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Equipment Discussions >> Refractors

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Pinbout
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284229 - 06/22/12 05:51 PM Attachment (34 downloads)

Quote:

I guess it's a draw Alan.




A draw? I think he really loves drawing TV scopes in secret...


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CounterWeight
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Re: What an ED refractor can and cannot do new [Re: Pinbout]
      #5284418 - 06/22/12 08:32 PM

I think it's odd to say because the definition was not in place prior years, it can not be put in place now. Often it is by defining new types that progres can occur, at least in clarification - and hopefully understanding. But for that to happen there needs to be some tangible agreement on what in general the definition would apply to and mean - I think that is certainly possible. Within type (sticking to refractors), I think there is room for inbetween if folks want it, I'm certainly not opposed as it's accurate if used properly and does have a tangible meaning. I've recommended ED doublets to folks.

I can't agree with the definition of 'apo' being in flux because of the quality of A-P scopes or Tak. That is a bit fanboy for me. Maybe the quality evolved to the difinition, but the difinition is there regardless.


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jmiele
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Re: What an ED refractor can and cannot do new [Re: CounterWeight]
      #5284566 - 06/22/12 11:00 PM

Jim, To me, the biggest change in the Apo world came as the CCD part of the hobby expanded. As it did, makers looked for ways to expand color correction outside traditional visible wavelengths. That drove the search for exotic materials and new designs to meet new demands. Those demands were for a higher level of correction across more of the color spectrum and a flatter imaging field.

The traditional Apo definition did not call for this level and that is were the biggest change has occurred - in the user requirements. So I'd have to agree that the traditional Achro definition has grown closer in capability to the "traditional" Apo standard. For goodness sake, folks are even doing AP with Achro's today....and good work at that. That said, Apo capabilities have expanded beyond the original definition.

Ah well, I know what Jon Isaacs would say about now - "shut up, and look up Joe"

Best, Joe

Edited by jmiele (06/22/12 11:17 PM)


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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284607 - 06/22/12 11:34 PM Attachment (29 downloads)

Quote:

Jim, To me, the biggest change in the Apo world came as the CCD part of the hobby expanded. As it did, makers looked for ways to expand color correction outside traditional visible wavelengths. That drove the search for exotic materials and new designs to meet new demands. Those demands were for a higher level of correction across more of the color spectrum and a flatter imaging field.

The traditional Apo definition did not call for this level and that is were the biggest change has occurred - in the user requirements. So I'd have to agree that the traditional Achro definition has grown closer in capability to the "traditional" Apo standard. For goodness sake, folks are even doing AP with Achro's today....and good work at that. That said, Apo capabilities have expanded beyond the original definition.

Ah well, I know what Jon Issacs would say about now - "shut up, and look up Joe"

Best, Joe




Joe:

A few random thoughts/experiences looking:

The place to start when discussing apochromatic telescopes:

Roland Christen: Musings on APOs

The reality is that modern apos do not meet the original definition of an apo.

- An achromat is an achromat... today's have no better color correction than those of yesteryear and in fact, since they are generally faster than those of 25 and 50 years ago, they most often show more.

- Greg is discussing the Vixen 102mm F/6.5 ED scope. One cannot generalize the performance of this scope to other "ED" scopes because color correction depends on design, the glasses and scales with focal ratio and aperture. My suspicion is that the Vixen does not use either Fluorite nor FPL-53... the Vixen 102ED F/6.5 has been around a long time, Orion sold them back before the advent of the affordable scopes like the ED-80.
- Over the years, in my pursuit of photographs of birds, I have taken many photos that are very similar to Gregs, birds and branches against a bright sky background. The vast majority were taken with a Coolpix 4500, the last of the standard digiscoping line that included the 900, the 950, and the 995.

In my experience, an 80mm F/7 with an FPL-53 based objective can take such photos without showing color fringing. Better glass, smaller objective, slightly slower focal ratio...

With most scopes, it is the out of focus color is really the question, in Greg's photos, the branches that are in focus are most likely color free, fringing is most often seen in the parts of the image that are out of focus, at least that is what I have decided...

As an aside, as Greg says, these sorts of daylight tests are tough. My favorite is a telephone pole about 100 yards away with a blue sky behind it. Crank up the magnification far enough that your eye is not acting as an aperture mask, in a 4 inch, at least 50x, 100x is better. The yellow and purple fringes will show quite nicely if the color correction isn't there.

(Photo taken with a Astro-Tech 102ED and Coolpix 4500)

Jon

Edited by Jon Isaacs (06/22/12 11:37 PM)


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5284626 - 06/22/12 11:48 PM

Quote:

My suspicion is that the Vixen does not use either Fluorite nor FPL-53... the Vixen 102ED F/6.5 has been around a long time, Orion sold them back before the advent of the affordable scopes like the ED-80.
Jon




That astrotech is performing very nicely, I dare say.

I've never been able to figure out what Vixen used in the fast EDs which they abandoned but Roland said it was NOT "unobtanium." Though only thing I know is that Vixen got the word they couldn't use lead in their urban factory and poof all their scopes became f/8 (more or less).

The color correction is good, maybe as good as the FS128, I don't have a way to quantify it. They Vixen EDs used to be fairly easy to find on Astromart but not recently.

In part, I think that's because owners of the Vixen f/6.5s don't want to part with them at prices driven by Chinese EDs and apos. Going rate on a used Vixen f/6.5 used to be around $1200 back around 2002 they were available new for something like $1800 or $2000. (I have noted that the FS102s seem to have trended a bit up lately, for a while they were at $1500 now more like $1800).

It's good enough, unfortunately, that whenever I think i should get the TEC 110 or a used 140 I just have to think about something else and the feeling goes away.

I always find it dicey trying to focus a camera with an automatic focuser that wants to do its thing while you're doing your thing. I don't know how you did the eagle shot but it's great.

regards

Greg N


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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5284827 - 06/23/12 05:47 AM

Quote:



That astrotech is performing very nicely, I dare say.

I've never been able to figure out what Vixen used in the fast EDs which they abandoned but Roland said it was NOT "unobtanium." Though only thing I know is that Vixen got the word they couldn't use lead in their urban factory and poof all their scopes became f/8 (more or less).

The color correction is good, maybe as good as the FS128, I don't have a way to quantify it. They Vixen EDs used to be fairly easy to find on Astromart but not recently.




The Vixen 102mm F/6.5 was sold at the same time Orion was selling the somewhat longer 102mm Fluorites which have become something of a cult scope. It has always been my suspicion that they were based on FPL-51 or something similar.

How does it look on Venus?

Jon


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5284905 - 06/23/12 08:40 AM

Jon,

I responded to Greg's post as he was using it to make a point we were discussing in another thread. He used that scope to make a "general" point and I responded to it generally.

I've read Roland's essays. Folks love to quote Roland and walk away. Their opinions on things are based on "what Roland said", rather than having there own ideas. I'm not saying he is wrong - at all. But my original point was made in one of Roland's essays linked by Alan and I. We closed our discussion quoting that article supporting my point. I made my case with my own thoughts and words first. This started as my issue with ED terminology and folks being mislead into believing that ED glass ensured an instrument was an Apo...per Roland, and Joe... it doesn't.

As an aside, when you open quoting Roland like that it comes off like you believe I need to do some reading before we will have a common frame of reference to discuss this. That doesn't sit well with me. If your point was to be helpful - apologies.

Best, Joe


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Alan French
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284925 - 06/23/12 09:05 AM

The reality is that most telescopes using ED glass actually do have significantly better color correction than an achromat. The exception is not common, as far as I can tell. Certainly there would be a lot of complaints if people bought a telescope advertised as an "ED" and found it had the secondary color of an achromat - which is pretty obvious to an informed buyer. If it's advertised as an "ED achromat," that's a different story (buyer beware).

Clear skies, Alan


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5284929 - 06/23/12 09:09 AM

Alan, We are 100% in agreement.

Joe


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RodgerHouTex
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5285045 - 06/23/12 10:46 AM

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. In addition this is a fairly short focal length doublet. Even with fluorite you really shouldn't go below f/8 according to Rutten and Vanverooj. I also don't consider a Nikon Coolpix as a high quality imaging device. The chromaticism in your pics could be from the camera.

In the case of color crossings, I believe AP actually looks at how far a color is while in focus and how far it is from the focal plane. Hence the 0.004 and 0.006 figures mentioned earlier. This translates into spot diagrams.

I had a FS-102, have a FS-152, and a TSA-102. The FSs were/are color free in focus probably because they are f8. The TSA is a bit better, but not that much.

Edited by RodgerHouTex (06/23/12 11:06 AM)


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Alan French
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Re: What an ED refractor can and cannot do new [Re: RodgerHouTex]
      #5285241 - 06/23/12 12:47 PM

I suspect the behavior at distances other than infinity depends a bit on the design, but 50 feet is not especially close. I just played with a 4" f/15 doublet apochromat, and the results barely budged when the object distance was decreased from infinity to 50 feet.

At 10 feet, it definitely took a big hit, but the Strehl was still greater than 0.8 from 436 to 656nm.

Clear skies, Alan

Edited by Alan French (06/23/12 12:52 PM)


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5285247 - 06/23/12 12:50 PM

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

Joe


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PhilCo126
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5285465 - 06/23/12 03:19 PM

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


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Alan French
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Re: What an ED refractor can and cannot do *DELETED* new [Re: PhilCo126]
      #5285525 - 06/23/12 04:01 PM

Post deleted by Alan French

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jmiele
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5285577 - 06/23/12 04:28 PM

Haha. It's crazy Alan..

Phil thanks for the link.. Joe


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Astropin
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Re: What an ED refractor can and cannot do new [Re: PhilCo126]
      #5285791 - 06/23/12 07:10 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




The Link says "LD" glass (low dispersion) NOT ED glass (EXTRA low dispersion) are used in achromats.

Again, I think there are very few Achromatic scopes using any ED glass in their system. Most are using LD glass.

Three Types of refractors:

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

I do think there are some ED doublets that would qualify as Apo's.......at least visually.

Edited by Astropin (06/23/12 07:10 PM)


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Astropin]
      #5285806 - 06/23/12 07:29 PM

Yes but your item 2 doesn't really exist as a classification. These type scopes can fall into either 1 or 3 (your list).

Joe


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ken svp120
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Re: What an ED refractor can and cannot do new [Re: Astropin]
      #5286550 - 06/24/12 10:58 AM

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.


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watcher
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Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5286587 - 06/24/12 11:20 AM

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.

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jmiele
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Re: What an ED refractor can and cannot do new [Re: watcher]
      #5286628 - 06/24/12 11:58 AM

ED is a glass material classification.
Doublet identifies the number of elements utilized.
Apo/Achromatic identifies the design primary characteristics.

I understand your concern Joe, however those are the facts. It does not however, dictate the level of performance achieved. You can have a great Achro and a poor Apo. Quality of the materials, specific design, figure, all contribute to the final end product.

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

Vixen I'm told, uses ED glass in there NA 140 Neo-Achro. That's an appropriate way to look at this. Note: I have yet to verify the Vixen NA 140 ED material for myself. But it makes sense to use an exotic element to cut CA in such a short FL instrument. You see, that's what started all this....short FL scopes and AP. When all the achros were F12 and longer, no one needed a "semi" this and "like" that. But folks wanted short FL scopes to do AP with the Apo's are expensive to make properly.

Best, Joe


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