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Equipment Discussions >> Refractors

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edwincjones
Close Enough
*****

Reged: 04/10/04

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


Reged: 07/24/09

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


Reged: 10/26/12

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
*****

Reged: 04/04/05

Loc: Limerick, Pa
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
*****

Reged: 09/01/08

Loc: North coast of Oregon
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
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
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
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
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
*****

Reged: 07/28/09

Loc: res publica caledoniae
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
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
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
super member


Reged: 11/19/10

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
super member


Reged: 12/02/09

Loc: UK Derby
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
Pooh-Bah
*****

Reged: 02/02/10

Loc: sebastopol, california
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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