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Are achromats enjoying a come back?

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#76 Pinbout

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Posted 14 November 2012 - 12:55 AM

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


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

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the ca is only noticable on bright defocused stars.

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but that stock diag. has to go. :grin:
 

#77 chboss

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Posted 14 November 2012 - 02:29 AM

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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#78 stanislas-jean

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Posted 14 November 2012 - 03:14 AM

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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#79 chboss

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Posted 14 November 2012 - 05:32 AM

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

best regards
Chris
 

#80 stanislas-jean

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Posted 14 November 2012 - 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
 

#81 Jon Isaacs

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Posted 14 November 2012 - 08:11 AM

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
 

#82 Mark Harry

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Posted 14 November 2012 - 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.
 

#83 Andy Taylor

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Posted 14 November 2012 - 09:19 AM

"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 :grin:


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

#84 Abb

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Posted 14 November 2012 - 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 :)
 

#85 stanislas-jean

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Posted 14 November 2012 - 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
 

#86 Sky Muse

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Posted 14 November 2012 - 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...:shocked:, 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...ified_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
 

#87 Astrojensen

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Posted 14 November 2012 - 03:05 PM

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
 

#88 Mark Costello

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Posted 14 November 2012 - 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?" :bigshock: :ohmy: :getem:
 

#89 Sky Muse

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Posted 14 November 2012 - 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. :thinking:

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
 

#90 Kentuckystars

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Posted 14 November 2012 - 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.
 

#91 Astrojensen

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Posted 14 November 2012 - 07:00 PM

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
 

#92 jrbarnett

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Posted 14 November 2012 - 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. :grin:

- Jim
 

#93 jrbarnett

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Posted 14 November 2012 - 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
 

#94 jrbarnett

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Posted 14 November 2012 - 08:18 PM

Sorry, Mark.

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

Let's start with this:

http://www.astromart...8245&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-for...hbarer-Achro...

Meade 90/1000 achromat (Taiwan): http://www.astro-for...1000-Model-3...

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

Celestron (Synta; China) 80/600 ED doublet: http://www.astro-for...&p=34104#pos...

107mm f/6.5 triplet (China): http://www.astro-for...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
 

#95 chboss

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Posted 14 November 2012 - 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
 

#96 jrbarnett

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Posted 14 November 2012 - 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
 

#97 Sky Muse

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Posted 14 November 2012 - 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.:scared:

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
 

#98 Sky Muse

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Posted 15 November 2012 - 12:10 AM

Jason,

Yes, this one...

http://www.chuckhawk...tron_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
 

#99 Sky Muse

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Posted 15 November 2012 - 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
 

#100 chboss

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Posted 15 November 2012 - 01:10 AM

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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