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Which brands produce "true" Apochromatic Refractors?

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#51 mikeDnight

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Posted 14 October 2020 - 10:59 AM

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

Colour correction is only one small part of a refractors amazing qualities. So much so in fact that many refractor users would still choose a refractor over a reflector, even if there is some residual CA. And of course any telescope that uses an eyepiece refracts light and suffers from some degree of CA. The trouble with reflecting telescopes is that, with very few exceptions, they suffer from other equally destructive aberrations and design issues.


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#52 garret

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Posted 14 October 2020 - 11:28 AM

 

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.

Ad three or four with Paracorr.

 

On Topic, Astro Physics, TEC, Takahashi; Agema, CFF, Baader; LZOS, Televue they all design and build 95-99% color free refractors.

Plenty to choose from.


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#53 barbie

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Posted 14 October 2020 - 02:25 PM

Colour correction is only one small part of a refractors amazing qualities. So much so in fact that many refractor users would still choose a refractor over a reflector, even if there is some residual CA. And of course any telescope that uses an eyepiece refracts light and suffers from some degree of CA. The trouble with reflecting telescopes is that, with very few exceptions, they suffer from other equally destructive aberrations and design issues.

These are the tradeoffs one makes when choosing a type of telescope optical configuration. In optics and optical physics, given our current level of technology, there are no free lunches.


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#54 Mitrovarr

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Posted 14 October 2020 - 03:57 PM

I mean, I get that eyepieces and paracorrs and such have lenses, but reflecting telescopes and the vast majority of catadiotric designs are completely false color free by any reasonable standard.

(Assuming the eyepiece isn't either ancient or junk.)
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#55 Jon Isaacs

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Posted 14 October 2020 - 04:10 PM

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....

 

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


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#56 Jon Isaacs

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Posted 14 October 2020 - 04:14 PM

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

 

What I meant was that the longitudinal focus error with FPL-51 is roughly 2x that of FPL-53 for an otherwise "identical" design. It actually some less that 2x.

 

Jon


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#57 Mitrovarr

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Posted 14 October 2020 - 08:28 PM

Of course, it's good to remember that the color advantage of good glass or more lenses is often used to make the scope faster.

#58 R_Huntzberry

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Posted 14 October 2020 - 08:45 PM

What?  Three pages and no mention of the legendary Zeiss Jena APQ's? 

 

Apparently the new APQ Jena "Fluorite Quadruplet Polychromat" is about as APO as you can get.

 

 

Some info on the new designs:

 

 

https://apq.de/img/p...on_APQ_JENA.pdf

 

https://apq.de/en/index.htm

 

 

-Richard



#59 weis14

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Posted 14 October 2020 - 09:12 PM

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 am very tempted to pull the trigger on a CFF 160 f/6.5, but my finances won't be ready for a couple of years.  My fear is that they will be out of production by then.  I guess there is always the used market.


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#60 csauer52

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Posted 14 October 2020 - 09:15 PM

I am very tempted to pull the trigger on a CFF 160 f/6.5, but my finances won't be ready for a couple of years.  My fear is that they will be out of production by then.  I guess there is always the used market.

Or you could just get a TOA-150. ;)


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#61 weis14

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Posted 14 October 2020 - 09:17 PM

Or you could just get a TOA-150. wink.gif

I'm sure I could live with a TOA too.



#62 gnowellsct

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Posted 14 October 2020 - 09:23 PM

What I meant was that the longitudinal focus error with FPL-51 is roughly 2x that of FPL-53 for an otherwise "identical" design. It actually some less that 2x.

 

Jon

Catalin writes dismissively of longitudinal focus error as a thing of the past easily controlled with the right glasses.  Of greater concern, he said, is spherochromatism.  I believe the link is somewhere in this thread.



#63 noisejammer

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Posted 15 October 2020 - 08:27 AM

I'm sure I could live with a TOA too.

funnypost.gif  I've been forced to live with mine for 12 years... Sometimes life can be so hard. lol.gif


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#64 Jon Isaacs

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Posted 15 October 2020 - 01:50 PM

Catalin writes dismissively of longitudinal focus error as a thing of the past easily controlled with the right glasses.  Of greater concern, he said, is spherochromatism.  I believe the link is somewhere in this thread.

 

It starts with longitudinal focus error.  It's not easily controlled unless you have an unlimited budget.

 

Jon



#65 Daniel Mounsey

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Posted 16 October 2020 - 08:59 AM

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


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#66 daquad

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Posted 16 October 2020 - 10:37 AM

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

Yeah, that's why I don't have any.  Don't want to be bested by a telescope.


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#67 Scott in NC

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Posted 16 October 2020 - 10:49 AM

Yeah, that's why I don't have any.  Don't want to be bested by a telescope.

On the contrary, I at least know that my eyes are the weak link in the optics chain.  It’s the same way with me for golf.  I’ve acquired some nice clubs over the years (all off the used market).  I’m horrible at golf, but once again I know that my equipment is not to blame, lol! :grin:


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#68 Daniel Mounsey

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Posted 16 October 2020 - 12:20 PM

The obsession beginners have with color correction is as bad as a golfer who returns a set of golf clubs because they didn’t par every hole. 


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#69 Wildetelescope

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Posted 16 October 2020 - 02:49 PM

The obsession beginners have with color correction is as bad as a golfer who returns a set of golf clubs because they didn’t par every hole. 

 I think part of the challenge is that beginners come looking for information in places like CN and jump into the middle of one of these perennial conversations and do not have enough context to understand the relative differences in performance from a PRACTICAL perspective.  

 

If I, as a newbie. come into a conversation between two really experienced guys who spent the previous evening comparing a Tak with a TEC to see which has a better color correction. I am likely going to get a rather skewed perspective in regards color correction.  And that happens ALL the time here.  

 

That is one of the reasons I always like Jon Issacs posts, because he is always careful to provide context with respect to how he is using a scope and what he is looking at, as well as the occasional chart:-).  To the OP's question, Jon's point about the original definition of Apochromatism is really key(post #55 I think).  At the end of the day, I have come to the conclusion that If I like the view at the eyepiece and/or picture that took last night, then the optics of the scope are good enough:-) 

 

JMD


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#70 Mitrovarr

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Posted 16 October 2020 - 02:52 PM

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.
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#71 doctordub

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Posted 16 October 2020 - 04:36 PM

My definition of a visual Apo: Telescope can produce CA free view of Planet, bright star, or the Moon at .5mm exit pupil when objects altitude is above 60 degrees and at focus on axis, not before or after focus. Period.

CS

Jonathan


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#72 Suavi

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Posted 16 October 2020 - 05:23 PM

Which brands produce "true" Apochromatic Refractors?

 

Disclaimer: I used to be a 99.9% astroimaging dude for the past decade and consequently have limited experience as an astrovisual dude.

 

My eyes (I switch between eyes when observing) couldn't see evidence of CA on Jupiter nor Saturn (and some doubles+DSOs) over the past few weeks of careful and regular observations. Both planets are currently ideally positioned near zenith after sunset, allowing me to push magnification as hard as my eyes allow for with my 105mm f/6 oil spaced triplet, if only for the fun of doing so. Still no colour, and even at 500x (2.5mm Toe with 2x TV barlow) near zenith, the 5 brightest moons of Saturn were still obvious at this magnification, as was the Cassini division. Please note that currently my favourite magnification with this small refractor is 126x (with 5mm Delite).

 

Drawing from my imaging experience with this f/6 triplet there is a very slight focus shift between green/blue and red filters, but it is an order of magnitude smaller than with my previously owned and cherished 102mm ED f/7 doublet from a popular and affordable manufacturer.

 

Given my observations with this telescope from the past 4 years, and putting textbook definitions aside, to my eyes and to my camera, at this stage in my journey, I consider my 4" f/6 CFF to be a true APO.


Edited by Suavi, 16 October 2020 - 05:26 PM.

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#73 barbie

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Posted 16 October 2020 - 06:20 PM

I like Jonathan's(doctordub) definition!!  My 72mm ED doesn't show any false color visually on the moon, bright stars, or planets at high magnification, at focus, with a small exit pupil and at 60 deg. or higher altitude and it's only an ED doublet.  Every ED doublet I've had in the last 10 years has shown no false color, VISUALLY.  I only image the moon with my Nikon Telephoto lens and it contains 3 ED elements!!!  As far  as imaging anything else like planets is concerned, I've already been there and done that with a wide variety of refractors and other types of scopes in varying sizes so I don't feel the need to go down that slippery slope again, EVER!!

As for what type of ED glass it uses, I don't care as it performs like an Apochromat should and also has excellent spherical aberration correction with a textbook perfect star test and very finely figured/polished optics.  The star test(on a night of good sky conditions) tells me everything I need to know!!  I've had several excellent views through the little 72mm ED Skywatcher Evostar and am looking forward to next year's moon/planet and double star observing!!


Edited by barbie, 16 October 2020 - 06:34 PM.

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#74 SandyHouTex

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Posted 16 October 2020 - 06:50 PM

If you really want to know why different glasses matter when designing a doublet or triplet, get the book “Telescope Optics” by Rutten and van Venrooij.  Chapter 21.13 talks about designing doublets, and chapter 21.16 explains the triplet design process.

 

To sum it up for a doublet, you need to look at Fig. 21.14.  It shows why Fluorite (and FPL-53) are the best choice for controlling SA.  The second paragraph under the figure explains that you need two glasses with the smallest delta P on the y-axis (a horizontal line), and a maximum delta V on the x-axis.

 

With triplets, you need to look at Fig. 21.21.  To create the best triplet you need three glasses that create the widest triangle on the P-V diagram.

 

Simple.  Back in the 90s I used this book to design a great doublet using FPL-53.  Then I priced it.  $1000 for a 6 inch blank 1 inch thick.  Needless to say I decided to pass.


Edited by SandyHouTex, 16 October 2020 - 06:51 PM.

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#75 barbie

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Posted 16 October 2020 - 07:48 PM

I did the same thing Sandy. As a former glass pusher, I know about what Rutten and Vanvenrooij have to say on the subject and I still have, and refer to their book often!!


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