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Equipment Discussions >> Refractors

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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: PhilCo126]
      #5286632 - 06/24/12 12:01 PM

Quote:

Extra-low Dispersion glasses are used to reduce chromatic aberration most often used in achromatic doublet refractors:
http://en.wikipedia.org/wiki/Low_dispersion_glass




Probably written by someone on this thread. GN


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jmiele
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5286636 - 06/24/12 12:05 PM

No, it's been there for some time. Maybe Roland posted it after he wrote his essay confirm it's a correct statement.

I think Alan may be working on an update though.

Best, Joe


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: watcher]
      #5286642 - 06/24/12 12:07 PM

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


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gdd
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Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5286643 - 06/24/12 12:07 PM

Quote:

Quote:


Three Types of refractors:

1)Achromatic's
2)ED Doublets (Some would say "semi-apo's")
3)Apochromatic's (Triplets and Quadruplets)





Incorrect.

Achromat has performance defined. Aprochromat has performance defined.

ED Doublet has no defined performance criteria - it is almost a meaningless description let alone a type of refractor. If you are going to say this is a type of refractor based soley on the fact that there is one element of ED glass, then a Flourite is a type of refractor too - and so too will there be a new type of refractor for every glass combination being used. Shall we start making a list of all the new types of refractors that have just been created?

This makes no sense and serves only to confuse newcomers. If you want to have some type of refractor between an achro and an apo then at a minimum you need to define exactly what level of performance it takes to qualify as such - and simply using the terms semi-apo or ED does not in and of itself accomplish anything toward this end. Until we define a level of performance for this, the use of these terms has done and continues to do more harm than good.




Wouldnit it be more correct to say these are the three types of refractors:

1. Chromatic (singlet, 1 crossing)
2. Achromatic (doublet, 2 crossings)
3. Apochromatic (triplet, 3 crossings)

The corrected colors are the number of crossings.
The Low dispersion glass is used to reduce the color error of the unfocused colors.
As mentioned before, spherical abberation control was aslo included in the definitions.

Gale

Edited by gdd (06/24/12 12:09 PM)


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5286652 - 06/24/12 12:13 PM

Quote:

Even if it's not a perfect experiment, it give excellent examples of how color presents.

Joe




thanks. It's not a fair test of the Vixen as an overall instrument, but it does demo a lot of what goes on. Because the tree is close the three dimensional spacing of the branches shows how the different colors come to focus at different points (and that there even is a branch that shows no color).

I don't have a triplet to play with (some day maybe) but my hunch is that a good one would certainly do better *on what is certainly a high stress and "unfair" test* from the point of view of astronomical work. And yes as the objector noted there is an awful lot of extraneous glass in the system. I suspect that the lowest quality glass was in the camera: everything else was Vixen and Pentax. Greg N


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watcher
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5286735 - 06/24/12 01:11 PM

Quote:

There is NO semi Apo - there is a great performing Achro. That's it. You have fallen prey to marketing hype.





I think something like Super achro would work just as well, but I still can't see semi-APO as being marketing hype. It may not describe the design, but it does describe the view at the eyepiece of these short refractors. I like the term ED doublet a bit more, because it describes the design, thereby giving the the level of color error to be expected. I think too, that quality is, and has always been a separate issue. Every discussion I've ever heard or seen about one scope type vs another has started with "All else being equal".

I have 2 80mm refractors. One is an AT 80ED, and the other is a TMB 80SS. False color wise, I have to look VERY hard to see much difference at all. I'd be very happy if I bought the AT80 as a NOOB because it was described as Apo like, almost Apo, nearly Apo, partially Apo,........semi-Apo. I'm absolutely certain that a similar focal ratio 80mm achro would be much easier to see the difference in.


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orion61

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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5286756 - 06/24/12 01:30 PM

I'm wondering if folks who paid 5 grand for a scope aren't getting just a little nervous because another "type" of refractor is getting pretty darn close to their performance level at 1/5th the price.
I still think the best view of Saturn I've seen was through an old Celestron/Vixen C80F. WOW BURNED into my memory.
or possibly just because It belonged to someone else, and put my C102 Acro to shame.


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CounterWeight
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Re: What an ED refractor can and cannot do new [Re: watcher]
      #5286777 - 06/24/12 01:39 PM

I dunno - classification does exist. If we took example from geometry, there would then only be parallelogram and triangle? Square and right triangle are certainly useful and sufficient terms. ED isn't entirely as arbitrary as some posts here would make one think. Using similar criteria one could also say it's all marketing hype past doublet and triplet if they choose. If were going to split hairs that is where I would. I'll continue using ED doublet in my way, IMO the shoe fits.

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Refractor6
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Re: What an ED refractor can and cannot do new [Re: orion61]
      #5286795 - 06/24/12 01:49 PM

The way I look at it is observers who own {or have owned} a wide variety of achros, ED doublets or apos..whatever you want to call them.. etc.. can tell right away at the ep on select targets what they've got in front of them in the field.

All the talk, hype, marketing hype, data graphs and such means nothing compared to what is actually seen at the ep on select targets for this visual observer.

Whatever you have you'll know right away how it performs regardless of the label that is put on it based on years of experience at the eyepiece with both types of refractors and some for just visual use that fit quite nicely in the middle ground.

Anyhow that's how I "see" it......


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Scott BeithAdministrator
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Re: What an ED refractor can and cannot do new [Re: watcher]
      #5286802 - 06/24/12 01:53 PM

Quote:

Quote:

There is NO semi Apo - there is a great performing Achro. That's it. You have fallen prey to marketing hype.





I think something like Super achro would work just as well, but I still can't see semi-APO as being marketing hype. It may not describe the design, but it does describe the view at the eyepiece of these short refractors. I like the term ED doublet a bit more, because it describes the design, thereby giving the the level of color error to be expected. I think too, that quality is, and has always been a separate issue. Every discussion I've ever heard or seen about one scope type vs another has started with "All else being equal".

I have 2 80mm refractors. One is an AT 80ED, and the other is a TMB 80SS. False color wise, I have to look VERY hard to see much difference at all. I'd be very happy if I bought the AT80 as a NOOB because it was described as Apo like, almost Apo, nearly Apo, partially Apo,........semi-Apo. I'm absolutely certain that a similar focal ratio 80mm achro would be much easier to see the difference in.




Super Achromat has already been used and it is described as a refractor with four color crossings instead of an apo's three.


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Scott BeithAdministrator
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Re: What an ED refractor can and cannot do new [Re: gdd]
      #5286806 - 06/24/12 01:56 PM

Quote:



Wouldnit it be more correct to say these are the three types of refractors:

1. Chromatic (singlet, 1 crossing)
2. Achromatic (doublet, 2 crossings)
3. Apochromatic (triplet, 3 crossings)

The corrected colors are the number of crossings.
The Low dispersion glass is used to reduce the color error of the unfocused colors.
As mentioned before, spherical abberation control was aslo included in the definitions.

Gale




Apochromatacism is a performance measure, not a doublet, triplet, or quadruplet design issue.
I understand where you are going with it, but design and performance are different animals.


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Uggbits
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Re: What an ED refractor can and cannot do new [Re: Scott Beith]
      #5286928 - 06/24/12 03:01 PM

Sorry to stray away from the definitional argument, but the original purpose of this thread appears to have been an attempt to educate people on the "limitations" of an ED doublet. As myself and anouther poster have pointed out the test itself is in serious question not just for the distance to target, but if it was conducted with the afocal method. If it was this means that the final image is then limited by the quality of both the eyepiece, and the camera lens. I looked up the camera lens and it appears to have chromatic aberration issues meaning that the OP hasn't necessarily given evidence that the false colour is originating in the doublet. Even my dslr with a lens designed to control CA and a much shorter focal length can display CA against back illuminated tree-branches.

Why this is important to be brought up, is because you could take an AP 130, or other scope with excellent correction, do the same test with the same methodology, and wind up with similar results just at a higher magnification. In either case it isn't indicative of the scope in question, but rather the limitations of the imaging system and test methodology.

As I mentioned before deep-sky photography is conducted at prime focus, so a test to display the limitations of a design for that process should be conducted under similar conditions.The OP mentioned that it the ED Doublet could be suitable for planetary work as it is a shorter exposure, but made the recommendation for a triplet for deep-sky work. Planetary photography can use the afocal method, but once again you are potentially misrepresenting the merits of the optic in question by introducing extrinsic error through the other sources. Even if it is a viable method unless the telescope is the limit of the test it is hard to make a recommendation based on it.

Arguing of the definition of a type of telescope is fine, but the original theme of the thread seemed to be about performance, and empirical performance at that. I call them "ED Doublets" since it is easy. As others have pointed out some have fantastic performance (Ie why the EON 120 vs ES 127 debate keeps coming up) while others are more on the entry level side, but the one thing I do agree with is that in terms of sheer performance they do not fit a tradition definition of "achro." You could point out that most triplet "Apo's" don't fit the traditional definition of an Aprochromat either, so it may be that the standard of Apochromatism has or is falling, while the standard of Achromatism is rising (with each side having traditionally accurate offerings, Ie. Modern AP triplets, or Tasco doublets)leading to the issues in defining a field that is more closely contested than 40 years ago.


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Astropin
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5286932 - 06/24/12 03:02 PM

Quote:

Quote:

Extra-low Dispersion glasses are used to reduce chromatic aberration most often used in achromatic doublet refractors:
http://en.wikipedia.org/wiki/Low_dispersion_glass




Probably written by someone on this thread. GN




But it DOES NOT SAY THAT. It says "Low Dispersion" Not "Extra Low Dispersion".....and there is a huge difference between the two.

Call them what you want but to me a scope using an ED element is not an Achro. It might not be an Apo either.

Hence the need for a new designation - ED Doublet, Semi-Apo, Neo Achro......something. Granted I think they (the manufacturers) should pick one. My preference would be for ED doublet....which seems to be the most prevalent anyway.

ED doublets are different from standard achromatic scopes using standard "Low Dispersion" glass.... and as such need to be differentiated in some way.


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BKBrown
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5286959 - 06/24/12 03:21 PM

This is an interesting discussion; my take is that we need to distinguish between design and performance. Several online dictionaries define the noun apochromatic objective/lens as: "a lens, consisting of three or more elements of different types of glass, that is designed to bring light of three colors to the same focal point, thus reducing its chromatic aberration" (Harper Collins here, for instance), these definitions may also add correction for spherical aberrations at two wavelengths. However, the definitions for the adjective apochromatic tend to state: "Corrected for both chromatic and spherical aberration, as a lens." (American Heritage Dictionary) or "apochromatic, adj. (Physics / General Physics) (of a lens) almost free from spherical and chromatic aberration" (Collins English Dictionary, unabridged). While they don't fit the dictionary definition of an apochromatic objective or lens, an awful lot of ED doublets do meet the criteria for the adjective. So are we talking optical design or performance. I have a TEC 140 that punches all of the buttons here, but I also own an extremely well corrected ED doublet (a Skywatcher 100ED f/9) that qualifies, being virtually completely color free in focus with no detectable SA, for the adjective...

Clear Skies,
Brian

Edited by BKBrown (06/24/12 03:24 PM)


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XyrcesFenol
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Re: What an ED refractor can and cannot do new [Re: BKBrown]
      #5287070 - 06/24/12 04:53 PM

Maybe it would help if manufacturers could present more information on the performance, such as Star tests and spot diagrams as samples below?

Sample Telescope service Triplet APO 90-600mm Carbon fibre

Sample APM Astrograph FPL-53 Triplet Apo 80 F/4.7

Is this helpful, or should we want other measurements?


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jmiele
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Re: What an ED refractor can and cannot do new [Re: BKBrown]
      #5287076 - 06/24/12 04:55 PM

That Brian, is an interesting approach. I will have to give it more thought. I will say however, that my posts (I think) have been clear that I was not considering performance as the ultimate deciding factor. Back to the - great Achro, poor Apo concept. Design does not guarantee performance, nor does material.

I think it's fine that folks see this as a grey area, however deciding that a good performing Achromatic instrument requires it's own or new categorization is not the answer.

To your earlier point...if we consider that the Apo/Achro terminology is a measure of performance, then the qualifying instrument would just be called an Apo. If we consider the terminology to identify and define design elements, then the instrument in question would be an Achro. At no point however, would I see the need to call a poor performing Apo a semi Apo, nor would I call a high performance Achro an Apo or semi Apo. I'm sorry, it's just ridiculous to me.

The entire thread has been interesting if nothing else. Many good points made. I will say your approach is better than any of us using a Wiki to be considered the defacto answer to anything. While I might consult it before doing my own car brakes, I'm certainly not using a surgeon who consults it.

Best, Joe


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Alan French
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5287302 - 06/24/12 07:34 PM

Quote:

Quote:

Extra-low Dispersion glasses are used to reduce chromatic aberration most often used in achromatic doublet refractors:
http://en.wikipedia.org/wiki/Low_dispersion_glass




Probably written by someone on this thread. GN




On first glance, I did not read the article carefully. As someone else pointed out, it is perfectly correct. The low dispersion glass referenced is crown glass, which is indeed used in achromats, and results in better color correction than a singlet.

Clear skies, Alan


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5287399 - 06/24/12 08:48 PM

Quote:

Quote:

Quote:

Extra-low Dispersion glasses are used to reduce chromatic aberration most often used in achromatic doublet refractors:
http://en.wikipedia.org/wiki/Low_dispersion_glass




Probably written by someone on this thread. GN




On first glance, I did not read the article carefully. As someone else pointed out, it is perfectly correct. The low dispersion glass referenced is crown glass, which is indeed used in achromats, and results in better color correction than a singlet.

Clear skies, Alan




Well yes of course. Because achromats (without color) were a huge step up from whatever junk it was they were using when they were making sixty foot telescopes.

Since achromats pioneered the two glass approach there is a sense in which all modern refractors are achromats and we should just leave it at that.

greg n

Greg N


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5287404 - 06/24/12 08:51 PM

I note that the people most insistent about color correction as being apo and then everything else being achro are, oddly enough, seeing in black and white.

On the coolpix: The colors in the pictures could be seen with the eye, and note that the same coolpix did the pictures of the moon which came out OK. As I have said, the exercise demonstrates certain principles; it does not assign a place to Vixen in the hierarchy of glass greatness.

Greg N


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Scott BeithAdministrator
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Re: What an ED refractor can and cannot do new [Re: Uggbits]
      #5287506 - 06/24/12 09:55 PM

Quote:

Sorry to stray away from the definitional argument, but the original purpose of this thread appears to have been an attempt to educate people on the "limitations" of an ED doublet. As myself and anouther poster have pointed out the test itself is in serious question not just for the distance to target, but if it was conducted with the afocal method. If it was this means that the final image is then limited by the quality of both the eyepiece, and the camera lens. I looked up the camera lens and it appears to have chromatic aberration issues meaning that the OP hasn't necessarily given evidence that the false colour is originating in the doublet. Even my dslr with a lens designed to control CA and a much shorter focal length can display CA against back illuminated tree-branches.

Why this is important to be brought up, is because you could take an AP 130, or other scope with excellent correction, do the same test with the same methodology, and wind up with similar results just at a higher magnification. In either case it isn't indicative of the scope in question, but rather the limitations of the imaging system and test methodology.

As I mentioned before deep-sky photography is conducted at prime focus, so a test to display the limitations of a design for that process should be conducted under similar conditions.The OP mentioned that it the ED Doublet could be suitable for planetary work as it is a shorter exposure, but made the recommendation for a triplet for deep-sky work. Planetary photography can use the afocal method, but once again you are potentially misrepresenting the merits of the optic in question by introducing extrinsic error through the other sources. Even if it is a viable method unless the telescope is the limit of the test it is hard to make a recommendation based on it.

Arguing of the definition of a type of telescope is fine, but the original theme of the thread seemed to be about performance, and empirical performance at that. I call them "ED Doublets" since it is easy. As others have pointed out some have fantastic performance (Ie why the EON 120 vs ES 127 debate keeps coming up) while others are more on the entry level side, but the one thing I do agree with is that in terms of sheer performance they do not fit a tradition definition of "achro." You could point out that most triplet "Apo's" don't fit the traditional definition of an Aprochromat either, so it may be that the standard of Apochromatism has or is falling, while the standard of Achromatism is rising (with each side having traditionally accurate offerings, Ie. Modern AP triplets, or Tasco doublets)leading to the issues in defining a field that is more closely contested than 40 years ago.




Thanks for getting us back on track!


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