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Which brands produce "true" Apochromatic Refractors?

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95 replies to this topic

#26 payner

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Posted 12 October 2020 - 07:07 PM

Agree and prescription is also very important.

Absolutely. Design is the foundation. I don't think that is studied and considered too much. Most concentration is on glass/CaF2.


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#27 drprovi57

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Posted 12 October 2020 - 07:10 PM

I found this information very useful:  

 

https://cfftelescope.../optical-design

 

Helped me make the decision to purchase a CFF refractor


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#28 Ken Sturrock

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Posted 12 October 2020 - 09:04 PM

Absolutely. Design is the foundation. I don't think that is studied and considered too much. Most concentration is on glass/CaF2.

 

Sure because most people don't want to think through a system. They want a single value: glass type, price, square footage, horse power, amperage, megapixels...


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#29 nicknacknock

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Posted 13 October 2020 - 01:17 AM

Sure because most people don't want to think through a system. They want a single value: glass type, price, square footage, horse power, amperage, megapixels...

And the system includes eyepieces, diagonal, thermal acclimation, system collimation (lens cell, orthogonality of focuser, orthogonality of diagonal). Anything and everything between the front element of the lens cell to the last element of the eyepiece need to work together.


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#30 Voyager 3

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Posted 13 October 2020 - 07:56 AM

I've seen some refractors described as super-achromats or even super-apos which are apparently a step up again in terms of colour control. I believe this has been defined as an objective that achieves 4 color crossings.

It does sound rather like a marketing term though hmm.gif

Even ortho APOs 🤨... Or are they of any specific level ?

#31 nicknacknock

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Posted 13 October 2020 - 09:09 AM

Are we talking about eye relief here? Kidney beaning? 



#32 Jon Isaacs

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Posted 13 October 2020 - 09:23 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.

 

I thought that would be the case. Still, as I understand it, very good optics.

 

Jon



#33 Jon Isaacs

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Posted 13 October 2020 - 09:34 AM

I've been curious for years about why a lot of people go on and on about "glass". Is it really a documented case that you can't produce a good refractor without using specific glasses? I have no idea what glasses are in my refractor trio, I only care about the views and the images that I get from them. I would expect that there are lots of ways to eliminate CA without using one particular "glass". Comments please.

Rgrds-Ross

 

Ross:

 

The color correction does depend on many things. But, for a given focal ratio, aperture and design (doublet or triplet), the color correction is very much dependent on the ED glass chosen.

 

The difference between glasses similar to FPL-51 and glasses similar to FPL-53 is about a factor of 2.  

 

Your A-P 155 EDF would be an FPL-53 triplet. The NP-127 uses unspecified glasses but given the color correction, design and cost, it's almost certainly based on FPL-53.

 

Jon


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#34 Scott in NC

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Posted 13 October 2020 - 09:39 AM

The difference between glasses similar to FPL-51 and glasses similar to FPL-53 is about a factor of 2.  

 

Sorry Jon, but I’m slow today.  Please explain what you mean by “a factor of 2.”



#35 rgsalinger

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Posted 13 October 2020 - 11:27 AM

I don't want to hijack the thread. What I wonder about is that my limited understanding is that you a set glasses which have different CA characteristics to make the entire visible spectrum come to a single focus. That may be incorrect. Based on that, I would think that you would mix and match and not just use a single type of glass. Now I know that this is from ignorance (or intuition). There must be something that I'm missing that would account for the use of the same glass in all three elements of the AP155 that I own. 

Rgrds-Ross



#36 Rich V.

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Posted 13 October 2020 - 11:46 AM

There must be something that I'm missing that would account for the use of the same glass in all three elements of the AP155 that I own. 

 

Whether a doublet or triplet, it's always a mating of higher and lower density glass; a crown (or fluoro-crown) glass and a flint glass.  

 

https://www.telescop...po_examples.htm

 

Rich



#37 Mitrovarr

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Posted 13 October 2020 - 11:47 AM

Most of the companies that produce refractors have very well corrected entries. It's just that many of them also produce things like FPL-51 doublets as well, and you need to make sure you get the right one.

When older scopes are also taken into account, I don't think there are many companies if any that only produced extremely well corrected scopes. Even Takahashi and AP have some entries with incomplete color correction. So you'll have to research the individual scopes to find out.

#38 FirstSight

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Posted 13 October 2020 - 06:16 PM

Thread re-opened for on-topic discussion.


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#39 Kunama

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Posted 13 October 2020 - 06:33 PM

By true I mean they contain FPL-53 glass or better, and show little to no color dispersion on bright objects. I edited these requirements because I realize now that this is entirely based on other factors of the telescope, not just glass type or number of lenses.

Budget really isn't a factor, I'm just wondering what people have to say from their own research or experiences, whether it's your dream scope, a scope you own, or a scope you once owned, tell me who you went with, or will go with, for your beautiful, unadulterated raw data!

I have seen many scopes that are labelled as being apochromats which in my experience were not apochromatic in the truest sense but sufficiently well corrected that for hobby/enthusiast purposes they suffice.  My experience of those that are essentially CA-free are the TOA, TSA and FOA  scopes from Takahashi, my friend's CFF140F7.5, my TMB-LZOS 152F7.9.

 

I haven't used TEC refractors or AP triplets but I expect they are pretty close to the TOA and LZOS optics....

 

Merely using FPL53 doesn't guarantee anything, I have used some FPL53 scopes that were not apochromatic....  I think everyone knows that the mating glasses and the achieved figure/polish are just as important if not more so, than the often bandied about Ohara variants....


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#40 gnowellsct

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Posted 13 October 2020 - 06:35 PM

By true I mean they contain FPL-53 glass or better, and show little to no color dispersion on bright objects. I edited these requirements because I realize now that this is entirely based on other factors of the telescope, not just glass type or number of lenses.

Budget really isn't a factor, I'm just wondering what people have to say from their own research or experiences, whether it's your dream scope, a scope you own, or a scope you once owned, tell me who you went with, or will go with, for your beautiful, unadulterated raw data!

CFF is as apo as they get.  So is TEC.  



#41 gnowellsct

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Posted 13 October 2020 - 06:37 PM

I found this information very useful:  

 

https://cfftelescope.../optical-design

 

Helped me make the decision to purchase a CFF refractor

People don't understand they may not be making these refractors forever.  Get 'em while they're available.  The lure of big aperture RCs and such is tugging on the CFF business model.   

 

I keep thinking I should order another one.  

 

Greg N


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#42 KBHornblower

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Posted 13 October 2020 - 07:47 PM

To answer the question about which companies make "true" apochromats, we need a good definition.  As I see it there is not an objective or precise technical definition.

 

As an exercise in etymology, it appears to me that apochromat is a coined word without a technically precise literal definition, as compared with achromat.  The latter is from prefix a- (without) plus root word chroma (color).  If used literally it would mean a lens that is perfectly corrected for chromatic aberration.  While no such lens has been made, the 18th century doublets were a state-of-the-art improvement over single lenses and the term stuck in widely accepted jargon.  Thus, when triplets that greatly reduced the residual aberration were invented, the optical community coined a new word to distinguish them from the old doublets.  Here we have the prefix apo- (away from or something like that) and the root chroma, not exactly a literal statement of superiority to an optical element literally described by the word achromat.  For better or worse the optical business has embraced invented definitions according to the number of different wavelengths brought to a common focus, which by themselves do not guarantee that one is superior to the other in color correction.  Specifically, a long-focus doublet with two lines at common focus can have less residual color than a fast, not so well made triplet that has three lines at common focus.  Yet by the letter of the “law”, meaning the invented jargon, the triplet is an apo and the doublet is not.

 

Enough petty pedantry on my part for one night.  I just had to get it off my chest.


Edited by KBHornblower, 13 October 2020 - 07:48 PM.

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#43 Mitrovarr

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Posted 13 October 2020 - 08:07 PM

Yeah, the achromat/semi-achromat/apochromat conversation is a mess. Personally I regard anything that has ED glass or better as an apo, with some apochromats having a fair bit of color and some being pretty color free. Marketers often use the term like that, so it's safest to assume any given "apo" may not be and actually do research anyway. Plus, semi-apo is devalued as a term even if some of them may be pretty great.

That being said I know others don't agree so I just call my SW150ED a "doublet".

#44 PKDfan

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Posted 13 October 2020 - 10:37 PM

Personally with the whole is it/is it not affair gets messed up with each persons subtly unique eyeball characteristics. I myself have(lucky for astronomy)an especially sensitive fovea to light intensity. In some great sky sites I have seen much deeper than my friends.
Actually was not truly believed when I first joined CN and posted how easy I saw the Horsehead and Flame with a low aperture tube. Caveats being ultra clean skies, a 'resolving' magnification and great optics. Personally with my Sky-Watcher F/9 100ed I have not seen one trace of false colour on the bright targets so far observed but have not gone specifically looking for it. If I hav'nt seen it on the moons edge at any magnification then its not a concern.
Wow! did SW do a great job on these Pro-ed/Evostar tubes or what!! Practically every review of one is ga-ga! AWESOME for our Astro community! Saw an ed80 at 99.1 Strehl on a russian site!! Anyways this 'discussion' is never going away till all our eyeballs are the same.

CS & GS
Edit:typos clarity

Edited by PKDfan, 13 October 2020 - 10:43 PM.

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#45 25585

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Posted 14 October 2020 - 04:45 AM

People don't understand they may not be making these refractors forever.  Get 'em while they're available.  The lure of big aperture RCs and such is tugging on the CFF business model.   

 

I keep thinking I should order another one.  

 

Greg N

I got a TSA-120 partly because it could be discontinued as was the TSA-102. Thus 120 is my truest apo, though I personally see no colour in my FC100DL either. 



#46 gnowellsct

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Posted 14 October 2020 - 08:09 AM

Enough petty pedantry on my part for one night. I just had to get it off my chest.


As the joke goes, think I heard it from Alan French, achromat means without color and apochromat means "this time we mean it. "
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#47 KBHornblower

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Posted 14 October 2020 - 08:15 AM

As the joke goes, think I heard it from Alan French, achromat means without color and apochromat means "this time we mean it. "

smile.gif


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#48 EJN

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Posted 14 October 2020 - 09:24 AM

Reflectors are true apochromats. All lens systems have secondary color. It is then a matter of how well it is controlled.
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#49 gnowellsct

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Posted 14 October 2020 - 09:34 AM

Reflectors are true apochromats. All lens systems have secondary color. It is then a matter of how well it is controlled.


Yes well color can become extremely well-controlled. To the point that it is undetectable to extremely sensitive imaging systems working over 30 and 60 minute imaging runs. That's well below the ability of the eye to see.

It's hard to get away from lenses when you drop in a wide field eyepiece you might be stacking 6 to 8 of them right in the focuser.

If refractors did not produce color, some more, and some a great deal less, there would be very little to talk about in this forum so there is that.
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#50 Traveler

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Posted 14 October 2020 - 10:03 AM

Reflectors are true apochromats. All lens systems have secondary color. It is then a matter of how well it is controlled.

Sorry,although i understand what you are trying to say, but apochromatic is a term used for a lenssystem only. Light is going through a glasssystem and therefore light will disperse more or less. A Reflector just reflects light and will only disperse by an eyepiece which has a glasssystems where light is going through....




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