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Equipment Discussions >> Refractors

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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: RodgerHouTex]
      #5287906 - 06/25/12 06:19 AM

Quote:


I know I'm a little late on this thread but this really isn't a valid test. The branches are at 50 feet. Telescopes are designed to focus light rays that are parallel (at infinity). Light rays coming from 50 feet are not parallel. Any telescope will perform poorly at such short range. Try this test with a triplet. I'm guessing you'll find similar shortcomings.




I have performed this test with many refractors. As Alan says, 50 feet is not particularly close, it's a good test and an apo doublet or triplet will be color free whereas a fast achromat (C.R. = 1.5) will show significant color and a slow achromat (CR=4) will show the chromatic aberration but it will be harder to see.

Jon


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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5287917 - 06/25/12 06:29 AM

Quote:

Quote:

I just don't understand the need to rigidly classify everything. It's a doublet. It uses different glass than a standard achromat. It has better color correction than a standard achromat, It's color correction is not as good, at least for imaging as an Apochromat. Semi-Apo or ED doublet are both pretty descriptive of what you get. As long as you don't get vendors trying to sell achros as ED doublets, or ED doublets as APOs, even a relatively uninformed buyer has some idea of what to expect when they buy one. It's at least as indicative of what you'll see at the eyepiece as the other "perfectly" defined terms. If you have a need to define everything to exact definitions, then you're right. You have to come up with a term for every glass type used in a scope.




Exactly




This seems to bring us back to the fact that there are ED refractors and then there are ED refractors. One has to be specific when drawing conclusions because they do perform differently. One cannot generalize that because this particular scope, a Vixen 102mm F/6.5 ED, has a certain level of chromatic aberration, that all ED refractors will perform in a similar manner.

When the Orion ED-80 hit the scene, the 80mm F/7.5 based on FPL-53, it was quite a shock because it provided what seemed to be perfect color correction. But previously, Orion had marketed an 80mm F/6.25 as an ED scope and I believe it used the same ED glass as the Pronto (FK-5?) which meant it was basically an achromat. Both "ED" scopes but quite different at the eyepiece.

Jon


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5288004 - 06/25/12 08:19 AM

Jon,
But Achromats none the less.

All,
So show of hands, who thinks we still have a definition issue? Do we all agree you can have ED glass in an Achromat and that ED isn't a new classification/design?

Scott,
I know you wanted to get back on track, however this is the core of the issue. Needing to "know what an ED refractor can and cannot do" is the result of confusion over this very issue.

Joe


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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5288022 - 06/25/12 08:36 AM

Quote:

All,
So show of hands, who thinks we still have a definition issue? Do we all agree you can have ED glass in an Achromat and that ED isn't a new classification/design?




The "ED Glass" used in the Pronto and others was FK-5 which was classed as ED but really offered no advantages in terms of color correction over an achromat. I think FK-5 is considered by most to be "fake" ED glass.

In general, if one goes to the trouble of using a "real" ED glass, (FPL-51 etc) then the color correction will be significantly better than that of a Crown-Flint Achromat.

There is a need for a name for scopes in this class, scopes that show some small amount of false color but are much better than an achromat of similar aperture and focal ratio.

These days such scopes are often called ED but scopes like the ED-80 seem to offer apo level color correction so it's a murky area.

Jon


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5288032 - 06/25/12 08:43 AM

OK, then why not the same classification for an achro designed at a long enough FL to eliminate color to the same level? IDK Jon, to me they are just great performing Achros. Why not call them all Apos and we can call the real Apo's SUPER. TEC's, AP's - They will be the SUPER class of Apo.

You'll all be please to know, I've tendered my resignation from this discussion. Jon it will be on your desk in the morning.

Best, Joe


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ken svp120
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5288037 - 06/25/12 08:46 AM

Joe, you know where my vote comes out. But I don't even think it comes down to a vote. It seems clear that a number of folks would like the designation. And I'm certain that marketing departments everywhere would love it.

As I said earlier, if you want a new type of refractor that somehow fills the middle ground between achromatic performance and apochromatic performance, all you neeed to do is define what level of performance needs to be met in order for a scope to be considered as achieving this new classification.

As yet - I have heard exactly zero people offer up that new definition. And no - simply saying ED is not a definition of performance. Until that happens any such designation is more than meaningless - it is a point of confusion that we need to continue taking our time to clarify so that folks have some hope of avoiding confusion.

Offer up a technical definition of what performance a scope must meet to be termed "semi-apo" or "ED" and I'm all ears. Until then its all pointless to discuss. I can take a piece of ED glass, mate it with a piece of clear plastic, and it suddenly is considered an ED scope and implies some level of performace to someone? What we have been offered so far is no more specific or informative than that.

So I would suggest to supporters of the semi-apo, or ED designation that rather than just using the term arbitrarily - lets get specific about a performance level that you're talking about and give this/these terms some sort of meaning that will relay something of value to their use.


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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5288052 - 06/25/12 08:54 AM

Quote:


OK, then why not the same classification for an achro designed at a long enough FL to eliminate color to the same level? IDK Jon, to me they are just great performing Achros. Why not call them all Apos and we can call the real Apo's SUPER.




apo is a definition, 3 color crossing... Doublets with good quality ED glass do have three color crossings though not all are necessarily in the visual range.

Even a 60mm F/15 shows false color on Venus, my Megrez II FD does not... A 60mm F/15 achromat performs as one would expect, the same 1 part in 2000 color correction, it is just less visible.

Jon


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watcher
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Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5288055 - 06/25/12 08:57 AM

I can get on board with that. Istar is doing basically that with their R30 lenses. While it's not the common ED glass used in ED doublets, it tells you what you're going to see color wise. R30=the same level of color as an achro with a 30% longer focal length. If you can make the same analogy for the glass you're using in your particular scope it would be a pretty good gauge of what you'll get. Providing the "standard" crown and flint achro is actually a standard that can be quantified.

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jmiele
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5288116 - 06/25/12 09:49 AM

Quote:

Quote:


OK, then why not the same classification for an achro designed at a long enough FL to eliminate color to the same level? IDK Jon, to me they are just great performing Achros. Why not call them all Apos and we can call the real Apo's SUPER.




apo is a definition, 3 color crossing... Doublets with good quality ED glass do have three color crossings though not all are necessarily in the visual range.

Even a 60mm F/15 shows false color on Venus, my Megrez II FD does not... A 60mm F/15 achromat performs as one would expect, the same 1 part in 2000 color correction, it is just less visible.

Jon




Jon, That really makes the point. "Not necessarily in the visual range." I will go to this great man's comments:
____________________________________________________________________________
"But any lens, be it a doublet, triplet, quad, air-spaced or Petzval, that has a peak visual null (~5550A - the green-yellow) with a Strehl ratio of .95 or better, coma corrected and is diffraction limited from C (red) to F (blue) with 1/4 wave OPD spherical or better, has good control of the violet g wavelength with no more than 1/2 wave OPD P-V spherical and optical spot sizes that concentrate the maximum amount of photons within the diffraction limit -- a result of the low spherical aberration, which can be seen with modern optical design programs, as the "spot rays" will be seen concentrated in the center of the spot, not evenly or worse, concentrated outside the center -- will satisfy the modern definition of "Apochromatism."

Lenses of this quality do not satisfy the Abbe definition, but for all intents and purposes, will be color free and will give extremely sharp and contrasty images." - Thomas M. Back
____________________________________________________________________________

He was ahead of his time in understanding where we were going with new designs and capabilities. He clearly sates however, they DO NOT meet the Abbe definition but that the would meet his modern day definition. I've never been 100% on board with his new definition because I felt it ignored the eye of the camera. If you are considering updating it based on new design and material capabilities, also consider new user requirements as well. But, as far and visual astronomy goes, I would say a could agree with his definition.

The entire essay is here: http://geogdata.csun.edu/~voltaire/tmb/definition.html

He does deal with most of what we are discussion here and breaks down the Abbe Apo definition. Further, he uses terms like "Semi-Apo and Super-Achro.. Not making much of a case for myself, am I.

Jon, don't process my resignation yet.

Joe

Edited by jmiele (06/25/12 09:55 AM)


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RodgerHouTex
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5288173 - 06/25/12 10:27 AM

I totally disagree that 50 feet is not close. My house is longer than 50 feet. Once again, if it's a valid test find an AP or TAK triplet and do it again. My guess is that they too will show color since all astronomical telescopes are designed to focus PARALLEL rays of light. These rays are NOT parallel. It cannot be concluded, based on this test, that doublets cannot be color free in focus.

Anyway the OP is trying to demonstrate that a doublet shows color. In focus I know of some that do not.


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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: RodgerHouTex]
      #5288208 - 06/25/12 10:51 AM

Quote:

I totally disagree that 50 feet is not close. My house is longer than 50 feet. Once again, if it's a valid test find an AP or TAK triplet and do it again. My guess is that they too will show color since all astronomical telescopes are designed to focus PARALLEL rays of light. These rays are NOT parallel. It cannot be concluded, based on this test, that doublets cannot be color free in focus.

Anyway the OP is trying to demonstrate that a doublet shows color. In focus I know of some that do not.




Roger:

As I said, I am not guessing here... I have performed this test with a number of scopes of varying configurations, achromats as well as ED doublets, apo triplets and the NP-101. My experience is that if the scope is color free at infinity, it will be color free at 50 feet and vice-verse. If it not color free at infinity, it will not be color free at 50 feet.

50 feet is about 23 focal lengths of this scope, it is in the far field.

Jon Isaacs


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gdd
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5288277 - 06/25/12 11:25 AM

I think the test shows a doublet will show color when out of focus. I suspect a triplet will also. The test should be repeated for a doublet, a triplet, and a reflector.

Gale


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: RodgerHouTex]
      #5288338 - 06/25/12 12:00 PM

Quote:


Anyway the OP is trying to demonstrate that a doublet shows color. In focus I know of some that do not.




I have to question whether you in fact looked at the original post. The whole point was that in focus the scope works fine and is very well corrected. That's what the moon pics were for: high power, low power, no color, except what is there due to refraction (the moon shows color to the naked eye, and can show red/blue rims just like Jupiter, Saturn etc. due to refraction).

I think the ED scope has been a real workhorse for me and after owning an achromat I consider it a BIG improvement.

Showing the tree shot was a way of demonstrating that *behind* and *in front* of perfect focus there is out of focus color. It is color like this, particularly in the blue wavelength, that frustrates the use of EDs in long exposure astrophotography.

Greg N

Greg N


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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5288370 - 06/25/12 12:18 PM

Greg

For A-p, the image is in focus so the out of focus performance is not an issue. The problem is that a camera looks at a much wider spectrum so that it sees the defocused IR and UV that the eye cannot see.

I am curious how much CA you see on Venus at 200x?

Jon


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Astropin
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Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5288525 - 06/25/12 02:32 PM

Quote:

Joe, you know where my vote comes out. But I don't even think it comes down to a vote. It seems clear that a number of folks would like the designation. And I'm certain that marketing departments everywhere would love it.

As I said earlier, if you want a new type of refractor that somehow fills the middle ground between achromatic performance and apochromatic performance, all you neeed to do is define what level of performance needs to be met in order for a scope to be considered as achieving this new classification.

As yet - I have heard exactly zero people offer up that new definition. And no - simply saying ED is not a definition of performance. Until that happens any such designation is more than meaningless - it is a point of confusion that we need to continue taking our time to clarify so that folks have some hope of avoiding confusion.

Offer up a technical definition of what performance a scope must meet to be termed "semi-apo" or "ED" and I'm all ears. Until then its all pointless to discuss. I can take a piece of ED glass, mate it with a piece of clear plastic, and it suddenly is considered an ED scope and implies some level of performace to someone? What we have been offered so far is no more specific or informative than that.

So I would suggest to supporters of the semi-apo, or ED designation that rather than just using the term arbitrarily - lets get specific about a performance level that you're talking about and give this/these terms some sort of meaning that will relay something of value to their use.




In a perfect world maybe this would be possible. I think their going to have to go with the materials used to manufacture the scope to designate it's class.

Example:
All scopes using flint glass ("Low dispersion") would be achromats.

All scopes using at least one ED element (in a doublet configuration) using greater than or equal to FPL-51 glass are "ED doublets"

All scopes using at least one ED element (in a triplet or quadruplet configuration) using greater than or equal to FPL-51 are Apo's.

NOW - I agree that performance can still vary wildly (so what's new?). You will end up will some well configured ED doubltes outperforming Apo triplets of poorer quality (so what's new?).

It will be up to the reviewers and owners to decide if the scope truly lives up to it's designation (so what's new?).

I understand that Apochromatic is a performance classification and you could very well end up with ED doublets at are Apo performers......so what new?

We will decide which designs succeed and which designs fail. The good designs will hold there value regardless of there designation (Apo or ED Dublet or Achro) and bad designs will tank in value and disappear........so what's new?

Edited by Astropin (06/25/12 04:35 PM)


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5288542 - 06/25/12 02:45 PM

Quote:

Greg

For A-p, the image is in focus so the out of focus performance is not an issue. The problem is that a camera looks at a much wider spectrum so that it sees the defocused IR and UV that the eye cannot see.

I am curious how much CA you see on Venus at 200x?

Jon




On the ED in focus there are still wavelengths that are out of focus that the camera picks up but not the eye. I think you know this...? This was a quick and dirty demo. If you went to the Vixen yahoo a few years ago there were astro ED pics and there were issues with color control FOR PHOTOGRAPHY.

200x is of course topping out the magnification on the Vixen. I seem to recall some out of focus color. On Mars when you if you out of focus either way you see a color haze, it actually helps adjust to correct focus (you shrink the haze to zero: achromats of course require a filter).

I remember checking the color correction against a Televue SDF and thought it better; I don't know if I have compared to an NP, there is only one of those in the club.

Incidentally the Stellarvue Lomo triplet apo that my friend just bought seems to me superior to my Vixen doublet, although in the applications to which they are both put (one is on a C11, the other a C14) they are functionally identical, when power is desired one uses the larger instrument. Daytime views in the Stellarvue-lomo seem to have an extra something that the Vixen lacks. I wish it weren't true but it is.

Greg N


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saemark30
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5288851 - 06/25/12 06:00 PM

That's an interesting way to show CA, but the only thing that matters is the view in focus.
Not all ED is the same, Synta 80 and 120 are outstanding.
But old Meade 6" f/9 ED shows a lot of color, can't remember if it was magenta or some greenish brown secondary.
Let's see what Markus can do with the Chinese glass.


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Refractor6
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Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5288958 - 06/25/12 07:14 PM

I agree its what the IN FOCUS image looks like that really matters. Who observes objects out of focus? {and the noted color differences on each side of focus}. Speaking of which even the much highly regarded flourite element FS-102 NSV shows different color in each diffraction image on each side of focus BUT the high contrast sharp in focus image is another story. They call that beauty an apo too by the way....

Agreed on the Synta 120 ED doublet...color rendition on Saturn last night at HARD FOCUS was simply lovely. Call it whatever you want since just like the fine FS-102 doublet made in the past this one works too for this observers eyeballs to get the job done at the ep.


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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: Refractor6]
      #5289030 - 06/25/12 07:50 PM

Quote:

Who observes objects out of focus?




Birdwatchers... My small, short focal length refractors double for birding.

For astronomy, the field is flat even though the objects are at wildly varying distances.

But I think Greg post this as a demonstration of what could be seen. A reflector will show no false color in or out of focus. The closest refractor I have seen to a reflector in this regard is my NP-101.

Jon


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sg80
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5289280 - 06/25/12 10:26 PM

Fk5 abbe number 70.40 might make a difference in long focal length Bk7 abbe number.64.17 I,m just reading and I don,t know if there,s other factors beside the abbe number.

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