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Equipment Discussions >> Refractors

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jmiele
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Re: What an ED refractor can and cannot do new [Re: Eddgie]
      #5283976 - 06/22/12 02:26 PM

Eddie, I agree with the concept of an evolving Apo definition. Time and materials have created more possibilities.

Joe


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5283984 - 06/22/12 02:32 PM

Quote:

Greg - nice images and interesting write up.

However,"ED" is not a type of refractor. I understand that many manufacturers/resellers brand their scopes with the "ED" designation and I think that has begun to confuse people. There is no such thing as an ED scope. There IS ED glass. And their are both achromats and apochromats that make use of an ED element. But "ED refractor" means nothing other than that it uses one piece of this type of glass - and the level of correction between all available scopes using ED and carrying that label on the tube varies quite a bit.

Don't get me wrong - I think you put up a nice post, it is informative, and I appreciate seeing it. I just hate seeing scopes referred to as an ED refractor when that term says nothing at all about how well the scope will perform.

Anyway - just my two cents.

Good job, nice post.




The problem is that no one has come up with a name for an ED glass doublet that is generally recognized except ED glass doublet. There is in fact a need for a designation that describes in-between triplet apo and standard achromat.

However, since even triplet apos vary quite a bit in quality, and some of the higher end doublets might well be better than some of the lower end triplets, the terminological issue is far worse than the debate over ED suggests. We do not resolve anything by eliminating ED and dichotomizing into triplet apos and achromats, for as I have indicated, not all triplets are equal.

I find it interesting that even though there apparently is no such thing as an ED doublet everyone knows EXACTLY what I am talking about. If I were going to sell my scope on astromart I would list it as an ED doublet. That would not only identify me in the market it would also help comfort the buyer that I had delivered what he wanted when he saw the big red letters "ED" on the dew shield. They must be four inches high.

If one were to dig down a bit deeper into this issue, my suggestion is not that ED is terribly inaccurate as a descriptor but that there have been a profusion of mostly (or entirely, I don't know) Chinese scopes that have done much to muddle performance criteria across all categories.

If it is a no-name ED from the mainland that's one thing. If it is a Vixen it likely is quite another. But it's the same thing in eyepieces. There's the spec (20mm 80 degrees), and then there's made-in-Japan (or Taiwan: Taiwan is honorary Japan). In the current state of things there probably is less to be gained in criticizing the label "ED Doublet" than there is in pointing out to folks that if you get a fluorite doublet from Tak or an ED doublet from Vixen you're likely getting more optical accuracy out of the doublet design than many of the other doublet options out there at half the price. But as a term ED doublet is probably at least as useful, as a practical matter, as triplet apo. Once you want to know more you have two choices: you can try to get the glass spec (but there is much more than just the glass to a good scope) or you can go by manufacturer's rep. Most of us know who the top names are, but it helps to put the list up from time to time (as I did in the other thread) so that newcomers can explore the other options.

I note that Vixen is currently selling a

"ED Apochromatic Quad Element Refractor" for which they put up a lens diagram and spot diagrams. They do *not* put ED in the name.

They've also got a an FPL 53 ED doublet which they call and ED115. Andthey put the glass in the spec sheet. They *do* put ED in the name. They *do not* provide spot diagrams for that one. So the term is not going away soon. I would interpret it to mean that the color correction is not as tight in the ED115 as in the AD103S which looks to be a triplet apo with a field flattener in the rear.

But I'm not sure that we'd be any better off if ED as a term did go away.

Greg N


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ken svp120
Carpal Tunnel


Reged: 12/19/04

Loc: Ohio
Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5284059 - 06/22/12 03:34 PM

Quote:

There is in fact a need for a designation that describes in-between triplet apo and standard achromat.





I'm not positive but I think I disagree.

As far as I know, the primary distinction between achromat and apochromat is that achromats bring two colors of light to focus at the same point and apochromats bring three to focus at the same point (as well as further control of other abberations).

It is impossible to bring 2 1/2 colors to focus and thereby be "semi-apo" or to say that somehow "ED" means this middle ground.

How are you going to define and set performance criteria? What specific level of correction for which specific abberations are you going to establish that a scope must meet in order to be "semi-apo" or "ED"?

If as you suggest we are going to establish a new classification for a refractor, then you must set a specific quantifiable definition before that classification can possibly have any meaning. Short of that its simply arbitrary. And "ED" doesn't define anything about performance or level of correction.

I'm not sure if I agree that even needs to be this designation. What's wrong with saying you have a well corrected achromat?

Maybe someone can better explain this to me...


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gnowellsct
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Reged: 06/24/09

Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5284071 - 06/22/12 03:47 PM

Quote:

Quote:

There is in fact a need for a designation that describes in-between triplet apo and standard achromat.





I'm not positive but I think I disagree.

As far as I know, the primary distinction between achromat and apochromat is that achromats bring two colors of light to focus at the same point and apochromats bring three to focus at the same point (as well as further control of other abberations).

It is impossible to bring 2 1/2 colors to focus and thereby be "semi-apo" or to say that somehow "ED" means this middle ground.

How are you going to define and set performance criteria? What specific level of correction for which specific abberations are you going to establish that a scope must meet in order to be "semi-apo" or "ED"?

If as you suggest we are going to establish a new classification for a refractor, then you must set a specific quantifiable definition before that classification can possibly have any meaning. Short of that its simply arbitrary. And "ED" doesn't define anything about performance or level of correction.

I'm not sure if I agree that even needs to be this designation. What's wrong with saying you have a well corrected achromat?

Maybe someone can better explain this to me...




This doesn't help either. Obviously a standard achromat which does two out of three has a good deal more color issues than an ED which "only" does two out of three. For that matter, the apos which purportedly do three out of three aren't all doing the same thing either.

One could pass a law requiring spot diagram decals to be on every refractor. Gre gN


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Alan French
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Reged: 01/28/05

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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284076 - 06/22/12 03:53 PM

Quote:

Alan I'm aware of the text. I was referring to scope like this using ED in the name and model number.

One other note. What the eye and camera see is different. So your virtual color free achro may be god awful in the eyes of the CCD...

Joe




Joe,

If the use of "ED" in the description means it uses a true ED glass, it is going to have far less secondary color than any achromat.

An archromat, by definition, unless it is a very small aperture or really long focal length, is going to have some visible secondary color. Even a 3" f/15 shows a bit.

Yes, when it comes to CCDs, you definitely need far less chromatic variation of focus.

You may be trying to clarify things, but by calling an ED scope an achromat you are simply muddling the terminology even more.

Clear skies, Alan


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jmiele
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5284085 - 06/22/12 03:59 PM

The reason for the confusion is the attempt to define the undefinable. An Apo brings the 3 together and an Achro only 2. While there are varying degrees of performance for both Apo and Achro, there remain only two definitions. The reason for price differences within the Apo and Achro world are because of these degrees of variation. On top of it all - the materials used guarantee absolutely nothing. The design and figure coupled with materials results in the quality (good, better, best) of the end product. Process and controls used to create these lenses is what determines the consistency of that product. IOW, good material and good design do not guarantee the products quality or capability.

Makers Like TEC and AP have good designs, materials and process and produce consistently high quality Apo's.... Wait for it..... Companies like D&G do the same with Achros. At the end of the day each has its capabilities but they are either Apo's or Achro's... Thats it.. We need no further definition.

Joe


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ken svp120
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Reged: 12/19/04

Loc: Ohio
Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284092 - 06/22/12 04:08 PM

Joe - precisely!

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ken svp120
Carpal Tunnel


Reged: 12/19/04

Loc: Ohio
Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5284100 - 06/22/12 04:11 PM

Quote:


One could pass a law requiring spot diagram decals to be on every refractor.




Man after my own heart! I know you are saying this in jest, but at least if you were to have objective information included with your scope then you would know something for certain about the performance of what you are buying.


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5284103 - 06/22/12 04:13 PM

Here is where the mud comes from Alan...

http://www.vixenoptics.com/refractors/ed100sf.htm

ED Achro... It's confuses folks that don't fully understand what they are getting. Sorry but I've no intension of backing off on the point. There are good makers and misleading makers. Good and bad scopes. I'm not BTW saying Vixen is bad. Sorry but this scope wasn't in the Berry text.

There are 100's more like this. You miss the point...that a 200 foot long achro is still and achro.. And again....that's not bad - it just is.


Best, Joe


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Alan French
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Reged: 01/28/05

Loc: Upstate NY
Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5284105 - 06/22/12 04:15 PM

If the manufacturers provided a graph of the chromatic focal variation, then it would be fairly easy to compare different refractors. Even the total variation from C to F (for visual) or r to h (for astrophotography) would be valuable.

Clear skies, Alan


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284109 - 06/22/12 04:18 PM

Quote:

Here is where the mud comes from Alan...

http://www.vixenoptics.com/refractors/ed100sf.htm

ED Achro... It's confuses folks that don't fully understand what they are getting. Sorry but I've no intension of backing off on the point. There are good makers and misleading makers. Good and bad scopes. I'm not BTW saying Vixen is bad. Sorry but this scope wasn't in the Berry text.

There are 100's more like this. You miss the point...that a 200 foot long achro is still and achro.. And again....that's not bad - it just is.


Best, Joe




Actually as far as advertising goes that ain't bad. They're specifically mentioning the f/9 ratio as a component of the color correction.

If those photos are real they're better than other ED pics I've seen.

Greg N


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5284115 - 06/22/12 04:20 PM

Quote:

Quote:


One could pass a law requiring spot diagram decals to be on every refractor.




Man after my own heart! I know you are saying this in jest, but at least if you were to have objective information included with your scope then you would know something for certain about the performance of what you are buying.




Well the law would have to include all telescopes. That would be something of a chore, I imagine. GN


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jmiele
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5284116 - 06/22/12 04:21 PM

Yes, that is not a misleading ad at all. I just wanted to show a prominent maker using ED in the model number, an ED glass element and identifying it as an achro..

Joe

Edited by jmiele (06/22/12 04:21 PM)


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Astropin
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Reged: 08/30/07

Loc: Michigan
Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284119 - 06/22/12 04:23 PM

But isn't the primary difference between an Achro and an ED doublet the fact that an ED doublet uses "Extra-low Dispersion" glass while achros do not?

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Alan French
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Reged: 01/28/05

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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284144 - 06/22/12 04:43 PM

Quote:

Yes, that is not a misleading ad at all. I just wanted to show a prominent maker using ED in the model number, an ED glass element and identifying it as an achro..

Joe




Joe,

I don't see the word "achormat" anywhere on that web page, and "Edit/Find" didn't either. So I am a bit confused now.

Clear skies, Alan


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Astropin
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Reged: 08/30/07

Loc: Michigan
Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5284185 - 06/22/12 05:12 PM

Quote:

Quote:

Yes, that is not a misleading ad at all. I just wanted to show a prominent maker using ED in the model number, an ED glass element and identifying it as an achro..

Joe




Joe,

I don't see the word "achormat" anywhere on that web page, and "Edit/Find" didn't either. So I am a bit confused now.

Clear skies, Alan




The Page I'm seeing say's "Vixen's apochromatic refractor has an extra-low dispersion (ED) lens"


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5284194 - 06/22/12 05:19 PM

Your right Alan, I'm loosing it.
Try this Skywatcher:
http://www.australiantelescopes.com.au/100mm-ed-achromatic-telescope-gold-tube-ota-f-6-skywatcher.html

Just quick googles and they pop up.
Joe


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jmiele
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284197 - 06/22/12 05:22 PM

Are you going to tell me they don't say ED glass? How's the novice to know that? You won't concede the confusion is from the makers? You don't see how this can be misleading?

Joe


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Alan French
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Reged: 01/28/05

Loc: Upstate NY
Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5284200 - 06/22/12 05:28 PM

Quote:

Your right Alan, I'm loosing it.
Try this Skywatcher:
http://www.australiantelescopes.com.au/100mm-ed-achromatic-telescope-gold-tube-ota-f-6-skywatcher.html

Just quick googles and they pop up.
Joe




Well, it's apparently not impossible.

But it looks pretty dumb in an advertisement.

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]
      #5284205 - 06/22/12 05:30 PM

That's so funny I had that link copied to paste. lol
http://geogdata.csun.edu/~voltaire/roland/ed.html
I guess it's a draw Alan.

Best, Joe


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