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Which brands produce "true" Apochromatic Refractors?

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#76 Bowlerhat

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Posted 16 October 2020 - 09:50 PM

There is a definition of apochromatism which was developed more than a 100 years ago. It does not mean the objective will be free of chromatic aberration or even free of false color visually.

 

http://www.csun.edu/...and/musing.html

 

"May I point out that this original definition: "bringing 3 wavelengths to a
common focus and be corrected for spherical aberration at two wavelengths" came
about historically because the very first apochromats actually did this -
thus the definition was made to fit the example."

 

Jon

I like this line "Finally, it is not the ED or Fluorite which determines the overall correction, rather it is the mating element." everyone is forgetting about the mating element.

 

 

Yeah, people get swept up in that one particular aspect of perfectionism. People might complain about a doublet because it has a tiny residual color error, but forgive a fancy triplet for being so expensive and heavy that you have to give up a bunch of aperture to choose it. Never mind that the aperture lost reduced the detail you can see much more than the doublet's color error did. And the figure is also a huge factor.

I really think that the only people who need to stress about perfect apochromatism are hard-core imaging folks and the few people for whom even the tiniest amount of purple grates on their nerves like an off-tune instrument. Which is a pity for them, because there are a lot of great achromats and imperfectly corrected apos out there.

Classic telescopes are never mentioned nowadays especially with newbies.



#77 Barologuy

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Posted 17 October 2020 - 12:46 AM

Speaking of Classic Telescopes, I have a relative with a Pentax 85mm ED APO, I think it's around f/10 or f/11... does anyone know anything about these scopes? I saw a thread on them from a while back, but I haven't seen much online about them.

#78 Suavi

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Posted 17 October 2020 - 03:12 AM

I like this line "Finally, it is not the ED or Fluorite which determines the overall correction, rather it is the mating element." everyone is forgetting about the mating element.

 

 

Classic telescopes are never mentioned nowadays especially with newbies.

IMHO classic telescopes are a little bit like classic cars - beautiful and classy, but modern well made telescopes (and cars) are just better at what they are supposed to do.



#79 EJN

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Posted 17 October 2020 - 10:55 AM

IMHO classic telescopes are a little bit like classic cars - beautiful and classy, but modern well made telescopes (and cars) are just better at what they are supposed to do.

 
Is your opinion based on experience? I have this 3" f/16 refractor which is 50 years old, and optically it is fully
equal to my 80mm f/7.5 ED doublet with an FPL-53 element. It is actually is slightly better on double
stars. Both show an essentially perfect star test.
 
It is the only telescope I own which will show pinpoint images to the edge of the field using "simple" eyepieces
like Plossls, or even Kellners.
 
post-12877-0-37706800-1600098485.jpg
 
 
For comparison, the C80 FPL-53 ED doublet.
Remember, "The enemy of good enough is more better gooder" to quote Rod Mollise.
 
ed80-5_fl.jpg

 

 

 

 


Yeah, people get swept up in that one particular aspect of perfectionism. People might complain about a doublet because it has a tiny residual color error, but forgive a fancy triplet for being so expensive and heavy that you have to give up a bunch of aperture to choose it. Never mind that the aperture lost reduced the detail you can see much more than the doublet's color error did. And the figure is also a huge factor.

 
You have escaped from the reality distortion field which permeates this forum.


Edited by EJN, 17 October 2020 - 11:41 AM.

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#80 TG

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Posted 17 October 2020 - 11:40 AM

Predates ED and SD glass. In the early days, Astro Physics used a special batch of glass made for NASA, and when that glass ran out, KzFSN-4. They switched to FPL-5x with the Starfire line.


The Blue Tubes *were* called StarFires and used two abnormal dispersion flints which are a mystery to this day. I own a 7” one and it does show a tiny bit of very deep violet at high magnification. Not a problem for the eye but for astrophotography you would need to use filters.

The Blue Tubes were oil spaced. Later a white tube version was offered which was air spaced. After two years of this ca. 1991-92 the EDT StarFires were offered.

TG
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#81 TG

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Posted 17 October 2020 - 11:54 AM

There was a reference earlier to spherochromatism. If intent is imaging, this is a serious concern especially if one also wants a faster f/ratio which exacerbates spherochromatism. With uncorrected spherochromatism, even when you use filters for imaging, you can have different star sizes at different wavelengths due to spherical aberration. The longitudinal chromatic error, which is what the definition of apochromatism is mainly concerned about, is determined by the glass choices, but to fix spherochromatism, the lens has to be figured aspherically. Astro-Physics does this as I’m sure the other high end makes such as CFF and TEC do this as well.

TG

#82 Jon Isaacs

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Posted 17 October 2020 - 01:16 PM

Today’s ED scopes perform better than the people using them.

 

:waytogo:

 

That's true of all my scopes.   The weakest link in chain is me, not the scopes, not the eyepieces, not the mounts.  

 

My stuff may not be the best but a better observer would see more than I can so my goal is to work on improving my observing skills, my equipment is plenty good enough.

 

Jonl


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#83 Jon Isaacs

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Posted 17 October 2020 - 01:26 PM

It is the only telescope I own which will show pinpoint images to the edge of the field using "simple" eyepieces like Plossls, or even Kellners.

 

 

A Barlow resolves this issue.

 

On the other hand the maximum field of view possible with a 3 inch F/16 with a 1.25 inch focuser (could be a 0.965 inch) is 1.3 degrees, a far cry from the 4.4 degrees possible with a ED-80.

 

In terms of color correction, a 76 mm F/16 achro's chromatic blur is 1.8 x the airy disk.. thevED-80, about 0.7x.

 

Jon



#84 REC

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Posted 17 October 2020 - 01:51 PM

So you _do_ have a budget.  <smile>

 

I have this scope (exactly, but with the better focuser, to avoid being tempted to get a Featherlite).  Works well.  M13 below.  FPL-53 from Ohara (there are different manufacturers of FPL-53).

 

https://www.teleskop...-Refractor.html

 

Bought mine from Karl Kloss.  He puts them on an optical bench, will give you the report on your specific scope, there may be a small fee.  Good "lemon" insurance.

 

https://www.teleskop...ed5f831c6ea8b76

 

attachicon.gifM13V19.jpg

That is one hell of a sharp image of M13, may the best one I have ever seen! Every star, right to the edge is sharp. What scope did it?


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#85 bobzeq25

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Posted 17 October 2020 - 02:06 PM

That is one hell of a sharp image of M13, may the best one I have ever seen! Every star, right to the edge is sharp. What scope did it?

Thanks.

 

TS 130 F7 with 3.7 focuser.  FPL53 triplet.  TSFlat3 flattener (no reducer, it's 1.0X), carefully spaced for the sharpest possible stars.  Atik 460EXM with Astronomik Deep Sky RGB filters.  No L on this bright target.  Bortle 7 skies.

 

It's no Tak or Tec, but it's less than half the price.  No noticeable color, regardless of what words are used to describe it.  <smile>

 

Better version of that image (than the required crappy CN jpg) here.

 

https://www.astrobin.com/351037/I/


Edited by bobzeq25, 17 October 2020 - 02:10 PM.

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#86 Suavi

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Posted 17 October 2020 - 09:37 PM

 
Is your opinion based on experience? I have this 3" f/16 refractor which is 50 years old, and optically it is fully
equal to my 80mm f/7.5 ED doublet with an FPL-53 element. It is actually is slightly better on double
stars. Both show an essentially perfect star test.
 

I thought this thread was about true APOs. From my experience, an f/7.5 ED doublet is not an APO.

 

Never mind, someone earlier suggested a reflector, even though the original question was about brands producing true apochromatic refractors.



#87 Ken Sturrock

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Posted 17 October 2020 - 10:20 PM

Back to that troublesome "true" word.


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#88 bobzeq25

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Posted 17 October 2020 - 11:07 PM

I thought this thread was about true APOs. From my experience, an f/7.5 ED doublet is not an APO.

Have you used that specific lens?  I started out in imaging with that lens.  It's fine, with very little false color.  Triplets don't automatically make an APO, design, glass, and lens quality count.  The 80 F7.5 has all three.  Some triplets don't.
 


Edited by bobzeq25, 17 October 2020 - 11:08 PM.

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#89 Suavi

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Posted 17 October 2020 - 11:26 PM

Have you used that specific lens?  I started out in imaging with that lens.  It's fine, with very little false color.  Triplets don't automatically make an APO, design, glass, and lens quality count.  The 80 F7.5 has all three.  Some triplets don't.
 

Obviously I have not had a privilege to use your specific lens. It's great that you are pleased with its performance. My f/7 ED doublet had plenty of false colour when imaging, but it was fine for visual and narrowband. Would really like to see LRGB images taken with this true (please excuse me for using a troublesome to some folks word) APO, if possible.



#90 Ken Sturrock

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Posted 18 October 2020 - 10:31 AM

...Please excuse me for using a troublesome to some folks word...


The words are only troublesome because neither "apochromat", let alone "true" are used in any kind of a standardized manner. Neither by manufacturers, nor by regular people, either. An "apochromat" is frequently described in quantitative performance terms as focusing three colors to the same point, but which colors are not consistently defined. Moreover, as Jon points out, that initial definition wasn't created to describe an ideal but was created to describe (and market) the performance of an actual instrument that was produced. Continuing on that theme: Today, "apochromat" is frequently described in material terms such as the OP did - but saying that an aprochromat consists of a three lens objective where some number of lenses is FPL-53 (or better...) doesn't guarantee any kind of performance and also leaves out other possible configurations that could achieve similar or better performance. Finally "apochromat" is usually used in a subjective and qualitative manner to describe something "better than an achromat" or an objective design which doesn't show false colors that some particular person can see in a focused (versus unfocused) image.
 

Few people, seem consistent about what "aprochromat" means, and adding "true" doesn't create any additional clarity. So, if you don't think that an f/7.5 doublet is an apochromat then that's fine. Many will agree with you. Unfortunately, there's no common definition in play so it's all opinion.
 
In this case, we know examples of instruments that you don't think are "true apochromats" but we don't know what you do think is a "true apochromat" because you never said which definition you're using. Other than that you think that it's the "true" one. To be fair, other than a working definition bravely put forth by the OP and a couple of others, most have not. Many have simply chosen to talk around it and explore the phenomenon because that's what we do in a casual discussion forum.


Edited by Ken Sturrock, 18 October 2020 - 10:46 AM.
Further pontification

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#91 j.gardavsky

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Posted 18 October 2020 - 12:23 PM

The "Apochromasie" has been finally defined and published by Ernst Abbe in 1886, and also with a reference to Carl Friedrich Gauss' investigation of the spherical aberration on the telescopes optics.

 

An  APO objective (telescope, microscope) has to obey simultaneously the two conditions:

1. The focused light rays of three different wavelengths meet in a common focus (The Abbe's condition)

2. The focused light rays of two (or more) different wavelengths have no spherical aberration (The Gauss' condition) ["... die sphärische Aberration für mehr als eine Farbe aufzuheben.] )1

 

The residual chromatic aberration and spherochromatism of an APO form the "tertiary spectrum",

as opposed to the secondary spectrum of an achromatic objective in which just two wavelengths meet in a common focus, and the spherical aberration has a minimum for just one wavelength.

 

Comments:

)1 Addresses the problem of the undercorrectied spherochromatism on the long wavelength,

and of the overcorrected spherochromatism on the short wavelengths.

 

(See the reprint dossier by Moritz von Rohr: ERNST ABBEs APOCHROMATE)

 

As the both conditions can be checked on an optics bench, it should not be a problem to find out which telescope is an APO (or full APO if you like), and which one is not.

 

Hoping this helps,

JG


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#92 Barologuy

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Posted 18 October 2020 - 01:26 PM

After reading through the thread a couple times, I've learned some incredibly valuable information, and among those things, there's a couple factors that I didn't know coming into it:


"True" is entirely colloquial and experience based.

Apochromatic, while being a scientific term, is also colloquial due to the nature of magnification, light, and everyone's own experiences both with a scope but also time, location, quality control, design, glass, eyesight, purpose, camera, and accessories, if any. This doesn't even include bortle scale, light pollution, or sky glow.


It would be nearly impossible to assess a telescopes ability from one experience with it, because so many factors exist, and among those is location, weather, and how the light (or light pollution) appears in the sky to begin with.

The theory of constraints applies to every piece of equipment in your imaging train. Basically, your experience is a function of the weakest part in your system. A Tak is great. Pair it with lesser parts, and your own experience, and photos may still end up sub par.

Inevitably, there are objective things, such as CA, that we can agree should be minimized. I think the key though is understanding that each piece is exactly that -- part of a greater system. I've seen images of people with just sky trackers that I've found to be impressive because they have darker skies, better cameras, and were able to gather it in less time too.

Basically, Astrophotography is like cooking. If you have a bad ingredient, it's going to affect your whole dish. If you have the best ingredients, but your technique is bad or you don't "follow directions" it's not going to turn out like you expect. If you have the right tools,setting, the right ingredients, and you know what to do with them to get your desired outcomes, you have a good chance of getting desired results.


Sounds obvious but it puts the fact that perhaps there's such conflict around terms such as "true" "apo", because we have no standard to off of. We aren't taking spot diagrams, calculating the circumference of them and saying "this is x% deviance light refraction from what is expected from a Tak or whatever example we would like to use"

Which is really how the comparison of glass should be getting made, because it's not "just glass", it's also how good do the lenses do their job based on design, spacing, and so forth. The question should really be, who puts together the best objectives and puts them in telescopes at as high of a QC as possible with respect to spacing and other things we care about, expressed as the spot diagram and other tests that exist, and then using those as sales material, rather than touting specific materials as the be all end all, because while that's important it's not the end of the story.


Just my two cents, after some thought.

At the end of the day, perhaps the only approach is taking this equipment out in the field and seeing if we enjoy the images from them.

Edited by Barologuy, 18 October 2020 - 02:41 PM.

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#93 bobzeq25

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Posted 18 October 2020 - 02:21 PM

+100 <grin>



#94 Suavi

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Posted 18 October 2020 - 03:35 PM

Yes, I have also truly learnt a great deal from this thread. It is truly encouraging that there is so much true wisdom that is freely shared amongst us waytogo.gif



#95 Simoes Pedro

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Posted 21 October 2020 - 04:50 AM

To OP, color correction is loosely correlated with a single glass choice.

 

Many Fluorite/FPL53 doublets are not apochromatic.

 

A 4" BK7/F2 at f50 is aprochromatic.

 

A telecope can have zero color error and be unusable. Spherochromatism is the main aberration in short focus triplets.


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#96 j.gardavsky

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Posted 21 October 2020 - 07:28 AM

To OP, color correction is loosely correlated with a single glass choice.

 

Many Fluorite/FPL53 doublets are not apochromatic.

 

A 4" BK7/F2 at f50 is aprochromatic.

 

A telecope can have zero color error and be unusable. Spherochromatism is the main aberration in short focus triplets.

It is frequently the spherochromatism, when somebody is crying about the chromatic aberration.

 

JG
 


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