get all the good equipment......


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: csrlice12]
      #5521411 - 11/15/12 03:17 PM


"...get a refractor"

I have one.

"...Get a Dob/Newt"

I have two of those.

"Want to see everything--get both."

I have both.

"Neither a reflector nor a refractor can do it all."

True.

"Neither is 'better' then the other..."

False.

"...just different."

True.

"They both have their advantages and disadvantages."

True.

Cheers,

Alan


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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521602 - 11/15/12 05:27 PM

If one design could do it all then it reasons that only one design is all we would have from which to choose. There would be no need for any variations in the telescope.

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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521614 - 11/15/12 05:35 PM

Quote:

Sir, you dare question the design of the Maker?




I had a private conversation with Her a while back. She said that after Newton arrived, he pointed out a few design errors and that given the chance, She would do it differently the next time.

Jon


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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5521623 - 11/15/12 05:43 PM

Are apo's better than achro's? Of course they are, the apo is an improvement of the achro's design. The apo is a child of the achro, but mom and dad are still around too so he knows where he came from.

Will apo's ever replace achro's altogether? Maybe, but the cost is going to have to come down to that of an achro. And I don't mean within a few hundred dollars but the SAME. Then the need to market achro's will no longer exist. Until then achro's will remain an option on our astronomical table.


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5522186 - 11/16/12 12:58 AM

Jason,

The two immediate advantages an apochromat possesses over an achromat is that chromatic abberation is kept to a bare minimum, and the ability to keep it so while at least halving the traditional focal length, including the physical tube. An f/8 apochromat is perhaps only a little better than an f/16 achromat in the supression of spurious color, therefore an apochromat's primary strength is that it's more manageable, with a mount just capable of sustaining an f/8 being inadequate for an f/16, though not by much, but enough to neccessitate the purchase of a larger, more expensive mount.

However, I was never too troubled over the chromatic aberration exhibited by my 80mm f/11 achromat on brighter objects, for just having a refractor, regardless of variety, instills a pride of ownership as no other design dare approach, primarily Newtonians of at least twice a given refractor's objective diameter. To be fair, Maksutov-Newtonians being somewhat removed in that they're graced by a lens on the front for corrections of the original design.

Closer to being so honoured as the refractor are mirrored arrangements of the Cassegrain design, particularly those employing a lens, also corrective, that is, the Schmidt and Maksutov-Cassegrains, with the Questar being paramount.

There are many varieties of mirrored arrangements, from the Maksutov-Newtonian to the Ritchey-Chretien to the Tri-Schiefspiegler, so many in fact that it seems indicative of as many attempts to get it right. The Tri-Schiefspiegler did manage to eliminate that pesky secondary obstruction, but at what cost, as it's most inelegant in appearance, combined with a frightfully excessive focal ratio.

Happily, with the refractor, you have only two varieties, just two, though not nearly as differing as those within the mirrored arrangements, and a testament to imperfect man's near-perfection upon the first attempt, with "semi-apochromats" excluded for reasons obvious. Thus, whether a refractor be an achromat or an apochromat, one's sense of pride in ownership remains absolute and unaffected.

Cheers,

Alan


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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5522198 - 11/16/12 01:37 AM

About apos here is a test on a famous model.
I leave you your own comments about.
http://www.airylab.fr/contenu/mesures/astro/rapport%202011-05002-public.pdf
On this site 4 units were tested 130 and 150 with the worst and the best.
Be aware of what is acquired actually and with regards to the investment.
In blue and sometimes red channels, frankly there is no value.
Stanislas-Jean


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5522340 - 11/16/12 07:07 AM

Quote:

However, I was never too troubled over the chromatic aberration exhibited by my 80mm f/11 achromat on brighter objects, for just having a refractor, regardless of variety, instills a pride of ownership as no other design dare approach, primarily Newtonians of at least twice a given refractor's objective diameter.




Oh well...

When it comes to pride of ownership, I take pride in scopes that I have built myself... I am much prouder of my 16 inch F/4.42 Dob than of my NP-101 or the 25 inch Obsession because the 16 inch was something that represents my efforts, not something I just bought.

Seriously, I have to wonder if you have actually done a side by side comparison between your 80mm F/11 achromat and decent 6 inch F/8 Newtonian...

Currently my collection includes three 80mm F/11s, all Japanese Meades, two of them Mizars. It also includes an RV-6... people compare RV-6s to 5 inch apos... no false color, small central obstruction, rapid cooling... The OTA only weighs 8 lbs...

When it comes to viewing the planets, splitting double stars, seeing faint DSOs... the Newtonian is so far superior there is really no contest. A more reasonable contest is between a good 80mm F/11 and a good 114mm F/8 Newtonian.

I enjoy a good refractor, I spend a lot of time looking through refractors but I don't make a fetish of it, I realize they are no more special than any good scope.

Jon "Not a SRF" Isaacs


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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5522443 - 11/16/12 08:42 AM

Yeah, enough debate. Let's go out and use our scopes whatever design they may be. As long as we're all happy with the view then they have passed the test.

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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5522447 - 11/16/12 08:43 AM

The only test that matters.

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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5522508 - 11/16/12 09:34 AM

My point being that a good achromat is not that far removed from a good apochromat so as to require moving heaven and earth to replace it. Contrast and sharpness are far more important than the occasional sighting of spurious colour.

Again, the Celestron XLT achromat would be a great choice for a refractor, if you're wanting a refractor.

Cheers,

Alan


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chboss
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5522517 - 11/16/12 09:40 AM

Quote:

About apos here is a test on a famous model.
I leave you your own comments about.
http://www.airylab.fr/contenu/mesures/astro/rapport%202011-05002-public.pdf
On this site 4 units were tested 130 and 150 with the worst and the best.
Be aware of what is acquired actually and with regards to the investment.
In blue and sometimes red channels, frankly there is no value.
Stanislas-Jean




Well I do not know anything about the company that did the test...
The linked test shows a Strehl that is rather low for such a premium refractor, was there a problem with the optic?
Was it checked with the manufacturer?
The red is already low at 0.88 while the blue at 0.68 is way too low.
I really do not know what the data of a single scope should proof.
As the owner of this exact scope I would want to have a word with the manufacturer... and I am sure he would do everything he can to resolve this.

For your comparison I give you this link to another AP telescope a 155EDT from the early 90's with a bit more information:
http://www.astro-foren.de/showthread.php?10849-APO-von-Astro-Physics-mit-Glas...

With a prism diagonal this telescope reaches a Strehl of 0.987 so you can extract as much "information" as you like and this across the complete visual spectrum.
The test proofs only one thing that the specific optic tested is excellent, it does not allow to draw conclusions regarding the quality of the next scope produced. Going with a premium manufacturer you can be sure that the quality differences from scope to scope are rather small.

But the question still stands how for example your ISTAR Achromat would do in the same kind of bench test. If perfectly figured it will show a very high strehl in the green light but by the nature of an Achro the red and blue wavelengths will have a much lower Strehl. This is a given fact.

best regards
Chris


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stanislas-jean
Carpal Tunnel


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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5522617 - 11/16/12 10:39 AM

The lab is independant, works for industry in europe and makes tests for amateur owners and professionals.
The results are not oriented.
Frankly the strehl got in G channel is excellent and will give superbe views classically in IL. Now if you are working in R channel this is not bad but we expect somthing better. In blue channel too, but refractors are degraded in that fields even apos.
The balance results between investment and prestation is too abnormal for getting such: unproper for venus works (for the unit tested, a C8 will do same).
Now what for correcting this, change some lenses.
Actually what is the cdc of that OTA, to get high contrast views in IL. At the spectrum edges we may have a good model, an average also.
A newtonian of 150-180mm will do better on the edges of the spectrum.
Now the achromat 150 owned here is PTV 7 with the zygo test, R channel, still good in G channel (by roddier test) degraded in B channel (so un-proper for venus works).
We know the limits, but are others knowing their OTA.
Reasons for waiving on other designs.
The website has other OTA tests on other designs and we have also surprises.
What is abnormal when delivering such expensive OTA this is the fact of absence of test results as given on that site with the buy.
Stanislas-Jean


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5522909 - 11/16/12 01:18 PM

Quote:

Contrast and sharpness are far more important than the occasional sighting of spurious colour.




Indeed... contrast (true contrast) and sharpness are both primarily functions of aperture and quality.... one might argue about sharpness but in my view, a scope that can resolve a 0.5 arc-second double is sharper than a scope that cannot...

Jon


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ThomasWos
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5522999 - 11/16/12 02:29 PM

I don't have any optical training, but I do have some sound engineering training. When certain frequencies of sound within the strereo image are disturbed, the stereo image becomes smeared to some degree.
When more colors from an object come to focus, that implies to me a truer image. Hence the real advantage of an apochromat to me.

Tom


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5523389 - 11/16/12 07:56 PM

That would be 100% correct if all designs cost the same to execute on the same scale.

There is one single "best" telescope design for visual observing, and if money were no object, it would be the only design any of us used.

Problem is, money is an object, which opens the doors to compromise designs (compromise performance per inch in favor of low dollars per inch).

Never take a diversity of designs for equality of those designs in absolute terms. More often design diversity is all about achieving certain characteristics for a price at which a better all-around design could not achieve those characteristics.

The other thing is this. That "best design" is not static. It will change with advances in technology. Today it's probably a mirrored or catadioptric design with adaptive optics. None of us amateurs have those. Tomorrow, I suspect it will be adaptable liquid lenses.

- Jim


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chboss
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5523402 - 11/16/12 08:02 PM

Quote:


The balance results between investment and prestation is too abnormal for getting such: unproper for venus works (for the unit tested, a C8 will do same).





I fully agree with this sentence. Something is not OK with this telescope and should be checked with the manufacturer. In this league you can expect better results. The test you showed us is rather the exception than the rule for high quality APO's.

You have to be aware that the same can happen if you buy an Achro or Newton. If the tolerances and quality controls are off you end up with a lemon. This has nothing to do with the type of telescope. Most manufacturers do not send you an interferogram with the optics.

If you discover such a problem, I would advise to have it checked by an independent lab and then go with the results to the manufacturer.
This is also the reason you find more "bad" results in the scopes tested, if the quality is initially OK the scope will most likely not end on the optic bench for analysis.

best regards
Chris


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jrbarnett
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Re: Are achromats enjoying a come back? *DELETED* new [Re: Mark Harry]
      #5523412 - 11/16/12 08:08 PM

Post deleted by KWB

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VNA
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5523479 - 11/16/12 09:23 PM

Talking about achros versus apos:

http://www.cloudynights.com/item.php?item_id=1281

It always come back to that "law of diminishing returns"


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5523521 - 11/16/12 09:53 PM

"...was something that represents my efforts, not something I just bought."

Yes, I know the feeling, as I've made things from scratch...

http://public.fotki.com/MississippiAL/miniatures/miniatures.html

"...between your 80mm F/11 achromat and decent 6 inch F/8 Newtonian."

I no longer have the achromat, as I gave it to a relative after I bought my apochromat in 2003. However, if you have said reflector to send, I'll compare it to my apochromat and give an objective opinion.

"Currently my collection includes three 80mm F/11s, all Japanese Meades, two of them Mizars."

I want one! ...and the best of the three, in your esteemed estimation, of course.

"It also includes an RV-6... people compare RV-6s to 5 inch apos...the Newtonian is so far superior."

I'm a bit skeptical, naturally, but if you say so I'll take your word, however, "superior" save that it exhibits reflections rather than direct views.

"...but I don't make a fetish of it..."

...not of refractors in any case.

"I realize they are no more special than any good scope."

If you say so.

Alan "Not a NC, ST or a MQ"

Although, I could be a MQ...


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: VNA]
      #5523536 - 11/16/12 10:09 PM

I think about that very refractor as an achromatic companion to my apochromat, but I don't want to think about the mount it would require.

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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: VNA]
      #5523546 - 11/16/12 10:19 PM

The problem with the "law of diminishing returns" is that, as applied to telescopes, it is entirely subjective.

For example, observer A and B hate false color. Observer B also hates reflectors. Observer C dislikes small aperture and also dislikes false color. Observer D doesn't mind false color doesn't mind reflectors, but would like to save as cash as possible. The four are on budgets of $1000 They are asked to consider four scopes; a 12" f/5 Dob for $1000, a 6" f/8 achromat for $500, a 4" f/15 achromat for $800 and a 3" f/6 apochromatic triplet for $1000 (all real life scopes, available today).

Let's apply the law of diminishing returns to these three observers. Observer A hates false color. His personal cost-benefit analysis says that he could go either 3" f/6 APO or 12" f/5 Dob, avoid dreaded false color, and get what he perceives to be "value" with either scope for his money the extra he spends to avoid his dislike (false color) is a small price to pay; in fact it is a necessary price for him to pay to be satisfied. Observer B has only one choice, because for him, reflectors don't cut it, and he goes with the 3" APO. For him, it is the only choice that makes sense given his preferences. Observer C, too, has only one choice. 3" won't cut it, so he goes for the $1000 12-incher. Observer D is tolerant of false color, and above all values economy. He could go foe either the 6" f/8, get more aperture and false color, but also pocket $500 or the 4" f/15 and get some, but a bit less false color, but less aperture and have $300 less left over. He goes for aperture and $500, and buys the 6" f/8. Each of these observers has applied a version of the law of diminishing returns (given the limited scope pool in the example) and come up with a different answer.

A "law" that, when applied, results in different answers, isn't all that useful as an analytical tool.

- Jim


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Daniel Mounsey
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5523558 - 11/16/12 10:27 PM

Just an FYI. Be careful using a dew zapper on the end of your dew shield like that. They will KILL your images since heat currents pour off them like a furnace. If you saw it in the streak test, it would shock you! Back on topic, I also love achromats.

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Abb
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Re: Are achromats enjoying a come back? new [Re: Daniel Mounsey]
      #5523695 - 11/17/12 12:19 AM

Or, one can simply avoid ""law of diminishing returns" or any other "laws" and buy whatever he or she wants and not care what anyone else thinks of their choice

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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5523856 - 11/17/12 04:32 AM

I donot share exactly the same view.
You could go to the fabricator with the test results and it will be also a non productive discussion mostly.
My feeling is that the scope tested 150mm apo is under the fabricator standard and at the margin of the tolerance standard. Apo doesn't mean 0 spherochromatism aberration even on a 10 000€ OTA. OK.
However the second OTA tested still same fabricator and diameter is on the max narrow possible tolerance. Don't imagine that we can get 80% of the fabrication serie like this, just few units.
The "bad" OTA is not a lemon because giving high contrasted views in integrated light (the B and R channels on that global view having a confidential influence visually) but the possible applications are limited.
It shall be difficult to find such OTA with more than 90% strehl in B channel, except built especially, and leaving less performance at the opposite end of the spectrum.
Frankly serious studies are undertaken with reflectors with applications possible from the UV until the IR channels.
For refractors achromats are well enough on planets, a 6" is collecting say already 80% the events occuring, they are cheap enough and enoughly performing. An excellent entry level OTA.
After a good reflector will help for second step performance at the edges of the visible spectrum.
Stanislas-Jean


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wh48gs
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5523958 - 11/17/12 08:51 AM

Quote:


For your comparison I give you this link to another AP telescope a 155EDT from the early 90's with a bit more information:
http://www.astro-foren.de/showthread.php?10849-APO-von-Astro-Physics-mit-Glas...




There is no comparison between f/9 and f/6.3 apo of a similar design - the difference in the magnitude of spherochromatism is huge (aperture size is much less of a factor, with the error being nearly proportional to it).

Both units are well corrected close to the optimized wavelength. Being 0.89 Strehl @635nm indicates good - possibly true apo correction level in the red - for the f/6.3. The RMS error and the longitudinal aberration plot indicate about 1/3 wave p-v of mainly pure secondary spherical (changes with the 4th power of zonal height longitudinally, as opposed to primary spherical which changes with the square) @475nm. Considering that the error typically increases significantly faster toward the violet end, the error @435 (g-line) is probably sufficiently larger than 1/2 wave p-v to prevent the objective to be considered a "true apo" according to the technical definition (that wouldn't make it "useless").

However, considering low eye sensitivity in this spectral range, it would not necessarily - and in this case probably wouldn't - prevent it from achieving better than 0.95 polychromatic photopic Strehl, which is a more reliable criterion of the correction level (the shift in eye sensitivity toward blue/violet under the typical observing conditions would push the Strehl down, but we can only guess to what extent).

Also, the French test is somewhat odd in that it gives Zernikes in microns (usually in units of wavelength when evaluating optics). For some reason, it accentuates the primary spherical term, while discarding piston, tilt and defocus. That doesn't make much sense, since any of these terms can be, and often are, used in modeling the wavefront. Any term alone can be more or less offset by other terms, hence its value alone is a poor indicator of its actual magnitude, whenever values of other terms are not negligible relative to it. For some reason, the (eye) pupil size is added to the Zernike data which, again, indicates that the software was set for ophthalmology, and that Zernikes were calculated from the error in diopters (any longitudinal aberration can be expressed in diopters). Since the pupil is 12% larger for the 473 wavelength, it could imply that the nominal error here is about 25% larger (proportional to the square of pupil diameter) than what it really is, or that the actual error is closer to 1/4 wave p-v (but that, on the other hand, wouldn't agree with the longitudinal plot).

Rohr's display isn't impressive either: a bunch of graphs, photos, interferograms, screen snapshots, and what not - including that no-good RC_Index (which, basically, compares apos and achros based on the spectral defocus at 0.707 zone while entirely neglecting type and magnitude of spherical aberration). All that is needed for evaluating correction level in a refractor is polychromatic Strehl and longitudinal aberration plot accompanied with the RMS error at selected wavelengths (corresponding p-v wavefront maps don't hurt).

Vla


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Binojunky
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Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5524112 - 11/17/12 10:40 AM

Well all I know is that the previous post lost me altogether, and after observing Jupiter, the Plaides and Orion last night with my lowly 4.5" Starblast and seeing a meteor or two with my refracting eyeballs I want no stinking apos in my back yard, DA.

Edited by Binojunky (11/17/12 10:45 AM)


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5524133 - 11/17/12 10:48 AM

I think the zernikes as you wrote are expressed in nm and it is easy to express them in wavelength, after the bulletin could be converted, but we have those and interresting to read anyway.
It is important to highlight the fact that 3 channels are explored and corresponding to the tricolor imaging set.
This is oriented to imaging (where aberrations are involving sensitive imbalances on the final image) and anyway the measure machine get the polynome that must be read as polynome.
These are bulletins but what are your conclusions about them and the OTA behind? excellence, lemon, can be improved, etc...
The average one is the result assembly of the fabricator having the last 3 lenses only available?
Stanislas-Jean


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chboss
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Reged: 03/24/08

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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5524770 - 11/17/12 05:44 PM

Quote:


Frankly serious studies are undertaken with reflectors with applications possible from the UV until the IR channels.





So your recommendation is, if you are serious, to buy a reflector?
Well each telescope has its use I guess and rest my case.

best regards
Chris


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rwiederrich
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5524853 - 11/17/12 06:31 PM Attachment (44 downloads)

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob


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chboss
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Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5524864 - 11/17/12 06:38 PM

Quote:


There is no comparison between f/9 and f/6.3 apo of a similar design - the difference in the magnitude of spherochromatism is huge (aperture size is much less of a factor, with the error being nearly proportional to it).




Hi Vla

I am fully aware of this fact. It was on purpose that I linked the 155EDT because it is an ideal visual telescope at f9 and Stanislas is always talking about visual.
You should ask him why he pulled out the test result for the AP 130EDF that is optimized for CCD work...

best regards
Chris


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iceblaze
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Re: Are achromats enjoying a come back? new [Re: rwiederrich]
      #5524924 - 11/17/12 07:15 PM

Quote:

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob




Fantastic! Love the chalk board

-James


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rwiederrich
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Re: Are achromats enjoying a come back? new [Re: iceblaze]
      #5525050 - 11/17/12 08:39 PM

Quote:

Quote:

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob




Fantastic! Love the chalk board

-James




I write down the nights images and exposure times on that.....it makes for great reference.....plus during outreach with the boyscouts it makes a good class room.

Rob


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orion61

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Re: Are achromats enjoying a come back? new [Re: Nippon]
      #5525088 - 11/17/12 09:03 PM

Quote:

There seems to be a resurgence in interest in the achromatic scopes. If so what are some of the reasons do you think. I have a Vixen A 105 M. I bought it because I wanted a 4" scope with good optical figure for doubles and planets. The color correction is good and I'm pleased with it. But part of my reason was just the classic appeal of a good 4" inch achro.



I woudn't say replace but the newer acros do perform to a higher degree in color correction and final figure.
But they will never replace the premium refractors where the additional performance comes in close to a year of secondary education cost.
Even ED's have dropped in price to where I bought one,
It has fantastic contrast and color correction.
but a 5k APO will NEVER beat a 5K DOB in performance,
A 20" tracking DOB with an 8" off axis mask will give outstanding performance on non Stellar objects..
I have a 5" Celestron Prototype from the 90's that is pretty good. I could never justify the cost! But I can
an ED, with the performance of it mmeade 102 ED/APO but we all know it is Possibly a "Semi" APO.
If I had money laying around I couldnt find anything else to use it on I might buy a Premo APO.
But remember there are those who buy the most expensive
items just because there are no others more expensive.


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mikey cee
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Re: Are achromats enjoying a come back? new [Re: rwiederrich]
      #5525413 - 11/18/12 01:46 AM

Quote:

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob


Rob my friend that is a real "says it all" pic! Mike

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rwiederrich
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Re: Are achromats enjoying a come back? new [Re: mikey cee]
      #5525427 - 11/18/12 02:01 AM

Quote:

Quote:

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob


Rob my friend that is a real "says it all" pic! Mike




Us old guys...need to use any *analogies* we can muster. that chair is a dental chair and it can go down nearly to the floor....perfect! Thanks bro.


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: rwiederrich]
      #5525436 - 11/18/12 02:10 AM

Now that is a "Great Refractor"!

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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5525568 - 11/18/12 06:17 AM

You see the question is we find good models and also some only average. When the good one is here, keep it.
Now my recommendation, if I can raise one, is to waive to a good reflector, an 8" will suffice for making similar or better. We find excellent corrector also for the the correction of the coma.
The website that I gave here was to highlight the fact that we can met, good units and average ones sold as good. For the investment performed it is a pitty and abnormal.
How many people knows that their models are average? This is disturbing me somewhere and the results brought considered as perfection so well eared should be considered out of concern. The problem is to acquire pertinent data, not only pretty.
Stanislas-Jean


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5525816 - 11/18/12 10:28 AM

A "great refractor" is a refractor with aperture that is on par with a medium-sized amateur Newtonian reflector.

- Jim


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5525885 - 11/18/12 11:12 AM

So true, with a "great reflector", that's really not so great after all, requiring a minimum of sixty inches, i.e. Mt. Wilson.

Alan


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wh48gs
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5526083 - 11/18/12 12:54 PM Attachment (35 downloads)

Stanislas,

Quote:

These are bulletins but what are your conclusions about them and the OTA behind? excellence, lemon, can be improved, etc...




If we just take the data as presented, the unit is visually close to "true apo" technically, and probably there if the criteria is a min 0.95 photopic polychromatic Strehl. Photographically, it probably doesn't exceed 1/4 wave p-v in the red r-line (706nm) and might be even within in the violet g-line (436nm), which would make it a "true apo".

But, the problem is that the test isn't convincing. The graph implies the possible design minimum of less than 0.003 wave RMS wavefront error of nearly balanced secondary spherical in the optimized wavelength (with the actual error about 50% larger, or less than 1/200 wave RMS). I know of no glass combination that would exhibit nearly as low spherical aberration in this fast triplet. If you look at the wavefront map, the actual spherical is no more than 1/3 of the RMS wavefront error, which is clearly mainly the wavefront tilt (i.e. resulting from minor alignment errors in test setup). Correcting for the tilt leaves in roughly this magnitude of error (0.02-0.03 p-v) mainly at the edge. In other words, this unit would be practically perfect in the e-line. At f/6.3? Hmmm...

The tester doesn't specify the glasses and, since AP did not publish them, the question is how did they get the data to obtain this plot? It might be that they obtained it from AP (note that the graph indicates different values than test, especially in the blue line). AP itself is very shy in this respect, only stating that the focal shift from g(436nm) to r(706nm) is less than +/- 0.006%.

The French graph, on the other hand, shows paraxial foci shift of +/- 0.1mm, i.e. +/- 0.012% from 0.473nm to 0.633nm already. So, my guess is as good as anyone's, as to what +/- 0.006% should imply. It sure looks too good to be true, but it's just another useless figure.

As a comparison, here's such a triplet made with FPL53 (nearly certainly the central element in the AP) and Schott's ZKN7, the best mach for it out of the available glasses. There is noticeably more spherochromatism, in the optimized wavelength, due to the strongly curved R2/R3, but the error is still only 0.018 wave RMS (graph in the middle is for the three tested wavelengths, for comparison, and at right is with the conventional five lines). In the actual unit, it would be hard to get below 0.03 wave, or 1/10 wave p-v of lower order spherical. Technically, this objective is not a "true apo", having the g-line error larger than 1/2 wave p-v (0.7 wave). However, it is more than compensated by a smaller error in the red, so it could be considered a true apo overall. Its polychromatic photopic Strehl is 0.965, which does make it true apo visually. It could be slightly bettered by optimization, but it wouldn't be of practical importance.

Sure, the scope could be using special melt for its matching glass, but it would make it very expensive. The conclusion is: we just don't have the data needed to asses specific level of correction, or to compare most of these instruments.

Vla

Edit: added values for the RMS in wavelengths in the text box top left, so that they are directly comparable with the values given with the graphs

Edited by wh48gs (11/19/12 10:30 AM)


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wh48gs
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5526103 - 11/18/12 01:03 PM

Hi Chris,

Quote:

You should ask him why he pulled out the test result for the AP 130EDF that is optimized for CCD work...




I think I know the answer: it is always interesting to try something new

Vla


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Andy Taylor
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5526253 - 11/18/12 02:32 PM

When all is said and done, I think it's also a question of your skies and seeing...

I live in what was a white/pink zone. New LED street lighting has improved matters somewhat - especially when they are dimmed by 50% at 11.00 pm.

My 8" SCT and RFT still show a bright background but my Carton 100mm F13 shows me fantastic contrasty views with a black background.

Why pay out for an APO or buy a light bucket?

For me, far more bang per buck with the Carton.

(Cost me 250 Uk pounds as a self build)


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5526258 - 11/18/12 02:37 PM

What'd it cost you to mount the Carton?

Regards,

Jim


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Andy Taylor
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5526272 - 11/18/12 02:46 PM

Quote:

What'd it cost you to mount the Carton?

Regards,

Jim




Uses the same Autostar modded EQ-5 as the SCT...

Total cost of mount so far 150 Uk pounds (including a Criterion "Golden Pyramid" tripod - rock solid...).


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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5526481 - 11/18/12 05:01 PM

So at the conclusions you wrote, the bulletin doesnot give enough data for expecting the OTA figure in use on the sky (under excellent conditions).
But the question was through the 4 data results regarding 2 units 130mm and 2 units 150mm same fabricator the great results difference between the 2 for each aperture. This is something disturbing somewhere.
That is making some questions about the QC follow-up or simply this is the standard offered.
The investigation inspection method could be discussed but the same was on the 4 units.
Stanislas-Jean


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5526629 - 11/18/12 06:59 PM

What's your saddle height, Andy?

Recently I swapped my CG5 from a 48" pier to a 60" pier for my ~95mm, ~f/15.7 Antares. I was getting irritated having to drop the observing seat to the lowest usable rung to view near zenith. The extra foot took the eyepiece from ~2-feet to ~3-feet off the pitch at zenith, but it also added to the post-focus settling time. I do use VSPs, as I find the setup almost unusable on hardscape without them, and they add leveling travel for the pier feet.

I don't have a picture of my solution yet, but here's a picture illustrating the problem:



Regards,

Jim


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John Huntley
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Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5526740 - 11/18/12 07:59 PM

Quote:

....For me, far more bang per buck with the Carton.

(Cost me 250 Uk pounds as a self build)




Was that one of the Skylight kits Andy ?

I thought they cost a lot more than that !


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wh48gs
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5527913 - 11/19/12 01:08 PM Attachment (14 downloads)

Quote:

But the question was through the 4 data results regarding 2 units 130mm and 2 units 150mm same fabricator the great results difference between the 2 for each aperture.




I wasn't aware of the other tests. But looking at it, there's not much difference between the two f/6.3 units. One is nearly perfect in the optimized wavelength, the other has about 1/10 wave p-v added to the optimized wavelength (under-correction as shown). This is quite possible consequence of suboptimal spacing, and it somewhat improved the other two lines (change of correction in the optimized wavelength always affect correction of the other wavelengths as well). The Strehls remain relatively close.

Still, I'm puzzled by the near-zero minimum possible for the error in the optimized wavelength that the graphs implied. The other two wavelengths do show quite a bit of spherochromatism (including strong secondary spherical in the blue line), even more than the FPL53/ZKN7 triplet I posted earlier. It does not agree with how spherochromatism normally behaves. Another thing is that the foci are reversed to the common scenario, with the blue (paraxial) focusing farther away. That must be some really odd glass combination, in addition to having correction in the optimized wavelength that just cannot be explained with the available glasses.

The focal shift is about 0.2mm in unit 1 and 0.23mm in unit 2, for about 0.025% (somewhat more if extended to the g-line, 436nm). That's more than twice the AP's "less than +/-0.006%" figure. Still don't know how the plots are obtained; measuring focal location for several zones at each wavelength, and interpolating? That can be sort of tricky way to calculate wavefront errors. The RMS for the optimized wavelength's s.a. implied by the graph is about equal to the RMS error shown with the wavefront map - it would imply that nearly entire aberration is spherical, yet the map shows some rather irregular shape, with the dominant tilt component (similarly to unit 1). And so on.

Those things don't add up, but there is not enough information to confirm or deny. Which leaves you two choices: either believe the units are correctly presented, or don't.

Vla


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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5528048 - 11/19/12 02:06 PM

Here is the principle of the lab test method (unfortunately in french only)
http://www.airylab.com/contenu/incertitudes%20v1.05.pdf
those following in english
http://www.airylab.fr/AirylabUS/index.php/optical-metrology/science-and-industry
http://www.airylab.fr/AirylabUS/index.php/optical-metrology/astronomy
They trace the optics in the 3 color channels and results are raw data not interpolated I think.
They had been followed by refrerence laboratories and own reference optics to follow their calibrations and references. The pdf booklet gives an example of comparison results between different labs and them.
For more info it should be interresting to contact them directly people knows english (in spite of the website in french). Hope this is an help.
Now results when read does not imply confidence, my feeling, even researching reasons why such status.
Please compare with the following test:
http://airylab.com/contenu/mesures/astro/Astreya/rapport%202011-50001-a.pdf
That is excellent after realignement of the lenses by the lab.
Stanislas-Jean


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csrlice12
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5528093 - 11/19/12 02:33 PM

I suppose the real question, is did they ever go out of style as they're still widely manufactured and sold.......

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wh48gs
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5528259 - 11/19/12 03:51 PM

Inconsistencies cannot be explained by a testing process setup, only by possible errors in it and/or the presentation. As for the quadruplet apo (Astreya 152mm f/7) it has unusual correction mode, more like Maksutov corrector. Color error is lower than in the AP 130 f/6.3 but, then, it wouldn't be vs. an f/7 lens. The RMS for the three wavelengths (0.473, 0.543, 0.635 micron) are just 0.023, 0.022 and 0.021 micron, or 0.049, 0.041 and 0.031 in units of the wave, respectively. However, the error in the green is larger than for triplet designs, more than double that in the FPL53/ZKN7 f/6.3 triplet, and another more than a double vs. AP's f/6.3 as tested (although the graph suggests it could be further reduced by 10-20% by correcting a slight alignment error), and could be still significantly larger in an f/6.3.

The plots are similar to that of Gauss doublet achromat, except, of course that the colors are brought together. So the objective could be a doublet consisting of two strongly curved achromatised menisci, each using low dispersion glass. A bit too fast (too much spherochromatism) for comfort, considering that the actual units will likely have more spherical in the optimized wavelength (which, with this design, means in all other wavelengths as well)) than the design minimum.

Vla


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rwiederrich
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5528560 - 11/19/12 06:44 PM

Quote:

Now that is a "Great Refractor"!




And the views are stunning as well......covered in a fantastic sage green.....

Thanks


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rwiederrich
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5528565 - 11/19/12 06:47 PM

Quote:

A "great refractor" is a refractor with aperture that is on par with a medium-sized amateur Newtonian reflector.

- Jim




10"..right up my alley.


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chboss
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Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5529217 - 11/20/12 12:33 AM

Quote:

As for the quadruplet apo (Astreya 152mm f/7) it has unusual correction mode, more like Maksutov corrector. Color error is lower than in the AP 130 f/6.3 but, then, it wouldn't be vs. an f/7 lens.




Hi Vla

Are you sure the tested tube is using the Astreya quadruplet?
The missalignment would be an indication since there have been some reports of instabilities of the lens cells in these optics in the past...
A&M (Now Officina Stellare) from Italy also manufactured tubes using the LZOS triplet but this would be either 152mm f6 or f8?
All a bit confusing.

best regards
Chris


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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5529292 - 11/20/12 01:46 AM

Yes it is a triplet (according my informations).
Now regarding the rms it is always a matter as the scaling of the measures being too coarse the results are from my opinion just indicative.
If we push the investigation on this website as there are numerous tests about different designs it should be interresting to make the comparison performance.
On an absolute value the data expressed by the buletins can be discussed, but the main topic remains the comparative evaluation between tests using the same measure means (it is an automatic machine i presume whare longitudinal flaws are collected.
Thanks Vlad for trying to interprete them accordingly.
We can argue for the actuality of a PTV, the surface figure quantitied by the polynome but the overall "shape" remains regarding each design and this is the point where I wanted to go the comparison of the scope quantified.
All the OTA also I presume are lambda scopes not selected, brought by owners that authorised the data publication for knowledge and who wanted to get a characterisation.
Facing this, being just a buyer, I am wandering a lot for a pertinent selection.
Stanislas-Jean


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Andy Taylor
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5529370 - 11/20/12 04:31 AM

Quote:

What's your saddle height, Andy?

Recently I swapped my CG5 from a 48" pier to a 60" pier for my ~95mm, ~f/15.7 Antares. I was getting irritated having to drop the observing seat to the lowest usable rung to view near zenith. The extra foot took the eyepiece from ~2-feet to ~3-feet off the pitch at zenith, but it also added to the post-focus settling time. I do use VSPs, as I find the setup almost unusable on hardscape without them, and they add leveling travel for the pier feet.

I don't have a picture of my solution yet, but here's a picture illustrating the problem:



Regards,

Jim




Saddle height is at my eye level (I'm 6'2").

The Criterion tripod legs have a good height range and it's very solid.

Yeah - eyepiece gets low at zenith but I usually target planets and moon with this scope.


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Andy Taylor
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Re: Are achromats enjoying a come back? new [Re: John Huntley]
      #5529375 - 11/20/12 04:39 AM Attachment (12 downloads)

Quote:

Quote:

....For me, far more bang per buck with the Carton.

(Cost me 250 Uk pounds as a self build)




Was that one of the Skylight kits Andy ?

I thought they cost a lot more than that !




No, I bought the objective and built the rest.

If you look closely at the pic you can see that I've been a bit crafty. It shares my single speed crayford focuser with my 8" SCT.

Just screws on the back...


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chonum
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5529377 - 11/20/12 04:41 AM

Hi guys,

I'm the guy who tested those instruments at Airylab.

The lab uses a Shack Hartmann wavefront analyser from Imagine Optic with three laser sources (DPSS 473nm, HENE 543nn, controlled LED 635nm), all being fiberized through a 4µm fiber core and therefore spatialy monomode.
Tests are performed against a 530mm ceramic flat mirror from REOSC (the that built VLT mirrors).

The analyser gives pure wavefront phase error in µm. Those values are not related to the wavelength as it is the case for interferometry. We need to take into account the wavelength only when processing the wavefront to get the MTF, the spot diagrams, the PSF and Strelh ratio.
Note that the wavefront is given WITHOUT any Zernike polynomial fit, and avoid any error and uncertainty due to this fit.
Then only a fit is done to get some information about the implied error terms splitting the error into legacy aberration terms (astig, SA, Coma, Trefoil...).

The spherochromatism (LSA) is calculated based on the Zernike fit for the 3, 5 and 7th order of SA. This is not done the the analyser, but in another code we made. It usually fits quite well manufacturer's data that is issued from Zemax. Except that it is reversed.

Spherochromatism in SC and triplet depends on the F/D ratio, and the aperture.
The classical triplet (lenses in contact or very close) loose one degree of freedom for the designer cannot use the lens spacing. Therefore those designs show spherochromatism, that is inevitable.
The only instruments that don't show any (besides reflectors) are Mak and special triplet designs such as the TOA cooke.

Here is a graph of the 3rd order SA over wavelength for three refractors :


Does that make the AP a bad refractor ? No for the polishing is excellent (the best telescope we tested on one wavelength was an AP130) and mecanic as well, not to mention a very large corrected FOV using the 4" flattener that just a few telescopes are able to achieve for imaging.

BTW The 152 was indeed the Astreya quadruplet version, prior to the triplet.

Frédéric.


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5529431 - 11/20/12 06:19 AM

Quote:


My 8" SCT and RFT still show a bright background but my Carton 100mm F13 shows me fantastic contrasty views with a black background.




At that same exit pupils? Sky brightness is a function of exit pupil... at 200x, my 60mm is pretty dark, the 25 inch is quite bright even under dark skies. With a 1mm exit pupil, both are similarly dark though the magnifications are quite different.

Jon


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chboss
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5529540 - 11/20/12 07:56 AM

Frédéric, thank you for the additional information regarding your testing process and the resulting information.

So it was an Astreya quadruplet at f7... seems like the cell has its problems holding collimation. It is not the first time I heard about readjustments on the lens level.

Fantastic collection of tests, just what Jim was looking for:
http://www.airylab.fr/AirylabUS/index.php/optical-metrology/astronomy-tests-r...

best regards
Chris


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Re: Are achromats enjoying a come back? new [Re: chonum]
      #5529660 - 11/20/12 09:30 AM

Hi Frederic,

Thanks for chiming in. I'll second Chris, your site is well done, with some very interesting information for us amateurs. But some things are still not clear to me.

Quote:

The analyser gives pure wavefront phase error in µm. Those values are not related to the wavelength as it is the case for interferometry. We need to take into account the wavelength only when processing the wavefront to get the MTF, the spot diagrams, the PSF and Strelh ratio.




That is quite unusual way to put it. "Wavefront" is geometric category, while "phase" is related to the wave (oscillation) itself. Kind of, don't stick together. Wavefront error is usually expressed either directly, in geometric units (like micron, nm, etc.) or relatively, in units of wave. Phase error, on the other side, is usually expressed in units of full phase, i.e. 2Pi radians for a full wave, either as "variance" (standard phase deviation squared) or "standard deviation", with the latter being the phase analog to the RMS (geometric) wavefront error (e.g. 0.1x2Pi radians phase error is equivalent to 0.1 wave RMS wavefront error).

Quote:

The classical triplet (lenses in contact or very close) loose one degree of freedom for the designer cannot use the lens spacing. Therefore those designs show spherochromatism, that is inevitable.
The only instruments that don't show any (besides reflectors) are Mak and special triplet designs such as the TOA cooke.





Any apo objective will show significant spherochromatism if pushed far (fast) enough (try to scale the TOA down to f/6). Air-spaced objectives are generally better in controlling aberrations; they do tend to produce somewhat less spherochromatism too, but the difference usually isn't really significant.

Quote:

Here is a graph of the 3rd order SA over wavelength for three refractors :




There's not much use of such graph, considering that faster apos typically have mixed 3rd and 5th order spherical (for instance, the AP 155 has mostly 5th order in the red, mixed 3rd/5th in the green - which means that the 3rd alone is nominally significantly larger than the combined error - and mainly 3rd order only in the blue).

I also still wonder how the longitudinal aberration plots are obtained.

Vla


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chonum
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Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5529807 - 11/20/12 10:51 AM

Hi Vla,

WFE is in µm (or nm) indeed. Nevertheless it is a phase map should you convert it to fringes using the lambda value. We prefer to use nm anyway as you don't need to take into account the wavelength.
But when i need to use diffraction, or even spot diagram to get the real scale then the lambda is obvioulsy required.

I don't agree that much about the TOA. Of course it's not very fast and it would show a less flat 3rd order SA curve at F/6. Nevertheless at 7.7 native the SA is flat and almost null. The big difference with a legacy triplet is that you can still use the reducer down to 5.8 and be almost spherochromatism free (at the expense of the field coverage but there's still plenty for most CCD sensors).
A guys i know use it in the CAK line for solar imaging and it's maybe the only refractor to be able to get there and stay clean.

In the case of the AP155, 5th and 7th order SA are quite the same over the spectrum. Only one AP130 shows some evolution of its high orders SA.

But it's true that sometime high orders can compensate partially the 3rd order, Mak are good example of this.

LSA is calculated by projection of the normal from the wavefront to the axis using tangent of the derivated zernike development 3, 5 and 7. That's just dull trigonometry, i was surprised not to find this somewhere in an optical receipe book. After doing it i recognize it's just a PITA to do
We are supposed to make an article about that in a book to come.


Frédéric.


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Re: Are achromats enjoying a come back? new [Re: chonum]
      #5530202 - 11/20/12 01:36 PM Attachment (19 downloads)

Frédéric,

Quote:

I don't agree that much about the TOA. Of course it's not very fast and it would show a less flat 3rd order SA curve at F/6.




Then we pretty much agree, except that I'd say wouldn't be nowhere near flat anymore.

Quote:

In the case of the AP155, 5th and 7th order SA are quite the same over the spectrum. Only one AP130 shows some evolution of its high orders SA.




That is not what the plots - including your own - show. It is very obvious that the 3rd and 5th order (transverse aberration; 4th and 6th order on the wavefront) are quite a bit mixed up (7th order is negligible). I've made a quick model of the AP 155 EDF triplet, with FPL53 and two crowns (Ohara equivalents to Schott's BK7 and K5), giving very similar plot 0.4-1 micron as the published one. It shows 5th order (changing longitudinally with the 4th power of zonal height) dominating in the blue, and 3rd order, to somewhat lesser degree, dominating in the red. The green is nearly perfectly balanced between the 5th and 3d, which means that each alone has nearly six times greater nominal RMS (of opposite signs).

Any time strongly curved surfaces generate significant 5th order spherical - rather common case with spo doublets, fast triplets and Maks - it has to bi minimized by balancing it with the 3rd order.

Quote:

LSA is calculated by projection of the normal from the wavefront to the axis using tangent of the derivated zernike development 3, 5 and 7.




What doesn't fit in is that the plots show smooth spherical WF deviation (thinking of AP 130 f/6.3 test), while the wavefront map shows that other - mainly random - forms of deformations dominate.

Do you plan on English edition for the book?

Vla


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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5530832 - 11/20/12 07:00 PM

Quote:

The problem with the "law of diminishing returns" is that, as applied to telescopes, it is entirely subjective.

For example, observer A and B hate false color. Observer B also hates reflectors. Observer C dislikes small aperture and also dislikes false color. Observer D doesn't mind false color doesn't mind reflectors, but would like to save as cash as possible. The four are on budgets of $1000 They are asked to consider four scopes; a 12" f/5 Dob for $1000, a 6" f/8 achromat for $500, a 4" f/15 achromat for $800 and a 3" f/6 apochromatic triplet for $1000 (all real life scopes, available today).

Let's apply the law of diminishing returns to these three observers. Observer A hates false color. His personal cost-benefit analysis says that he could go either 3" f/6 APO or 12" f/5 Dob, avoid dreaded false color, and get what he perceives to be "value" with either scope for his money the extra he spends to avoid his dislike (false color) is a small price to pay; in fact it is a necessary price for him to pay to be satisfied. Observer B has only one choice, because for him, reflectors don't cut it, and he goes with the 3" APO. For him, it is the only choice that makes sense given his preferences. Observer C, too, has only one choice. 3" won't cut it, so he goes for the $1000 12-incher. Observer D is tolerant of false color, and above all values economy. He could go foe either the 6" f/8, get more aperture and false color, but also pocket $500 or the 4" f/15 and get some, but a bit less false color, but less aperture and have $300 less left over. He goes for aperture and $500, and buys the 6" f/8. Each of these observers has applied a version of the law of diminishing returns (given the limited scope pool in the example) and come up with a different answer.

A "law" that, when applied, results in different answers, isn't all that useful as an analytical tool.

- Jim




Was thinking more simply in terms of equal diameter only:

A 6 inch refractor apochromatic versus a 6 inch refractor achromatic

The price will be 6 times or more for equal aperture the only variable will be that "chromatic aberration" that can be corrected for little money.


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Re: Are achromats enjoying a come back? new [Re: VNA]
      #5530910 - 11/20/12 07:44 PM

Quote:


Was thinking more simply in terms of equal diameter only:

A 6 inch refractor apochromatic versus a 6 inch refractor achromatic

The price will be 6 times or more for equal aperture the only variable will be that "chromatic aberration" that can be corrected for little money.




OK, I'll bite... How can the chromatic aberration of a 6 inch achromat be corrected for very little money?

The least expensive way I know of to correct the CA is the no longer available Chromacorr and that was not totally satisfactory. One can try to eliminate the defocused colors with a filter but that is not correcting the color, to correct the color, it must be refocused so it ends up where it belongs, in focus with the rest of the colors.

There are virtues and limitations to just about every telescope ever made... it really makes no sense to argue over personal preferences as which compromise makes the most sense. The important thing is just to recognize the virtues and limitations, try to make the choices that best an individuals needs and desires and move on.

Last night I had my 100mm F/6 achromat out in the back yard. The seeing was so so but it was giving me a nice clean split of the double-double. Later I tried to split delta Cygni but I just couldn't get it. I pulled out my 80mm F/7 FD and was able to get a nice clean split.

The right scope for the job...

Jon Isaacs


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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5530975 - 11/20/12 08:24 PM

Well Jon I was wondering about the same point....

How to correct the chromatic aberration for little money.
If there is many people would love to know.

best regards
Chris


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Re: Are achromats enjoying a come back? new [Re: rocketsteve]
      #5531019 - 11/20/12 08:51 PM

Achromats are enjoying a never-went-away.

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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5531440 - 11/21/12 01:42 AM

By the use of narrow band filters, for example the tricolor set filters.
If you are interested by some studies (visually)
calcium K: for venus still enough transparency otherwise the B filter tricolor during day is well enough,
RGB: all planets,
baader continuum: all planets,
O3: venus ashen light,
Halpha all planets in the bandwidth and for venus ashen light,
Now a simple synta will fail in some field as the presence of spherochromatism, but well designed doublets can do more than honestly.
This is not to collect pretty views but data.
Stanislas-Jean


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chboss
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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5531483 - 11/21/12 03:06 AM

Well yes, narrow band filters may do the trick in some cases....
E.g. some fantastic solar imaging is done with Achromats filtered accordingly.

However for the full visual spectrum that our eye can see on planets, this is not the solution.
Since I like pretty pictures.

best regards
Chris


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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: chboss]
      #5531486 - 11/21/12 03:14 AM

This is always a matter of what everybody is aiming, data collection or pretty views with a bank account.
Stanislas-Jean


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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5531662 - 11/21/12 08:21 AM

Quote:

Quote:



Last night I had my 100mm F/6 achromat out in the back yard. The seeing was so so but it was giving me a nice clean split of the double-double. Later I tried to split delta Cygni but I just couldn't get it. I pulled out my 80mm F/7 FD and was able to get a nice clean split.

The right scope for the job...

Jon Isaacs




Congratulations Jon! You must be beside yourself for having finally split Delta Cygni with an 80mm 'scope (having posted negative findings in this aperture class several times in the past even in your excellent climate).

Delta Cygni is fairly easy in both my 80mm achromats (f/9 and f/11).

I used to own a 100mm f/6.5 achromat and my records show that this system was also easy to split from my less than idyllic vantage with this instrument.

http://www.cloudynights.com/item.php?item_id=1515

Elephants never forget.

Cheers,

Nelly.

Edited by astroneil (11/21/12 08:24 AM)


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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5531703 - 11/21/12 08:44 AM

Quote:

By the use of narrow band filters, for example the tricolor set filters.




Filters do not correct chromatic aberration, the eliminate the offending colors...


Jon


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Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5531711 - 11/21/12 08:47 AM

Quote:

This is always a matter of what everybody is aiming, data collection or pretty views with a bank account.
Stanislas-Jean




If one is collecting data, the obvious solution is a reflector.. no filters needed. In fact for viewing the planets, the obvious solution is almost always a reflector, particularly if one is trying to do it economically.

Jon Isaacs


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Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5531728 - 11/21/12 08:57 AM

Quote:


Congratulations Jon! You must be beside yourself for having finally split Delta Cygni with an 80mm 'scope (having posted negative findings in this aperture class several times in the past even in your excellent climate).




I am very appreciative of the wonderful climate and the often excellent seeing. But this does not mean the seeing is always great and in this case, with Delta Cygni low over a warm roof, the seeing was not so hot...

I might have gotten it with one of my 80mm F/11's, certainly it is easier in the 80mm F/7 apo. Last night I had out my NP-101, it really pulls the faint companions out of the bad seeing. Delta Cygni was an easy split. Rigel was barely above the horizon and very unstable, flashing colors, and I was able to pick out the companion at 60x... Almost like a Newtonian.

I don't know about other fast achromats but in my experience, there is enough chromatic aberration and general muck in the ones I have used that apochromats with their superior color correction and most likely the superior optics certainly are better performers on doubles, particularly when the seeing is sketchy.

Jon Isaacs


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stanislas-jean
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5531736 - 11/21/12 09:03 AM

See the us alpo site for the use of filters, this is not for the improving contrast research (the strehl anyway is).
Sure you know this.
Stanislas-Jean


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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5531743 - 11/21/12 09:10 AM

Quote:

Quote:


Congratulations Jon! You must be beside yourself for having finally split Delta Cygni with an 80mm 'scope (having posted negative findings in this aperture class several times in the past even in your excellent climate).




I am very appreciative of the wonderful climate and the often excellent seeing. But this does not mean the seeing is always great and in this case, with Delta Cygni low over a warm roof, the seeing was not so hot...

I might have gotten it with one of my 80mm F/11's, certainly it is easier in the 80mm F/7 apo. Last night I had out my NP-101, it really pulls the faint companions out of the bad seeing. Delta Cygni was an easy split. Rigel was barely above the horizon and very unstable, flashing colors, and I was able to pick out the companion at 60x... Almost like a Newtonian.

I don't know about other fast achromats but in my experience, there is enough chromatic aberration and general muck in the ones I have used that apochromats with their superior color correction and most likely the superior optics certainly are better performers on doubles, particularly when the seeing is sketchy.

Jon Isaacs




I find this all very convenient.


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Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5531839 - 11/21/12 10:05 AM

Quote:


I find this all very convenient.




Indeed, apos are very convenient.

To get that same sort of views in an achromat, it would need to be very long and even then the color correction would not be up to the reflector-like performance of the NP-101...

The real convenience the NP-101 is the ability to provide both a 4.5 degree field that is flat and the near perfect high power views all in a compact package.

Last weekend, I had great fun with the 100mm F/6 achromat out under dark skies, I can enjoy just about any telescope around and it does a good job. But as much fun as I had, pulling out the NP-101 reminded me of just how perfect it is...

Jon


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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5532037 - 11/21/12 11:31 AM

Well, I just bought one yesterday. The Omni 102XLT. Couldn't afford an APO. To be truthful, Acromats never went away. I have no doubt if someone looked at sales stats, Acromats probably far outsell Apochromats. If nothing else, due to the costs.

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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5532727 - 11/21/12 05:47 PM

Jon, don't tell me you are a member of the worldwide anti-achromat conspiracy. Your contrary views suggest as much.

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Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5532807 - 11/21/12 06:51 PM

Quote:

Quote:

Quote:



Last night I had my 100mm F/6 achromat out in the back yard. The seeing was so so but it was giving me a nice clean split of the double-double. Later I tried to split delta Cygni but I just couldn't get it. I pulled out my 80mm F/7 FD and was able to get a nice clean split.

The right scope for the job...

Jon Isaacs




Congratulations Jon! You must be beside yourself for having finally split Delta Cygni with an 80mm 'scope (having posted negative findings in this aperture class several times in the past even in your excellent climate).

Delta Cygni is fairly easy in both my 80mm achromats (f/9 and f/11).

I used to own a 100mm f/6.5 achromat and my records show that this system was also easy to split from my less than idyllic vantage with this instrument.

http://www.cloudynights.com/item.php?item_id=1515

Elephants never forget.

Cheers,

Nelly.




Hmm, Last week I split Delta Cyg 3 nights in a row with my humble 75mm F/16 achro. Wasn't difficult either. I've routinely observed it with my 60mm f/15 Achro. Bill


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Re: Are achromats enjoying a come back? new [Re: Bonco]
      #5532913 - 11/21/12 08:23 PM

Yessiree Bonco Delta Cygni is definitely kindergarten level. I too have split it in a 60mm Sears. I only look at it with my 10" when I'm bored and that's been getting more often than not lately. Mike

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7331Peg
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5533268 - 11/22/12 01:38 AM

Quote:



I don't know about other fast achromats but in my experience, there is enough chromatic aberration and general muck in the ones I have used that apochromats with their superior color correction and most likely the superior optics certainly are better performers on doubles, particularly when the seeing is sketchy.

Jon Isaacs




Not sure what the problem is down there in your neck of the woods, but it has nothing to do with achromat vs. apochromat. I've split Delta Cygni with achromatic apertures ranging from 60mm to 152mm more times than I can count.


John


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Re: Are achromats enjoying a come back? new [Re: 7331Peg]
      #5533384 - 11/22/12 05:30 AM

Quote:



Not sure what the problem is down there in your neck of the woods, but it has nothing to do with achromat vs. apochromat. I've split Delta Cygni with achromatic apertures ranging from 60mm to 152mm more times than I can count.




Done it with any 100mm F/5 or F/6 achromats?

Jon


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Re: Are achromats enjoying a come back? new [Re: Joe Bergeron]
      #5533400 - 11/22/12 06:05 AM

Quote:

Jon, don't tell me you are a member of the worldwide anti-achromat conspiracy. Your contrary views suggest as much.




Joe:

I think of my views as realistic rather than contrary... In a thread like this one, I get a lot of flack. There's a lot of good scopes out there, most any scope I put my eye to provides enjoyable views. But honest evaluations require discussions of both the positive and negative aspects of the various designs. A few comments:

I own more achromats than apos. I own two of the Meade-Mizar 80mm F/11s that are essentially identical to the 80mm F/11 that someone was trying to sell for something close to $1000. They are good performers but I get better views with my 80mm F7 FPL-53 doublet.

I enjoy achromats, last night I spent with the 80mm F/5 which provides a 6 degree TFoV and the aforementioned 100mm F/6.. I also understand their limitations, the false color on Jupiter and the moon definitely affected the contrast. A fast achromat is no match for an apo at high magnifications and a slow achromat is no match for an apo in the widefield. That is just how it is.

In terms of performance, I like to think I play no favorites... In my world, refractors are small scopes, when the seeing is excellent, any refractor I would want to use is simply too small to really take advantage of it, half-arc second seeing needs at least a 10 inch and probably larger scope to do it justice. Rob's 10 inch would be a great scope but it's just too long and requires too large a mount to be portable. For such tasks, a larger Newtonian is my scope of choice, no problems with CA, easily mounted, well corrected at F/6, correctable at faster focal ratios...

The reason I mentioned Delta Cygni was simply that I have been unable to split it with the 100mm F/6 achromat, the optics seem quite reasonable, last night the seeing was adequate, at 150x and 225x I was seeing a clean image with stable diffraction rings but no companion.

And yes, delta Cygni is child's play, best suited for challenging smaller scopes. In a pedestrian 10 inch Newtonian, it's just a looker, not a challenge.

YMMV

Jon


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Mark Harry
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5533507 - 11/22/12 08:18 AM

I have one question about this-

-IF-,

Comparing a long achro where the relatively narrow field isn't a hindrance, how would the comparison work out against an apo? Would it be close as far as color error and contrast is concerned?
I don't have an apo, or an ED for comparisons, but I do have a smattering of achros.
M.


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Crayfordjon
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Re: Are achromats enjoying a come back? new [Re: Mark Harry]
      #5533723 - 11/22/12 10:48 AM

Its fashionable to have an apo, especially if it has a designer label. We live in the age of designer labelled goods. To have a 90mm apo by you know who means you have "arrived" into the ranks of the elite. Now the achro is definately and quaintly "retro" nobody uses them any more, so the recieved philosophy goes. In reality the apo only supresses the secondary colour, it is still not perfect, you have to use a reflecting scope for that, so what do we have left in the apo, tertiary colour aberration, but it is so small as not to be noticed. The achro is reletively cheap, does an excellent job and is quite good enough for general observing. I go for achros every time, I have looked through apos and say, so what!. When I was night assistant at the 28 inch refractor at the Old Royal Observatory at Greenwich, I used to look through the seven inch guide refractor bolted to the 28 inch tube, it was a Cook doublet, yes and achro, the colour correction is astoundingly good and images rival those seen through a reflector.

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Re: Are achromats enjoying a come back? new [Re: Crayfordjon]
      #5534012 - 11/22/12 02:04 PM

I liked your post and its point. Although I am fascinated by the quest for state of the art perfection, and would love to
have a top line apo, I am quite happy with my four achromats and my 6"f/8 Planetary Newtonian.

Thanks for sharing your comments and experience,
Robert
SV80 Aplanat (f/7)
SV80/9D
SV102F11
Unitron 75,f/16
Homemade 6"f/8 Planetary Newtonian


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The Ardent
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Re: Are achromats enjoying a come back? new [Re: Crayfordjon]
      #5534091 - 11/22/12 03:24 PM

Here in the US sometimes I look thru the BBC Sky at Night Magazine. All the ads and equipment reviews are for imported scopes. The are the same ones imported in the US sometimes with a different name.
My question is: What telescopes is the UK exporting? Where are your achromats and apos that are in high demand? Who are the UK lensmakers comparable to D&G?


Quote:

Its fashionable to have an apo, especially if it has a designer label.




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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5534726 - 11/23/12 01:37 AM

Quote:

Quote:



Not sure what the problem is down there in your neck of the woods, but it has nothing to do with achromat vs. apochromat. I've split Delta Cygni with achromatic apertures ranging from 60mm to 152mm more times than I can count.




Done it with any 100mm F/5 or F/6 achromats?

Jon




I got Delta Cygni with the old Stellarvue Nighthawk 80mm f/6 achro a few times, but I've never owned a 100mm f/5 or f/6. I can see where that could be a problem with those scopes, though.

Actually, I was thinking of your comment that you might have gotten Delta with one of your 80mm f/11's. There really shouldn't be a problem at that focal length, given cooperative seeing.


John


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: 7331Peg]
      #5534793 - 11/23/12 02:50 AM

Quote:

Actually, I was thinking of your comment that you might have gotten Delta with one of your 80mm f/11's. There really shouldn't be a problem at that focal length, given cooperative seeing.




I have split Delta Cygni a few times with my 80mm F/11s but it takes better seeing than it does in my 80mm F/7 apo.

Jon


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: Crayfordjon]
      #5534799 - 11/23/12 03:00 AM

Quote:

Its fashionable to have an apo, especially if it has a designer label.




I imagine there are those who choose their scopes based on fashion but I am certainly not one of them... I base my choices on what I see at the eyepiece and I assume that the rest of the members of this forum do likewise. To assume otherwise is disrespectful.

When it comes to a discussion between apochromats and achromats, costs somehow always seems to enter into the equation. Achromats are better deals, apochromats are better telescopes... I like them both.

Jon Isaacs


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Crayfordjon
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5534811 - 11/23/12 03:21 AM

Hi ICUCinVA. The reason why we are not exporting scopes from the UK is that the US have a stranglehold on telescope trade, There is one very good 21st Century hign tech advancement in telescope design, an APO of 200mm aperture at 1/10 the cost all ready for production, but it cannot get off the ground because the market is dominated by US products. The same goes for focusers, they are either Chinese mousetraps or US products.

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Astrojensen
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Re: Are achromats enjoying a come back? new [Re: Crayfordjon]
      #5534823 - 11/23/12 03:39 AM

Quote:

the US have a stranglehold on telescope trade... the market is dominated by US products.




Hmmm. Last time I checked, there were four major players in the apo refractor market: TEC, AP, APM and Takahashi. Two are US based and do their own optics, APM is located in Germany and use Russian or Chinese optics, and Takahashi is in Japan and do their own optics. APM and Takahashi outsell the two US based companies by a wide margin.

A US stranglehold on telescope trade? Come on...


Clear skies!
Thomas, Denmark


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astroneil
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5534935 - 11/23/12 07:14 AM Attachment (16 downloads)

Quote:



I have split Delta Cygni a few times with my 80mm F/11s but it takes better seeing than it does in my 80mm F/7 apo.

Jon




That's simply not true in my experience. Indeed, there are several postings going back a few years now, from different people at different locations, which have reported the exact opposite.

Edited by astroneil (11/23/12 07:15 AM)


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Bob Abraham
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Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5535186 - 11/23/12 10:16 AM

I'm with Jon on this one. In my experience, given equal optical quality, and assuming thermal effects are not in play, focal ratio makes no difference to image stability and color error can be a bit distracting in an achromat in some cases. If cost doesn't enter into it and optical quality is good for both I'd personally rather have a well corrected apochromat than an achromat for pretty much any use.

Of course, the trick is that cost generally matters and its easier to make a high quality achromat, and a long tube can help with thermals sometimes so I can see why some people really like their achromats.

Bob


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Crayfordjon
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Re: Are achromats enjoying a come back? new [Re: Bob Abraham]
      #5535209 - 11/23/12 10:32 AM

I am aware that other countries make scopes but in the UK the scope trade is practically all US in origin, Need I tell what these brands are. Designer label syndrome again, if it aint either a wotsit or a doodad, then it aint any good, and these brands dominate. Same goes for focusers, but the problem here is that nobody is willing to compete with the US focusers, and this simple piece of engineering would not pose a problem for an enterprising entrepreneur.

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astroneil
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Re: Are achromats enjoying a come back? new [Re: Bob Abraham]
      #5535213 - 11/23/12 10:34 AM

Then ladies and gentlemen,

Let us have tests. Take a good 80mm f/11 and compare it to a a shorttube apochromat; the more complex the better.

Take them out at the same time; aim them at a tricky double star and observe the images.

Come back with reports. No cheating and let's form a consensus.


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Astrojensen
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Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5535508 - 11/23/12 01:25 PM

Quote:

Then ladies and gentlemen,

Let us have tests. Take a good 80mm f/11 and compare it to a a shorttube apochromat; the more complex the better.

Take them out at the same time; aim them at a tricky double star and observe the images.

Come back with reports. No cheating and let's form a consensus.




History has a habit of repeating itself, right?


Clear skies!
Thomas, Denmark


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: Crayfordjon]
      #5535595 - 11/23/12 02:09 PM

I'd love another achromat, the Skylight 4" f/15 in particular, however while there are undoubtedly many apochromats marketed so fashionably, I consider, albeit few, that are above common marketing practices; Astro-Physics, for instance, and among mirrored arrangements, Questar. Another case in point, and perhaps even further removed, are the positively retroactive series of refractors from Takahashi...

http://www.google.com/imgres?q=takahashi+fs-102&start=508&um=1&hl...

Note its quaint, toy-like appearance, hearkening back to the days of Unitron and those achromats of other Japanese manufacturers, where many a generation whiled away the hours, gliding their hands over the textured fittings upon standing before them.

Alan


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astroneil
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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5535652 - 11/23/12 02:37 PM

I don't know why I waste my time Thomas.

It's a lost cause anyway.

Regards,

Neil.


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7331Peg
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Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5535722 - 11/23/12 03:22 PM

Quote:


I imagine there are those who choose their scopes based on fashion but I am certainly not one of them... I base my choices on what I see at the eyepiece and I assume that the rest of the members of this forum do likewise. To assume otherwise is disrespectful.

When it comes to a discussion between apochromats and achromats, costs somehow always seems to enter into the equation. Achromats are better deals, apochromats are better telescopes... I like them both.

Jon Isaacs





Of course cost enters into the equation. Depending on the aperture a person is considering, as well as the particular brand of the scope, the cost differential is usually very significant. And that difference in cost has kept more than one person out of this hobby.

A few years ago, someone new to astronomy and refractors would have read one comment after another about the "dreaded color" in an achromat. Fortunately that's changed, especially in the last year, thanks to the efforts of Neil and many other people to draw attention to the positive aspects of achromats. No doubt the slow economic recovery in most countries is responsible for some of that as well.

It's good to see the healthy debate. Both types of scopes work quite well when used to their strengths. But you certainly can't expect cost to not be part of the discussion -- in many ways, it represents the greatest difference between the two kinds of lenses.


John


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ken hubal
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Re: Are achromats enjoying a come back? new [Re: 7331Peg]
      #5535882 - 11/23/12 05:14 PM

After owning several apochromats in the 80mm-127mm range, the long focus achromat has made a comeback in MY collection of refractors beacause I have simply learned to see beyond the rediculous marketing hype perpetrated by individuals here and on other internet forums, glossy ads in S&T(which is no longer the definitve astronomical journal it was years ago), and the continuing praddle that drives irrational people to spend thousands of dollars on small apos when FAR better views may be enjoyed by larger reflectors, which by the way are color free AT ANY FOCAL RATIO!!

One could always build a Newtonian of F8 or longer focal ratio with a smaller secondary, obtain a Mak-Cass, or build a 5 inch achro around one of D&G optical's lenses, which are outstanding performers at F12 or longer to obtain well color corrected performance rather than spending a fortune on a small apo which will simply not perform as well as longer focus achros, Maks, or long focus Newtonians.

CLEAR SKIES!


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: ken hubal]
      #5535920 - 11/23/12 05:33 PM

Ken, has your perception of this fast apochromat changed from when you provided the report that follows?

http://www.cloudynights.com/item.php?item_id=1477



- Jim


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ukcanuck
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Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5535923 - 11/23/12 05:35 PM

Wow this thread has some legs...

Ultimately, to me the quality of the optics are key...and if it happens to be a long achromat tube, then so be it. It never ceases to amaze me when I get reactions from people who are stunned when they look through a long refractor as if they had no idea an 'achromat' (said with some scorn for full effect) was capable of such good views.

As long as the sight of that graceful long tube captures the imagination and calls to people, and the views from within are there to back it up...reports of the demise of the long achromat will continue to be exaggerated.

Quote:

I'd love another achromat, the Skylight 4" f/15 in particular... Alan




I'm working on that...


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ken hubal
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Re: Are achromats enjoying a come back? new [Re: ukcanuck]
      #5535936 - 11/23/12 05:43 PM

Jimmy...YES!! Your perception doesn't seem to have changed however since you continue to profess the same exact findings about apos as you did two years ago when we last exchanged pleasantries!! I have been reading(with much laughter) your posts concerning optics, optical performance, and other comments. My conclusions are still the same now as they were then! I've also seen others review your comments with much skepticism. Perhaps a re-read of Neil English's excellent work is in order for you!! I also find it interesting(and quite obsurd) that you must quote from a review that was done more than 6 years ago!! I KNOW you can do better than that my old friend!! By the way, have YOU written a book like Neil's yet?
Seidel Aberrations!!



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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: ken hubal]
      #5535967 - 11/23/12 06:05 PM

Now that's just odd, Ken. I mean we have a guy like you that tends to preface his comments with a recitation of his (alleged) decades of experience, who a few years ago was raving (literally) about a cheap, fast, apochromat, yet when interest in slow achromats came back into vogue in the couple of years, you shift your raving to that design instead, contradicting your prior statements about fast apochromats like the one you reviewed here on CN. If you could be that badly and easily fooled by that speedy little Astro-Tech, Ken, what are your (claimed) multiple decades of experience actually worth?

If one's perceptions are accurate and factual, and one actually does own and use the scopes one claims to own, those perceptions should *not* change with fashion. It's only when you make stuff up, Walter Mitty style, that you tend to contradict yourself.

Say, any pictures of your with your D&G long focus achromat? We've been waiting for those for four years now.

- Jim


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ken hubal
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5535975 - 11/23/12 06:11 PM

Jimmy.. Walter Mitty is a person you should be able to identify with quite easily! You have, and continue to contradict yourself repeatedly in every one of your posts!! BTW, how's the oil in your triplet holding up?

As for fashion, apos are simply that, fashion. Only through years of owning and testing have I gained more up to date knowledge about their advantages(few to none) and disadvantages(many). When you show me YOUR BOOK, I'll show you my pictures of the D&G!!
So long, OLD FRIEND!!



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The Ardent
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Re: Are achromats enjoying a come back? new [Re: ukcanuck]
      #5535987 - 11/23/12 06:18 PM

Skylight Telescopes:

This somewhat answers my previous question about refractors make in the UK, but there are more questions.

From reading the Skylight description, it looks very nice, but why are the lenses imported? Why is no one making objective lenses in the UK?

Last month I viewed thru a 6" f/15 refractor made by an amateur from North Carolina. Very nice views. It seems that all the American "boutique" refractors originated from an enthusiastic amateur telescope maker.

Having owned a 6" D&G, I remember the optics, build, and appearance highly pleasing. And its just a lowly achromat.

Quote:

Ultimately, to me the quality of the optics are key...and if it happens to be a long achromat tube, then so be it. It never ceases to amaze me when I get reactions from people who are stunned when they look through a long refractor as if they had no idea an 'achromat' (said with some scorn for full effect) was capable of such good views.






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la200o
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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5535998 - 11/23/12 06:23 PM

Quote:

Quote:

the US have a stranglehold on telescope trade... the market is dominated by US products.




Hmmm. Last time I checked, there were four major players in the apo refractor market: TEC, AP, APM and Takahashi. Two are US based and do their own optics, APM is located in Germany and use Russian or Chinese optics, and Takahashi is in Japan and do their own optics. APM and Takahashi outsell the two US based companies by a wide margin.

A US stranglehold on telescope trade? Come on...


Clear skies!
Thomas, Denmark




Thomas,

Aren't you forgetting Tele Vue? Or are you only considering triplets? (TV makes only ED doublets and Petzvals, as I'm sure you know). Also, it's my understanding that Canon provides the optics for Takahashi.

Regards,

Bill


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: ukcanuck]
      #5536051 - 11/23/12 06:49 PM

"Ultimately, to me the quality of the optics are key...and if it happens to be a long achromat tube, then so be it."

Richard, I couldn't agree more. At a given aperture, assuming rational design and choice of glasses, the refractor with the better optics will be the one that shows you more, fast or slow, apochromat or achromat. You can't go wrong with quality.

I'm looking forward to your upcoming offerings.

Regards,

Jim


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Deep13
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5536103 - 11/23/12 07:07 PM

Well, I certainly like the views in my DIY (not by me) 5" f/12 w/ D&G lenses. Unfortunately, it is huge for its size. And it exhibits false color around Jupiter. Plus, under magnification, the image gets dark pretty quickly. My TV 101 has no false color, of course, so fewer of its photons are wasted. It gives sharp, high contrast images, as does the 5". Still, the second set of lenses in the 101 takes a long time to cool off. What could provide a better image than either of these fine instruments? My garden variety 8" f/6 Dob at 1/5 the price of the TV101 (without the $600 mount). It shows diffraction spikes, of course. But even with that, I get more detail out of the 8" than either refractor. Aperture rules. Period.

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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: Astrojensen]
      #5536113 - 11/23/12 07:17 PM

"APM and Takahashi outsell the two US based companies by a wide margin."

That might be correct in the case of Takahashi, but it might not be in the case of APM. I'm not sure how many refractors APM sells a year under its own brand, but for a given aperture, the production numbers seem to be pretty small.

But Takahashi and APM are not like TEC or Astro-Physics. Neither firm makes their own refractor optics. Takahashi uses Optron, a Canon subsidiary, for lenses, but makes its own tube assemblies and focusers. APM, though, is really no different than Stellarvue, Astro-Tech or the other branders. They take bits and pieces from various sources and assemble them into complete telescopes.

TEC and Astro-Physics pretty much make everything in the scope, nose to tail. This is more akin to the 19th century artisanal model. This is what Alvan Clark & Sons did, for example. So for entirely in-house refractors, I'd say the US does almost have a stranglehold on that tiny niche. As for telescopes in general, US brands are dominant, but most of the scopes sold under those brands (Meade, Celestron) are not made in the US.

Regards,

Jim


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Sky Muse
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Re: Are achromats enjoying a come back? new [Re: ukcanuck]
      #5536264 - 11/23/12 09:03 PM

"Wow this thread has some legs..."

...yes, especially the "chart phase".

"...reports of the demise of the long achromat will continue to be exaggerated."

...rather, for as long as the objectives do not come out of Japan; or arise from native soil, decidedly during a collective, nationalistic shout, the present lack of which I've read bemoaned?

"I'm working on that..."

Yes, I noticed, as it's sold out. It will require an equatorial, perhaps a Losmandy G-11 later in the coming year. It's to be given very serious consideration.

Cheers,

Alan


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ThomasWos
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Re: Are achromats enjoying a come back? new [Re: ken hubal]
      #5536273 - 11/23/12 09:09 PM

Quote:

As for fashion, apos are simply that, fashion. Only through years of owning and testing have I gained more up to date knowledge about their advantages(few to none) and disadvantages(many).






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coopman
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Re: Are achromats enjoying a come back? new [Re: ThomasWos]
      #5536349 - 11/23/12 10:01 PM

I've seen a lot of discussion about the various 6" f/6.5 models in the last year or so. I also see that a lot of their owners put them up for sale a few months later. I think that maybe this is due to the large size & weight of the OTAs. These scopes can really be a severe test for many mounts.

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Scott in NCModerator
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Re: Are achromats enjoying a come back? new [Re: coopman]
      #5536383 - 11/23/12 10:32 PM

Hmm...right now my achromat to apochromat count is tied at 3-3. Although I just sold one of my apos, so once I mail it off the score will be 3-2 in favor of achromats. So in my household, yes, the achromats are enjoying a comeback!

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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: Scott in NC]
      #5536582 - 11/24/12 02:04 AM

Fact One: An apo will put up a better image than a fast (short-tube) achromat. Much less color, more detail. WE ALL KNOW THIS. It's simple physics.

Fact Two: Though a subject of much debate, the image of an apo will be slightly better then the image of a long focus achro. If SLIGHTLY more detail is seen in the apo it is probably due to more time and expense in perfecting the optical figure while it is being made. We all should expect as much for the price of entry.

Fact Three: Apo's are expensive, sometimes very expensive. Achro's much less so.

Fact Four: I am not a doctor or lawyer, and it would be fair to say the same for many of us. In fact, I would say many of us have to watch our budgets in these times. Taking out a second mortgage to buy a scope does not make sense if you are on a budget, and maybe you do not want to save in the piggy bank for 5 years to get a scope. Then for average working folks like us that want a refractor (before we die) an achro fits the bill. And it will show us AT LEAST 90% of what an apo of the same aperture will show with decent quality optics on both sides.

Fact Five: Achro's are what people wanting a refractor in the days before the development of the apo used. It is safe to say they were plenty happy with them.

Fact Six: If you are on here debating over known facts then you are not using the scope you have. No wonder your not happy. Use the scope you have no matter what it is, it's the best scope in the Universe when your eye is to the eyepiece. Use Scope=Happy Astronomers

Hey, some of us can't afford the best of the best in refractor technology but still want a refractor for various reasons of our own. We ARE aware that better scopes exist, you don't need to keep reminding us. Really......WE KNOW.


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Crayfordjon
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5536626 - 11/24/12 04:04 AM

Ken, it is refreshing to see that there is another ATM on the forum who has the same views as myself, another little boy who sees that the king is stark naked. You are righr there is a helluva lot of praddle guffed about the merits of APOs, why use em when you have reflectors? I will give a bit of practical information here for those who love to step out of the rut and experiment,try using a small short focus achromat ( A bino OG), or a well corrected reducer just inside the focus, you will find that the very small amount of secondary colour will be supressed significantly in fact you will practically have an apo. I have done this and boy does it work!.

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WRAK
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Re: Are achromats enjoying a come back? new [Re: Crayfordjon]
      #5536639 - 11/24/12 04:35 AM

Question to those in the know: Increasing focal ratio of an APO with a Barlow for example from 7.5 to f/15 - will this give at least some of the discussed benefits of long Achros like increased focal depth besides the side effect of loosing some light due to the additional optical element?
Wilfried


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Mark Harry
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Re: Are achromats enjoying a come back? new [Re: WRAK]
      #5536743 - 11/24/12 07:53 AM

Fact 7:
I've asked a question, and can't get an answer due to all the debate. Must be some truth to fact 6!
M.


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Pete-LH
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5536864 - 11/24/12 09:20 AM

"Use the scope you have no matter what it is, it's the best scope in the Universe when your eye is to the eyepiece. Use Scope=Happy Astronomers"

This really is the key ...

My impression from threads like this is some of us(Me being as guilty or more than most) spend more time worrying about small differences in performance between scopes and more time looking at their scope than looking through the lens and studying Universe.

I have spent too much time and money searching for the next telescope and not enough time observing and sharing what can be seen ... shame on me!

That being said I just bought and I'm waiting to receive a nice used 90mm f/11 Achromat. Can't wait to compare it to my 60, 80, 90, 100, 102 and five inch refractors(None are short focust true APOs) as well as my two SCT's.

Actually some of the best sharp images I've viewed were through an f/8 C6R which was a great value and I let it go in search of a "better" refractor. For myself being red-green colour blind it makes less sense than for most here.


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PhilCo126
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Re: Are achromats enjoying a come back? new [Re: Pete-LH]
      #5536967 - 11/24/12 10:17 AM

Achromats didn't need to make a comeback... A 20 cm Comet Hunter refractor is a great way to enjoy a large refractor!


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vahe
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5537033 - 11/24/12 11:00 AM

Quote:

Fact Four: I am not a doctor or lawyer, and it would be fair to say the same for many of us. In fact, I would say many of us have to watch our budgets in these times. Taking out a second mortgage to buy a scope does not make sense if you are on a budget, and maybe you do not want to save in the piggy bank for 5 years to get a scope. Then for average working folks like us that want a refractor (before we die) an achro fits the bill. And it will show us AT LEAST 90% of what an apo of the same aperture will show with decent quality optics on both sides.





Well, for starters you and I have something in common, me too, I am not a doctor or lawyer and as far back as I can remember I had to work on two jobs, a regular daytime and a permanent moonlight.

But the similarity end here, my first encounter with an apo happened about 25 years ago, a fellow member from our astronomy club had just bought a large Astro-Physics, this one was from pre ED era which is really not a real apo, but close enough.

I went for a look through that scope, there were no planets on the night, he had it pointed at some galaxies, next to the refractor he had his 10” Meade SCT pointed at the same object, what I saw in that refractor changed me forever, I could not get that image out of my mind until, years later, I bought a AP apo.

As for the Meade, it also changed my opinion of SCT’s, compared to the apo the SCT produced an image that to my eyes looked simply horrible.

Large apos are expensive and certainly out of reach for most of us “average working folks” but I know many average wage earners who sacrificed a lot and purchased one of these “designer” scopes, it is just how you prioritize your hobby, I am driving a 36 year old car, I am willing to sacrifice on that but when it comes to scopes my standards are a bit higher.

Vahe


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5537089 - 11/24/12 11:31 AM

"Though a subject of much debate, the image of an apo will be slightly better then the image of a long focus achro."

This one is not a fact. If the APO has the better figure, it will produce the better figure. Color error is only one aberration that affects visual image quality. There are other figure-error induced aberrations that are considerably more damaging to the image than defocused light in particular wavelegths.

"Apo's are expensive, sometimes very expensive. Achro's much less so."

This one too, is not a fact. Because quality, and not design, is what is most important, and quality costs money, the best achromats are more expensive than the more modest apochromats. Basically, whether achromat or apochromat, you generally get what you pay for. There is no free lunch. There are any number of 4" apochromats that are less costly than a 4" Skylight long focus achromat. When Antares was still selling the long focus achromats using "Vixen Spec" optics, those scope typically cost a couple of hundred dollars *more* than FPL-53 ED doublets of like aperture.

I pretty much agree with everything else you wrote, though. Stephen Stills said it best - "Love the one you're with!"

Regards,

Jim


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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5537118 - 11/24/12 11:47 AM

My "facts" were not meant as hard rules, more like general guidelines. There are always exceptions to the convention in this world, a "hard line" rarely exists with anything. The constant apo vs. achro debate gets on my nerves though.

You have to ask yourself: What kind of astronomer am I? One who studies the heavens, or one that spends much precious time studying opitcal quality.

I think I will keep looking up. Besides the pursuit of perfection is only an illusion. The closer you get the further you seem.


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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: vahe]
      #5537136 - 11/24/12 12:00 PM

I had an ED80 once, really liked it. But a period of unemployment forced the sell of all my equipment last year. I am currently employed and looking to enter the hobby agian when I get my money back from Uncle Sam in February.

All I have to use right now is an old Sears 60mm/700mm FL refractor. EP's stink but the objective seems to be of very good quality. I have ordered some better oculars for it but have been quite surprised at the detail seen on Jupiter. This scope is the reason I see little fault with a slow achro and why my new scope in a few months will be a long achro, just bigger. Budget is to tight for an apo, not that they are not great. But I have to get everything else too, just not room in budget for an apo. I am sure I will be happy though, looking at the Celestron Omni 102.


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jrbarnett
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5537140 - 11/24/12 12:03 PM

"What kind of astronomer am I? One who studies the heavens, or one that spends much precious time studying opitcal quality."

Why not "both"? Particularly if understanding one helps facilitate and make more productive your pursuit of the other.

"Besides the pursuit of perfection is only an illusion. The closer you get the further you seem."

Thank goodness our Olympians don't think that way!

- Jim


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ThomasWos
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Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5537163 - 11/24/12 12:16 PM

Quote:

You have to ask yourself: What kind of astronomer am I? One who studies the heavens, or one that spends much precious time studying optical quality.






Don't forget the title of this forum..."Cloudy Nights"!!!

I live where clear skies are rare, let alone clear skies with good seeing, (very rare indeed).

To me, an important part of my "study of the heavens" is learning more about optics and optical quality,
so that my precious observing time is as good as it can be.

Peace

Tom


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Kentuckystars
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Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5537164 - 11/24/12 12:17 PM

I know enough to tell if my scope is collimated, and after that I can