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Equipment Discussions >> Refractors

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gnowellsct
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What an ED refractor can and cannot do
      #5283565 - 06/22/12 10:09 AM Attachment (394 downloads)

You can get a pretty good photo without color. (This is with a coolpix 995) And that's pretty much what teh moon will look like in an ED. Note that a little bit of blue (and red) fringe is not a system optics effect. It is refraction, and is actually a true color as far as the optical system goes. The moon was not high when this was taken.

Edited by gnowellsct (06/22/12 10:11 AM)


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283577 - 06/22/12 10:17 AM Attachment (254 downloads)

Hi magnification exaggerates color effects, but here you can see that at higher magnification this scope is doing pretty well. Again the blue fringe is "real" as far as the scope system is concerned. These are of course short exposures, and I'm shooting through an eyepiece, so the image dims out with magnification.

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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283587 - 06/22/12 10:24 AM Attachment (329 downloads)

The moon is, of course, one of the most brutal optical tests in the sky, with the exception, I think, of Venus and possibly Sirius. Now then let's take a closer look at what this color is all about. You wanna see some of that color?! Here it is:

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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283594 - 06/22/12 10:30 AM

Now let's take a look at that picture carefully. Branches with bright light behind them (whether the moon, as here, or a bright blue sky in daytime) are as brutal, or more brutal, a test than the moon and Venus. Try your binoculars sometime. The branches are at a distance of about fifty feet from the telescope. Here are some things to notice.

1. The branch with the arrow pointing to it is pretty good! It's the branch closest to the scopes focus point.
2. Other branches are lime green and purple! What's happening here is that the tree is a three dimensional bundle of branches. Some of the branches are in front of the focus point, and some of the branches are behind the focus point. Depending on their position they are revealing out of focus color.
3. A true top end triplet apo would do a better job with this test. Maybe perfect, but remembering that we are at very short distance rather than infinite focus maybe not so perfect. But certainly a triplet would do better!
4. But there's even more....


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283610 - 06/22/12 10:37 AM Attachment (174 downloads)

If we dim down the light source by moving a little bit away from the moon, the branches lose their color regardless of whether they are in or out of focus. Remember this is shot at night so we are not *real far* from the backlight just a bit to one side. This is still a pretty bright part of the sky. And since I had no shutter control on this camera (in any of the pictures) it is messing with the shutter speed and f/stop pretty much as it sees fit. So we're getting a longer exposure here.

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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283612 - 06/22/12 10:42 AM

So, to conclude, this sequence of pics shows a lot about what EDs can and cannot do. They can render color free images at fast exposures, but much of what we want to do in astronomy requires slow exposures. It's particularly the case that people using four inch refractors like this one (Vixen f/6.5 ED102SS) are usually going after larger deep sky objects that require longer exposure times. Out of focus color will begin to show on the brighter stars, usually as blue or purple haze. So they are really better advised to pop the extra $3k for a genuine triplet apochromatic.

In focus the color correction of an ED doublet will be excellent. The human eye does not care about the wavelengths that a camera will pick up.

I was just goofing around with a camera for a few days and make no pretense about being an astrophotographer. All the pics were shot through XW eyepieces and the camera optics were what they were, but it was an ordinary camera for general purpose photography.

regards
Greg N


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jmiele
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283620 - 06/22/12 10:52 AM

Greg, Yes - but that ED scope is still an Achromatic instrument. While better in many respects to other Achro's, it's still an Achro. Many folks have fallen into the trap of believing an ED scope is color corrected.

None of what I'm saying means that this and others like it aren't wonderful instruments. They're just not Apo's.

Joe


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5283633 - 06/22/12 10:58 AM

Quote:

Greg, Yes - but that ED scope is still an Achromatic instrument. While better in many respects to other Achro's, it's still an Achro. Many folks have fallen into the trap of believing an ED scope is color corrected.

None of what I'm saying means that this and others like it aren't wonderful instruments. They're just not Apo's.

Joe




Joe, they're not triplet apos. I will even agree they are not apos. But they they're not achromats. Looking through an achromat is a very different experience, and to say that anything not an apo is an achromat would suggest for example that one might as well buy a $500 achromat as a $2k ED. As a general rule, there is value--measured in color correction--to be had from getting the visually well corrected instrument. GN


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Binojunky
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283664 - 06/22/12 11:19 AM

Then how come if a ED doublet isn,t an APO, then a TeleVue doublet is called an Apo??, to be honest myself I find this constant argument tireing, no offence to the posters intended, all scope designs have compromises, be it cooldown time, narrow fields of view,defraction spikes etc, some of my most memorable views have been through well made achromats and in the same breath I seen some very mediocre APO,s. DA.

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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: Binojunky]
      #5283681 - 06/22/12 11:34 AM

Quote:

Then how come if a ED doublet isn,t an APO, then a TeleVue doublet is called an Apo??, to be honest myself I find this constant argument tireing, no offence to the posters intended, all scope designs have compromises, be it cooldown time, narrow fields of view,defraction spikes etc, some of my most memorable views have been through well made achromats and in the same breath I seen some very mediocre APO,s. DA.




Actually I wasn't trying to engage the definitional issue. I was trying to clarify what an ED (represented by the Vixen ED102SS f/6.5) could do. GN


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jmiele
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283734 - 06/22/12 12:15 PM

Greg, I see your point as to the value of what I would call "improved" achromatic performance. Matching your astronomical goals and false color sensitivity to an instrument means not having to spend a boatload of money to accomplish your objective(s). We are in agreement - I think.

Joe


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Alan French
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5283773 - 06/22/12 12:37 PM

Quote:

Greg, Yes - but that ED scope is still an Achromatic instrument. While better in many respects to other Achro's, it's still an Achro. Many folks have fallen into the trap of believing an ED scope is color corrected.

None of what I'm saying means that this and others like it aren't wonderful instruments. They're just not Apo's.

Joe




Joe,

No, a scope with ED (Extra-low dispersion) glass is not an achromat. An achromat is a doublet with vanilla flint and crown glasses and a chromatic focal variation of about 1 part in 1800. That is, the focal length from C to F will vary by about 0.05" in an achromat of 90" focal length.

Unless they're not using a true ED glass, an ED doublet is going to have considerably better color correction than an achromat. If the aperture is small enough, or the focal length is long enough, such a lens can be essentially color free.

I suggest picking up a copy of Smith, Ceragioli, and Berry's "Telescopes, Eyepieces, and Astrographs."

Clear skies, Alan

Edited by Alan French (06/22/12 12:38 PM)


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simpleisbetter
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5283788 - 06/22/12 12:46 PM

Very good summary and examples Greg. Also very accurate and faithful to what I was through my old 102 f/7 ED. Not true APO and not one I'd choose if I were going to do AP work. But I always felt these scopes excelled for visual work and especially their quick cooldown times for a quality grab-n-go. Also their cost at the time they were on the market made them a good choice for visual observers. Thanks.

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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: simpleisbetter]
      #5283855 - 06/22/12 01:21 PM

Quote:

Very good summary and examples Greg. Also very accurate and faithful to what I was through my old 102 f/7 ED. Not true APO and not one I'd choose if I were going to do AP work. But I always felt these scopes excelled for visual work and especially their quick cooldown times for a quality grab-n-go. Also their cost at the time they were on the market made them a good choice for visual observers. Thanks.




Thank you for the kind words. In addition to grab and go an ED (by whatever name) also makes a good addition to larger scope, i.e. put one on a c11 or c14, very wonderful field combination for visual astronomy.

I don't know whether an ED could be used for guiding but I suspect it could. Once you get a triplet apo there is as much temptation to use the SCT for guiding as any other way.

Greg N


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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283873 - 06/22/12 01:31 PM

Great effort Greg at trying to tackle a difficult issue - and the interesting points not least of which is objects / magnifications and here I think why for visual only these ED variants can be really a great buy. There certainly is value there for visual, and I'm glad it has caught on with the vendors

I think if talking refractors (for imaging only) - a great reality test would be similar images with a decent triplet? The down side is whatever you try to conclude some folks can argue just for the sake of argument. I think with folks that do actively image and have seen for themselvs, the choice is made... go with mirrors (OK JUST KIDDING!)


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Re: What an ED refractor can and cannot do new [Re: CounterWeight]
      #5283901 - 06/22/12 01:48 PM

Greg,
Thanks for doing the test - it's always nice to see real world examples of what telescopes are capable or rather than anecdotal reports. If I may though I just have a couple questions about your testing methodology.

Were you using prime focus? You said you were using a coolpix by Nikon, and the reason I ask this is that even my dslr starter lens will display chromatic aberration similar to what the branches illustrated in daytime viewing. If you were shooting afocal you could be introducing both lateral colour from the eyepiece, as well as colour from a potentially undercorrected camera lens. Passing light through three refractive sources is a tall order, which is why most long exposure work is done with prime focus.

The reason why I point this out is that recommending that one should spring for a "real" apochromat, or a triplet for long exposure work should be based on a test that is done under the same conditions that the suggested telescope would be operated under. For myself I can say that I would never engage in long exposure or deep sky photography unless I was operating with a prime focus as introducing more refractive systems could compromise the final result (and nullify the advantages of a highly figured "premium" optic if the subsequent systems were of lower quality).

Note - I looked up the camera you used and can't really tell whether the swivel lens comes off allowing it to be used at prime. If not a note (wikipedia so I don't have great confidence here) did mention issues in controlling chromatic aberration.

I apologise if any of this irks you; I am really happy that someone is taking the time to do such tests in order to illustrate what different optics are capable of. I may just be in school mode where the nit-picky side of my head has taken over or something.

Regards,
Dan


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ken svp120
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Re: What an ED refractor can and cannot do new [Re: CounterWeight]
      #5283909 - 06/22/12 01:54 PM

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.


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CounterWeight
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Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5283917 - 06/22/12 01:58 PM

Whoops, I took to mean ED doublet, I agree - it's marketing when it comes to the glass type.

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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5283945 - 06/22/12 02:13 PM

I think it is rather difficult to draw the conclusion that an ED scope can't give APO performance from this test.

First, whent the scope is not being used at infinity, it can color (pun intended) the result of this kind of test.

Next, the scope used is unusually fast for an ED doublet.

Keep in mind that all but the 130mm Astro-Physices triplets are f/7.5 or slower. If it were possible to make triplets completly color free, one would think that AP would make them faster than f/7.5.

Even the 130mm AP is listed as having color correction that is somewhat inferior to the bigger models. AP list the color correction for the 130 as less than + - 0.006% focus variation from 706nm to 430nm (r to g wavelengths), while the larger scopes are listed as less than + - 0.004% focus variation from 656nm to 430nm (c to g wavelengths).

As this example illustrates, the faster scope has poorer color correction over a narrower range of spectrum than its larger, slower stablemates.

Is a f/6.5 ED doublet not a true APO? AP used to define APO as having 1/8000th variation in spectrum. By this standard, even some of the older Astro-Physics scopes (which were vaunted as APOs in APs marketing materials) would perhaps not meet APs current definition of being an APO.

But to do this test on such a fast scope one could expect to find that it has some color and may indeed not even meet the 1/8000th AP guideline of many years ago.

But that is not to say that many modern ED doublets would not fare as well.

Making even a triplet at very fast focal ratios and keeping them color free would be a real challange. To expect it from a doublet of course would be unfair.

Are these other ED scopes "APO" enough? Who knows. I have used 100ED f/9 scopes and found them to be excellent performers. I am good with calling them APOs.

Some of the faster (f/7.5) ED scopes? Surely not achromats, but close enough to APO that I think that for most people, they will work well enough for most use.

I do know this though... If you did this test with my 28 year old f/8 AP tripet, it would "fail." It is too demanding a test. At infinity though, the scope works quite beautifully, and I am content to call it an APO even as others may be inclined not to.

Roland Christen called it an APO once apon a time. If he called it one (and I have a copy of the marketing brochure stating that it is an APO) then who am I to argue the point.

So, while it is true that even Roland Christen might not call his older scopes APOs today, perhaps the definition of what constitutes an APO is somewhat in flux.


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5283973 - 06/22/12 02:23 PM

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

Edited by jmiele (06/22/12 02:29 PM)


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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
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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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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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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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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
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5289886 - 06/26/12 10:34 AM

Okay. If the point of all this was to demonstrate that doublets show color out of focus, I would agree. But I'm pretty sure most people already knew that. I'm pretty sure most people also know that for imaging purposes a triplet is better due to the blue and violet colors being better controlled.

So what was the point of all of this? I for one only use my scopes when they are in focus.


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jmiele
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Re: What an ED refractor can and cannot do new [Re: RodgerHouTex]
      #5289908 - 06/26/12 10:46 AM

The problem is they are recommended all the time for astrophotography. When that is done under the "semi Apo" label, folks might think they will have more success then is possible. Because that lack of control will result in bloated stars - at best.

And before anyone gets in a huff, I know there is much very nice work being done on ED doublet Achro's err ah Apo's, I mean semi something or other. No need to post pictures. I submit, nice pictures can be taken with them - in some cases.

Joe


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hfjacinto
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5289924 - 06/26/12 10:59 AM Attachment (35 downloads)

Quote:

The problem is they are recommended all the time for astrophotography. When that is done under the "semi Apo" label, folks might think they will have more success then is possible. Because that lack of control will result in bloated stars - at best.

And before anyone gets in a huff, I know there is much very nice work being done on ED doublet Achro's err ah Apo's, I mean semi something or other. No need to post pictures. I submit, nice pictures can be taken with them - in some cases.

Joe




I was going to post pictures showing sharp focused stars with an ED. But since you said not to, I definitely will, the argument is so over the top to not even merit justification. An ED can be used for Astrophotography and is a great visual scope. Period end of debate.


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: RodgerHouTex]
      #5289967 - 06/26/12 11:22 AM

Quote:

Okay. If the point of all this was to demonstrate that doublets show color out of focus, I would agree. But I'm pretty sure most people already knew that. I'm pretty sure most people also know that for imaging purposes a triplet is better due to the blue and violet colors being better controlled.

So what was the point of all of this? I for one only use my scopes when they are in focus.




For cameras, even when the ED doublet is in focus, it is still out of focus.

This thread began to help out s.o. in another thread...oh never mind. Refractor guys all out to lunch anyhow.

Greg N


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Sean Puett
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Re: What an ED refractor can and cannot do new [Re: sg80]
      #5289986 - 06/26/12 11:35 AM

I think ED doublet is pretty clear. Better than an achro at the same focal ratio, is all it means to me. Some flourite doublets have amazing color correction i.e. FS102 and some other brands/models are not quite as good. It is the same with triplets. My guess is the(even though i have not compared the 2 scope) that an fs102 would have better color correction than an ES ed102 which is a triplet. So if a high quality flourite doublet cannot be an apo, then why can the budget triplets be called apos? Here is another definition i found: apochromatic, adjective. Having something undesirable neutralized.
If cameras pick up color, then maybe the AP telescopes need their own classification. Visual astronomers shouldn't have to pay for camera perfect color correction. I think takahashi does this with their orthoscopic apos. They don't make the fs line anymore though do they? Don't get me wrong, i am glad that AP guys get the scope they need. It is a shame that we lose some availability of good visual scopes in the process.


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Eddgie
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5289999 - 06/26/12 11:48 AM

Quote:

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.




As far as I know (and having owned two of these), the Vixen 140 does not use ED glass.

The Vixen is an achromat and is not nearly as well color corrected any of the ED scopes I have owned.

The design uses a relativlly slow achromat doublet (maybe f/12) which would have far less CA tha an f/5.7 doublet of 140mm, and a Petzval corrector to achieve the final f/5.7 focal ratio.

Even a casual glance on a bright target would quickly show that this is still an achromat. It is a fairly large achromat with very good color correction, but not in the same class as ED/APO scopes I have owned.


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jmiele
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Re: What an ED refractor can and cannot do new [Re: hfjacinto]
      #5290096 - 06/26/12 12:53 PM

Eddgie,

The reason I said not to was because a recognize that it is possible. Unfortunately, between the marketing hype and the miss-information in some postings found here and within other threads, folks are buying and re-buying instruments and not always getting what they pay for.

That fine with me at this point. My goals for were the following:

Establish that ED elements are used in Achromats.

Establish that NO such definition as ED exists, defining telescopic instruments and their design parameters. Beyond slang, that it.

Establish that the use of ED elements does NOT make an instrument an Apo.

Observe that Eddgie can take a pretty picture of an open cluster with an instrument that includes an ED element.

The first three points were satisfied with comments by myself and others along with documented essays by Roland C. and Thomas B. The last was proven with your most excellent picture Eddgie, however, I'm not so sure your "PERIOD" at the and of your posting ends discussion here.

All in good fun.....Eddgie, I really do like you, you know.

Best, Joe

Edited by jmiele (06/26/12 12:55 PM)


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hfjacinto
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5290131 - 06/26/12 01:10 PM

Eddgie doesn't take pictures.

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Re: What an ED refractor can and cannot do new [Re: hfjacinto]
      #5290163 - 06/26/12 01:23 PM

Actually, I do image. Just not very often. 98% visual, so I don't call myself an imager.

But you are correct, I did not take the pictures in this thread.


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Eddgie]
      #5290181 - 06/26/12 01:32 PM

Quote:

Actually, I do image. Just not very often. 98% visual, so I don't call myself an imager.

But you are correct, I did not take the pictures in this thread.




Eddgie,
Well it's a nice image just the same. Also, I have read your report on the Vixen 140 NA. Nicely done. I do understand from a friend that color is quite well controlled in that instrument. I appreciate the effort you have always made to formally document your equipment Ed. We don't always see eye to eye, but I do respect your work.

Helder! We just don't hear enough from you lately. I hope all is well. I find I'm getting in way more trouble these days without your guidance.

Best, Joe


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jmiele
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5290189 - 06/26/12 01:34 PM

OMG, I don't now why I thought Eddgie posted the picture and the "over the top comment" : ) LOL

Sorry Ed... Helder.... You're right, I can be over the top.

Best , Joe

Edited by jmiele (06/26/12 01:35 PM)


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RodgerHouTex
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Re: What an ED refractor can and cannot do new [Re: hfjacinto]
      #5290329 - 06/26/12 03:20 PM

I was wondering when you were going to post some of those really out of focus pictures you've been taking hfjacinto with your EON 120 semi-achromat.

Just kidding. As always the proof is in the pudding and your photo is really breathtaking.

Thanks,


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: hfjacinto]
      #5290710 - 06/26/12 09:21 PM

Very nice pic. Low power view, nothing too bright, maybe not so long exposure--not a stress test. But it definitely shows that you can do photographic things with an ED (as I did with the moon). One can work around some of the limitations with shorter exposures and dimmer subjects. It's all in the OP.

Some objects might be considerably more difficult. For example, NGC 6888 would probably require a longer exposure to come out in good detail and the dense star field would likely begin to bleed blue during the process. But that's a very nice pic of M46. Maybe it could be done with multiple short exposures.

Greg N


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5290879 - 06/26/12 11:06 PM

http://www.astrosurf.com/sguisard/Pagim/CentaurusA-FS128.html Really beautiful pic but the stars are showing blue.

http://www.vixenoptics.co.uk/Pages/Image%20Gallery%20Pages/global_cluster_m13.htm (glob is showing some purple, exposures are pretty long)

To be fair I found some pics that were pretty impressive by both scopes. I think one can say that one can do some great astrophotography with an ED but one has to take into account instrument limitations to minimize false color. GN

Edited by gnowellsct (06/26/12 11:06 PM)


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Astropin
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5291037 - 06/27/12 02:17 AM

Quote:

Eddgie,

The reason I said not to was because a recognize that it is possible. Unfortunately, between the marketing hype and the miss-information in some postings found here and within other threads, folks are buying and re-buying instruments and not always getting what they pay for.

That fine with me at this point. My goals for were the following:

Establish that ED elements are used in Achromats.

Establish that NO such definition as ED exists, defining telescopic instruments and their design parameters. Beyond slang, that it.

Establish that the use of ED elements does NOT make an instrument an Apo.

Best, Joe




And I think the whole point of this thread is to show that achromats use "LD" glass not "ED" glass.

That the definition of ED is in fact "Extra Low Dispersion". Which is not used in most if not all achromats.

That many "ED" doublets can in fact be considered Apo's due to their excellent design and lack of CA in focus on even the brightest of objects......at least visually.

That by and large ED doublets outperform Achromats. While they often (not always) fall short of Apo Triplets they are Much, Much closer to the performance of Apo's then they are to Achro's.


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Re: What an ED refractor can and cannot do new [Re: Astropin]
      #5291112 - 06/27/12 05:00 AM

Sky At Night magazine July 2012 has a review of the Altair Astro 102 ED doublet and they call it a "semi-Apo" great for both visual and AP

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ken svp120
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Re: What an ED refractor can and cannot do new [Re: PhilCo126]
      #5291204 - 06/27/12 08:33 AM

While I agree that some achromats are fine for imaging, the review's use of semi-apo just further propogates the confusion and is indicative of either irresponsibility or of a deliberate attempt to promote its use without assigning any real meaning to the term. Exactly the marketing I expect to see in a review.

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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5291232 - 06/27/12 09:03 AM

Quote:

While I agree that some achromats are fine for imaging, the review's use of semi-apo just further propogates the confusion and is indicative of either irresponsibility or of a deliberate attempt to promote its use without assigning any real meaning to the term. Exactly the marketing I expect to see in a review.






Semi-apo is a term that has been abused for so long that it just needs to be forgotten, lost, sent to outer space. 80mm F/6 achromats have been called "semi-apo's"....

Jon


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Jon Isaacs]
      #5291462 - 06/27/12 11:52 AM


+1

Exactly, that's the only point trying to be made. Some tend to get defensive when this type discussion takes place. The intent is not to harm, but arm.. Educated astronomers will have more luck getting the correct instrument to meet their objective through open discussion. Happy astronomers, stay in the hobby and that's good for everyone.

Joe


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Re: What an ED refractor can and cannot do new [Re: Astropin]
      #5291536 - 06/27/12 12:45 PM

Quote:



And I think the whole point of this thread is to show that achromats use "LD" glass not "ED" glass.

That the definition of ED is in fact "Extra Low Dispersion". Which is not used in most if not all achromats.

That many "ED" doublets can in fact be considered Apo's due to their excellent design and lack of CA in focus on even the brightest of objects......at least visually.

That by and large ED doublets outperform Achromats. While they often (not always) fall short of Apo Triplets they are Much, Much closer to the performance of Apo's then they are to Achro's.




This is probably the best and simplest description of the differences between refractor lens types, and I agree 100% with the last sentence.


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Re: What an ED refractor can and cannot do new [Re: SteveG]
      #5291583 - 06/27/12 01:11 PM

Quote:

Quote:


That by and large ED doublets outperform Achromats. While they often (not always) fall short of Apo Triplets they are Much, Much closer to the performance of Apo's then they are to Achro's.




This is probably the best and simplest description of the differences between refractor lens types, and I agree 100% with the last sentence.




That's my view. I think EDs are wonderful and are much closer to triplet apos than to achros. So you don't have to pay for performance you don't need. To have someone say well it's just an achromat implies you're getting bright purple and/or green around Jupiter and such. Not so.

Nonetheless, if a newbie is under the impression that in buying an ED doublet he or she is getting the same performance as a triplet apo, that's a mistake. There are some domains where the ED won't be able to keep up.

Greg N


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ensign
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5291752 - 06/27/12 03:28 PM

I've just been reading a biography of Isaac Newton. It's interesting that he created the mirror-based telescope design that bears his name because of blurring caused by different wavelengths of light not coming to focus at the same point with telescopes made with lenses.

So is not color fringing more of an aesthetic consideration and clarity of the resultant image the real goal of the apochromatic optical design?

While perhaps a gross oversimplification, would it not be possible to define perfect apochromatism in terms of an idea such as "all wavelengths of light coming to precisely the same focus" and "no spherical aberration at all" and then to describe any particular instrument in terms of deviation from that standard?

A threshold could be set for the maximum deviation from perfect apochromatism beyond which, from a particular application's point-of-view, the deviation from perfection interferes with performance (and only, perhaps, one or a few aspect(s) of performance). Even such a standard would, to some extent, be arbitrary.

If my ignorance of this subject is obvious, please be gentle. I'm an absolute noob to the refractor wars.


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REC
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5291755 - 06/27/12 03:31 PM

Wow, just came across this post, look at all the comments!

Anyway I will ad my 2 cents as just last night I was thinking along those lines. I had my 5" Mak and the 80mm ED out last night for a quick evening of viewing. Always enjoy the 1st. qtr. Moon and plus we had pretty good skies as well. I was using the Mak for close-ups of the Moon and Saturn. I kept going back to the refractor because I thought it looked more crisp than the Mak. At 100x in both, they were close, but I like the 80mm better.

Looking at stars and clusters I kept thinking what a nice pin point images it throws up. I bought this scope used and so glad I added it to my collection!

Bob


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hfjacinto
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Re: What an ED refractor can and cannot do new [Re: REC]
      #5291972 - 06/27/12 05:46 PM

I was going to post a couple more pictures but the truth is that I don't really care what anyone thinks an ED can do, I own 2 EDs and had a chance to buy a Tak 130 and didn't because tin focus there wasn't a difference in view. You can read here http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4991870/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

But whether you agree or not if you don't own the scope don't say it can't be done.


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gnowellsct
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Re: What an ED refractor can and cannot do new [Re: hfjacinto]
      #5292208 - 06/27/12 08:48 PM

Quote:

I was going to post a couple more pictures but the truth is that I don't really care what anyone thinks an ED can do, I own 2 EDs and had a chance to buy a Tak 130 and didn't because tin focus there wasn't a difference in view. You can read here http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4991870/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

But whether you agree or not if you don't own the scope don't say it can't be done.




I own TWO doublet designs. The FS128 and the Vixen f/6.5 ED. Which are basically all I've posted about. Only triplets in my life are owned by friends. GN


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Alan French
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Re: What an ED refractor can and cannot do new [Re: RodgerHouTex]
      #5292209 - 06/27/12 08:48 PM

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.




Rodger,

I ran a design for a 4" f/15 apochromat (since I had one handy). Changing the focus to 50 feet barely had any effect on its performance.

Clear skies, Alan


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saemark30
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5293407 - 06/28/12 02:46 PM

What would you think of the old Meade 152 ED?
It's halfway between a FS152 and normal 6" f/15 achromat, or similar to what AP was before ED triplets.
Markus apparently has a ED refractor (doublet) than performs to a similar level.


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drollere
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Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5293752 - 06/28/12 06:53 PM

my usual reaction to a prolonged debate over definitions is that, outside the legal system, debated definitions have little practical significance.

my limited understanding of refractor optics is that the primary optical issues are (in order of importance):

1. focal ratio
2. number of elements
3. optical prescription (optical surfaces and glass qualities)

in other words, the difference between an ƒ/4 and an ƒ/40 refractor far outstrips the difference between two doublet refractors of the same aperture and focal ratio with our without an ED element, or between refractors made with two vs. three elements.

in fact, i'd suggest that the heightened attention to the various nuances of design and fabrication arise precisely because commercial designs emphasize shorter focal ratios, which make differences in optical design more significant.

i recently enjoyed viewing through a 5" ƒ/15 D&G achromat, and the image quality was remarkably achromatic on the moon, saturn and many bright double stars. it compared very favorably to my recent experiences with an ƒ/7 TEC 140 ED apo. focal ratio substantially minimized any optical differences in the glass and number of elements.

the discussion above also illustrates the point that i think is made in thomas back's posting on achromats, which is that the visual performance eludes a simple optical definition. and we have pretty good testimony above that visual uses and photographic uses put entirely different demands on an instrument. so performance criteria, like optical definitions, don't seem to provide any basis for categorization.

the valid point is that marketing communications need to be candid and interpretable, and that ED by itself does not provide any product quality assurance. but i don't think marketing has ever been much impeded by definitions, or actual product performance, especially in a market where product claims are unregulated.

becoming more aware of and wary of both marketing claims and the faddish claims of user segments is just something you have to learn as part of astronomy.


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jmiele
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Re: What an ED refractor can and cannot do new [Re: drollere]
      #5293840 - 06/28/12 07:45 PM

I would agree with your assessment of definitions and wouldn't be bothered, however in this case they are abused. Misleading advertisements exploiting the grey area here lead to misinformed and mislead astronomers. That's bad for the hobby.

Next we can take on "diffraction limited". That's the next load of hooey plaguing this hobby.

Best, Joe


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Jim Curry
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5293864 - 06/28/12 08:11 PM

"the valid point is that marketing communications need to be candid and interpretable, and that ED by itself does not provide any product quality assurance."

True enough but APO, short for apochromatic, does not in itself provide product QA, either. As a consumer I have certain expectactions for the three marketing labels used for refractors, achromat, ED, and APO. These labels provide mental benchmarks as to what the color correction MIGHT be. Unless there is something seriously wrong I would expect an ED to be putting up less color than an achro and less correction than an APO. Keep in mind 20 years ago todays ED was an APO but experimention with different glass combinations elevated the apochromatic scope performance to a level that left a pretty large performance hole that 2 element scopes with newer glass types could fill.

Regards,
Jim


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RodgerHouTex
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5294560 - 06/29/12 09:46 AM

By any chance Alan could you share the spotplots with us for infinity and 50 feet in color? Also please include the Airy disk superimposed if possible.

Thanks,

Edited by RodgerHouTex (06/29/12 09:47 AM)


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Alan French
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Re: What an ED refractor can and cannot do new [Re: RodgerHouTex]
      #5295332 - 06/29/12 06:57 PM Attachment (29 downloads)

Quote:

By any chance Alan could you share the spotplots with us for infinity and 50 feet in color? Also please include the Airy disk superimposed if possible.

Thanks,




Rodger,

Here are spot diagrams from 436 to 656nm for the 4" f/15 doublet apochromat for infinity, 50 feet, and 25 feet. Note that the size of the Airy disks will increase as the object distance decreases and the focus moves outward. At 50 feet it is working at about f/16.6, and at 25 feet at about f/18.8. At 50 feet the Airy disk sizes will increase by ~10% and at 25 feet they will increase by ~25%.

Clear skies, Alan


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RodgerHouTex
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5295484 - 06/29/12 08:54 PM

I would say that those look pretty good for a f/15 apo but I believe the scope that the OP took the pictures with was an f/6.5. There of course will be a huge difference in the spot plots at that f ratio. Could you run them at f/6.5. I think that would be very informative.

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Alan French
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Re: What an ED refractor can and cannot do new [Re: RodgerHouTex]
      #5295498 - 06/29/12 09:02 PM

Quote:

I would say that those look pretty good for a f/15 apo but I believe the scope that the OP took the pictures with was an f/6.5. There of course will be a huge difference in the spot plots at that f ratio. Could you run them at f/6.5. I think that would be very informative.




Rodger,

I ran them with that scope because I had the design handy. Things have gotten pretty hectic here, and company arrives tomorrow, so my time is going to be otherwise occupied. When I get a chance, I'll see what I can do. I'm curious too.

Clear skies, Alan


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PhilCo126
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5295863 - 06/30/12 04:08 AM

In summary:
A true APO-chromatic refractor realises a crossing of three colors at the spot, while a Super APO-chromatic realises a four color crossing.
When putting a DSLR or CCD chip camera on an ED Doublet, some chromatic aberration will show...
Astrophotography = an ED doublet will work but preferably get a true APO (air-spaced triplet).
Visual observations = an ED doublet will do very fine!


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RodgerHouTex
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Re: What an ED refractor can and cannot do new [Re: Alan French]
      #5296045 - 06/30/12 09:06 AM

I look forward to seeing them too Alan.

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jrbarnett
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Re: What an ED refractor can and cannot do new [Re: drollere]
      #5297240 - 07/01/12 01:34 AM

Or, more simply, "TEC is best".



- Jim


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phucdt
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Re: What an ED refractor can and cannot do new [Re: jrbarnett]
      #5300161 - 07/03/12 04:44 AM

I am curious about the number of ED elements in an Apo triplet refractor: why most triplet apo refractor has only one ED element, and the two others are crown and flint but they are too expensive? Am I missing something?

Edited by phucdt (07/03/12 04:47 AM)


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jrbarnett
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Re: What an ED refractor can and cannot do new [Re: phucdt]
      #5301307 - 07/03/12 09:46 PM

As ED blank size goes up, price of the blank, too, goes up exponentially. And that's just the materials cost. The real expense in a big, high quality triplet is the man-hours it takes to fabricate and test the optics. The larger the optic, the longer it takes to make to a given level of quality.

There are largish, cheap triplets. Figure those likely use either lower cost glass blanks, aren't figured as accurately or tested as thoroughly, or some combination of the foregoing. When you opt for a large, hand-figured, premium/artisanal refractor, you pay a lot but you also get what you pay for.

Regards,

Jim


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gdd
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Re: What an ED refractor can and cannot do new [Re: phucdt]
      #5301637 - 07/04/12 01:19 AM

Quote:

I am curious about the number of ED elements in an Apo triplet refractor: why most triplet apo refractor has only one ED element, and the two others are crown and flint but they are too expensive? Am I missing something?




I believe it is best if all 3 elements are of different types of glass in order to achieve the 3 ray coinciding crossings at 3 wavelengths.

Gale


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Alan French
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Re: What an ED refractor can and cannot do new [Re: gdd]
      #5305243 - 07/06/12 11:39 AM

An ED triplet can be, and often is, a fluor-crown and two crowns. The two crowns can be the same glass.

Clear skies, Alan

Edited by Alan French (07/06/12 11:40 AM)


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saemark30
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Re: What an ED refractor can and cannot do new [Re: hfjacinto]
      #5309896 - 07/09/12 02:13 PM

What ED scope was that image taken with?

The 80ED f/7.5 is definitely in the APO category.
Is the 120ED f/7.5 in the same league?


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dscarpa
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5310019 - 07/09/12 03:38 PM

I wish there was a very good not too expensive 5" doublet available. I would like to have a bigger refractor in addition to my WO ZS-110 but without the expense and weight of 5" triplets. The 120 Eon and 120 SW Pro are too close size wise to my ZS. I'm visual only. David

Edited by dscarpa (07/09/12 03:40 PM)


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kevint1
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Re: What an ED refractor can and cannot do new [Re: dscarpa]
      #5310138 - 07/09/12 05:12 PM

Quote:

I wish there was a very good not too expensive 5" doublet available.




Have you looked at the ES AR127?


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dscarpa
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Re: What an ED refractor can and cannot do new [Re: gnowellsct]
      #5310212 - 07/09/12 05:56 PM

I would want a ED doublet. I do a lot of lunar-planetary so good color correction is a must. David

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hfjacinto
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Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5310500 - 07/09/12 09:12 PM Attachment (25 downloads)

Quote:

What ED scope was that image taken with?

The 80ED f/7.5 is definitely in the APO category.
Is the 120ED f/7.5 in the same league?




This was taken with the F6.25 Orion 80MM

Edited by hfjacinto (07/09/12 09:13 PM)


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hfjacinto
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Re: What an ED refractor can and cannot do new [Re: hfjacinto]
      #5310504 - 07/09/12 09:14 PM Attachment (25 downloads)

This was also taken with the 80MM

Edited by hfjacinto (07/09/12 09:14 PM)


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hfjacinto
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Re: What an ED refractor can and cannot do new [Re: hfjacinto]
      #5310506 - 07/09/12 09:15 PM Attachment (29 downloads)

This was with a 120MM EON

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wh48gs
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Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5310650 - 07/09/12 11:09 PM Attachment (47 downloads)

Quote:

What would you think of the old Meade 152 ED?
It's halfway between a FS152 and normal 6" f/15 achromat, or similar to what AP was before ED triplets.




It uses Hikari's FK01 ED glass with KF3 in front. Visual polychromatic Strehl is 0.915 (design limit), which compares to 0.78 for the 6-inch f/15 achromat. This glass combination has some residual secondary spectrum (less than 1/5 of the standard achromat), but it matters less than spherochromatism.

Similar glass combination was probably used for the Vixen f/6.6 ED102SS. There is, actually, no better choice for such a fast doublet, since matches with near-zero secondary spectrum (including FPL53/ZKN7)don't have sufficient Abbe number differential to supress spherochromatism below 0.04 wave RMS in the optimized wavelength. A 102mm f/6.6 KF3/FK01 objective has 0.021 wave RMS in the optimized wavelength and 0.87 visual polychromatic Strehl design limit, which is quite good considering its speed (OSLO file attached).

Vla


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Jeff B
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Re: What an ED refractor can and cannot do new [Re: wh48gs]
      #5311135 - 07/10/12 10:01 AM

Ok, now that we've completely settled the glass and doublet versus triplets issues and are all in agreement, I think we need take up the air spaced versus oil spaced issue.

Jeff


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jmiele
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Re: What an ED refractor can and cannot do new [Re: Jeff B]
      #5311260 - 07/10/12 11:35 AM

Nooooooo! Not again..

Joe


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jmiele
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5311262 - 07/10/12 11:37 AM

If I concede that all ED Doublets are Apo's can we skip the Air vs Oil discussion.

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ken svp120
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5311289 - 07/10/12 11:58 AM

As an Aside: After years on this forum, there most certainly are recurring threads such as...

Air spaced v. Oil spaced
Aluminum v. Carbon Fiber
Doublet v. Triplet
What do Glass types mean
Is ED or Semi Apo meaningful/valid

It would almost be nice if the moderators would start an individual thread for each of these recurring items, let it go on indefinately, and redirect any such future threads/posts to the appropriate thread. That way, we could debate on and on, all the info would be in one place, and these items wouldn't keep cluttering up other threads.

Just a thought...


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Jeff B
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Re: What an ED refractor can and cannot do new [Re: ken svp120]
      #5311450 - 07/10/12 02:23 PM

Quote:

As an Aside: After years on this forum, there most certainly are recurring threads such as...

Air spaced v. Oil spaced
Aluminum v. Carbon Fiber
Doublet v. Triplet
What do Glass types mean
Is ED or Semi Apo meaningful/valid

It would almost be nice if the moderators would start an individual thread for each of these recurring items, let it go on indefinately, and redirect any such future threads/posts to the appropriate thread. That way, we could debate on and on, all the info would be in one place, and these items wouldn't keep cluttering up other threads.

Just a thought...




Don't for get Achro vs APO...or whatever you term an APO to be..DOH, it's going to start all over again..my bad.


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csrlice12
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Re: What an ED refractor can and cannot do new [Re: Jeff B]
      #5311514 - 07/10/12 03:09 PM

Quote:

Quote:

As an Aside: After years on this forum, there most certainly are recurring threads such as...

Air spaced v. Oil spaced
Aluminum v. Carbon Fiber
Doublet v. Triplet
What do Glass types mean
Is ED or Semi Apo meaningful/valid

It would almost be nice if the moderators would start an individual thread for each of these recurring items, let it go on indefinately, and redirect any such future threads/posts to the appropriate thread. That way, we could debate on and on, all the info would be in one place, and these items wouldn't keep cluttering up other threads.

Just a thought...




Don't for get Achro vs APO...or whatever you term an APO to be..DOH, it's going to start all over again..my bad.




You forgot the favorites: Dob vs Refractor, and Ginger or MaryAnn?


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ckwastro
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Re: What an ED refractor can and cannot do new [Re: csrlice12]
      #5311530 - 07/10/12 03:17 PM

Quote:

You forgot the favorites: Dob vs Refractor, and Ginger or MaryAnn?




1. Dob *and* refractor

2. MaryAnn....definitely!


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ken svp120
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Re: What an ED refractor can and cannot do new [Re: ckwastro]
      #5311550 - 07/10/12 03:29 PM

+1 for MaryAnn - no argument there.

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saemark30
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Re: What an ED refractor can and cannot do new [Re: wh48gs]
      #5311568 - 07/10/12 03:40 PM

Can you workout the Synta 120ED refractor?
We know it uses S-FPL-53 at least.
How good is it?


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Eddgie
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Re: What an ED refractor can and cannot do new [Re: csrlice12]
      #5311641 - 07/10/12 04:30 PM

Ginger... Tina Louise was a bombshell compared to Dawn Wells. Gilligan's Island over-did her and ruined her career. She has been compared to Jane Mansfield (who was originally offered the part I think).

This to me seems more interesting than the original premise of the thread that took a look at one of the fastest ED refractors ever sold and attempted to make a generic statement regarding ED scopes in general by using an extreme sample of the class as being represntitive of all ED scopes.

I wonder why Astro Physics makes their 140 triplets f/7.5? Gee, couldn't they have made it f/6.5? That would be more better, eh?

And what about the 175??? couldn't they have at least made it f/7??? I mean its a triplit! Wouldn't it be just perfect if it were f/7? And yet they made if f/8.. It's a triplet, but a slow one. Why oh why didn't they make it f/6.5?

I still think using such a fast doublet and inferring that the result applies to all ED scopes is a little bit of a stretch.

It is very difficult to make even triplets color free if the system gets to fast. Bending light is easy. Getting all the wavelenths bent to the exact same focus is almost impossible. And the faster the system, the more difficult it becomes.

Tina Louise in a hearbeat though. She should have been a big screen bombshell, but Gilligan's Island robbered her of that chance.

Just like the OPs post is attempting to do to ED telescopes in general by choosing an extreme sample and making a blanket statement that implies that all ED scopes are inferior to all triplets.

Quote:

A true top end triplet apo would do a better job with this test.




"D'uh" of the Decade award for this statement....

Edited by Eddgie (07/10/12 04:33 PM)


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wh48gs
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Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5311664 - 07/10/12 04:46 PM

The triplet? It is almost certainly FPL53, because it is probably the cheapest full-fledged ED glass available these days. More or less, it would be similar to downscaled TEC 140, which has 0.95+ polychromatic Strehl when visually optimized, and 0.92 claimed officially (which should be due to some compromises toward photo-visual correction and/or factoring in greater actual error in the optimized wavelength than the design minimum). With only little more expensive matching glass (ZKN7/FPL53/ZKN7 instead of K10/FPL53/BK7) it could reach 0.97 visual Strehl at 140mm aperture, which gets closer to 0.98 at 120mm. In any case, its design limit is probably 0.96-0.97 Strehl, which would boil down to about 0.95 with quality fabrication.

Vla


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saemark30
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Re: What an ED refractor can and cannot do new [Re: wh48gs]
      #5311745 - 07/10/12 06:01 PM

NO I was wondering how well the Skywatcher 120ED f/7.5 doublet performs, like its smaller 80mm ED f/7.5 sibling.

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cjc
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Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5311871 - 07/10/12 07:33 PM

Quote:

NO I was wondering how well the Skywatcher 120ED f/7.5 doublet performs, like its smaller 80mm ED f/7.5 sibling.



They perform well acording to owners and look at the tests here:

http://translate.googleusercontent.com/translate_c?hl=en&ie=UTF8&prev=_t&rurl=translate.google.com&sl=de&tl=en&u=http://www.astro-foren.de/showthread.php%3F6084-Verzeichnis-optischer-Berichte%26p%3D32999&usg=ALkJrhjdhOKfrg5kCdT0uxAm2znuTP2uQg#post32999

In passing one would also need threads on the quality of chinese manufactured optics and importance of ED glass...


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Jeff B
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Re: What an ED refractor can and cannot do new [Re: wh48gs]
      #5311872 - 07/10/12 07:35 PM

Sorry, but Lovey gets my vote.

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wh48gs
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Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5311944 - 07/10/12 08:32 PM

I read it has fluorite element, but it doesn't really need it, so I'm not sure. The same site (vendor) states that FPL53 is Schott glass, and that this SW doublet has Schott crown as the positive front element (since this element has higher dispersion, it has to have lower optical power in order for the two elements to cancel out each other's longitudinal chromatism; and if it would be the positive element, such doublet could only produce diverging rays).

So those sources don't seem to be awfully reliable. But there is no real difference, FPL53 or fluorite, in the correction level. The latter merely has more matching glasses available for near-zero secondary spectrum and, since has somewhat higher relative partial dispersion, it could be stretched out to somewhat faster objectives than FPL53. It will also, in general, has less of secondary spectrum than FPL53 with less-than-perfect matching glasses suitable for fast objectives, but secondary spectrum is nearly negligible here compared to spherochromatism.

For example, if a common inexpensive crown like K5 is combined with FPL53 in a 120mm f/7.5 doublet, it will produce 0.933 limiting visual polychromatic Strehl (0.966 at 80mm). If combined with fluorite, it will have almost twice smaller combined error in F and C, but will also have about 50% larger error in the optimized wavelength, due to more higher order spherical. According to OSLO, its limiting design Strehl is only negligibly higher: 0.936 and 0.969, respectively.

As for the 3%, or so, difference between 80mm and 120mm aperture Strehl, it is not really significant either. It is comparable to the difference between a perfect aperture and one with 1/10 wave p-v of primary spherical. As long as the actual Strehl is over 0.90, there will be little difference in the performance level. A single most important factor for agood paper design to come close to its design level in the real objective is keeping the optimized line correction as close to the design minimum as possible.

Vla


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What an ED refractor can and cannot do new [Re: wh48gs]
      #5311999 - 07/10/12 09:06 PM Attachment (22 downloads)

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.




Lots of misconceptions in this thread...

Lets sort out some simple truths first. Two element objective (so called ED doublet) CAN bring three colors to focus. Here's one I've just cooked up in Zemax. FPL52+ZKN7. Three crossings. Triplets can be designed to bring four colors to focus (in some cases even FIVE).

BTW, I would not take the number of wavelengths that are simultaneously focused too seriously as a quality measure. A doublet with two color crossing and tight chromatic focal shift where it counts is vastly preferred than three color (but at wrong wavelengths and huge CFS).


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bratislav
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Re: What an ED refractor can and cannot do new [Re: wh48gs]
      #5312041 - 07/10/12 09:28 PM

Quote:

I read it has fluorite element, but it doesn't really need it, so I'm not sure. The same site (vendor) states that FPL53 is Schott glass, and that this SW doublet has Schott crown as the positive front element




I've read in some German forums it uses combination of ZKN7 and FPL53. Not really that much different from what you suggested (K5+FPL53).


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wh48gs
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Re: What an ED refractor can and cannot do new [Re: bratislav]
      #5312206 - 07/10/12 11:02 PM

Quote:

Lets sort out some simple truths firts. Two element objective (so called ED doublet) CAN bring three colors to focus. Here's one I've just cooked up in Zemax. FPL52+ZKN7.




Of course it can. And just a look at the relative partial dispersion diagram tells which ones will bring F, C and e lines together. Some doublet combination actually bring four widely separated wavelengths to a comomn focus (e.g. FPL53/KZFSN2-ouch!). On the other hand, if a triplet or quadruplet is made of two glasses that can't do it in a doublet, they won't do it as well. Combining more than two glasses can do it, but there's not too many viable combinations. For a visual instrument it doesn't even matter.

As the RPD diagram indicates, FPL52/ZKN7 secondary spectrum is practically non existent. The focal shift plot, showing paraxial focus deviation with wavelength, now indicates the magnitude of spherochromatism, which bends the LA (longitudinal aberration) lines enforcing the separation of paraxial foci in order to have best foci of these wavelengths come as close one to another as possible. Only in the absence of spherochromatism, the paraxial foci would have come together.

Crossings do indicate that the wavelengths are bunched up together, but what really matters is the amount of spherochromatism they have in them and, even more, how close the crossings are to their respective best focus locations.

Vla


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wh48gs
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Re: What an ED refractor can and cannot do new [Re: bratislav]
      #5312227 - 07/10/12 11:18 PM

Quote:

I've read in some German forums it uses combination of ZKN7 and FPL53. Not really that much different from what you suggested (K5+FPL53).




It would up the Strehl to 0.935 design limit (vs. 0.933) (but ZKN7 is about 2.5 times the K5 price).

Vla


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jrbarnett
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Re: What an ED refractor can and cannot do new [Re: ckwastro]
      #5312720 - 07/11/12 11:40 AM

"1. Dob *and* refractor

2. MaryAnn....definitely!"

Pshah!

1. Dobs *and* refractors *and* catadioptrics

2. MaryAnn *and* Ginger *and* Lovey Howell *and* ...*and*...*and*...definitely!



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ckwastro
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Re: What an ED refractor can and cannot do new [Re: jrbarnett]
      #5312789 - 07/11/12 12:49 PM



Jim, I would expect nothing less from.......The Most Interesting Astronomer in the Universe.

(Although I'm not sure about Lovey)


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saemark30
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Re: What an ED refractor can and cannot do new [Re: ckwastro]
      #5312883 - 07/11/12 04:01 PM

Since we like doublets so much
+2. MaryAnn *and* Ginger


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elwaine
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Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5313573 - 07/11/12 11:23 PM

Quote:

Even the 130mm AP is listed as having color correction that is somewhat inferior to the bigger models. AP list the color correction for the 130 as less than + - 0.006% focus variation from 706nm to 430nm (r to g wavelengths), while the larger scopes are listed as less than + - 0.004% focus variation from 656nm to 430nm (c to g wavelengths).




Anyone who can see the difference between + - 0.006% focus variation from 706nm to 430nm and less than + - 0.004% focus variation from 656nm to 430nm has got some seriously exquisite rods in their eyeballs. I cannot see any CA in my A-P 130mm, in spite of its poor correction to only + - 0.006% focus variation from 706nm to 430nm (r to g wavelengths).

And, BTW, I've done a lot of AP with an A-P 130mm and never encountered focus problems due to the incredibly small amount of residual CA.

As for what one should call an ED refractor with very good, if not perfect color correction: How about Anicescope?

Regards,


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saemark30
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Re: What an ED refractor can and cannot do new [Re: elwaine]
      #5315981 - 07/13/12 12:47 PM

FPL-52 is no longer available.
So with a new 6" f/8 doublet being produced by APM, should we expect to see lower prices for used TEC160, FS152 even?


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Stacy
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Re: What an ED refractor can and cannot do new [Re: saemark30]
      #5316589 - 07/13/12 08:49 PM

I tried to read the whole thread, but I got a little over half-way before my brain started bleeding.

Please help me understand! I understand an Achro has two lenses and an APO has three. How can an ED doublet bring three wavelengths (typically red, green, blue) to focus? It seems impossible.

Also what is an ED Triplet? Is there any argument that ED Triplets are not "true apo's" ?

http://www.reference.com/browse/Apochromat?s=t


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gdd
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Re: What an ED refractor can and cannot do new [Re: Stacy]
      #5316618 - 07/13/12 09:18 PM

Quote:

I tried to read the whole thread, but I got a little over half-way before my brain started bleeding.

Please help me understand! I understand an Achro has two lenses and an APO has three. How can an ED doublet bring three wavelengths (typically red, green, blue) to focus? It seems impossible.

Also what is an ED Triplet? Is there any argument that ED Triplets are not "true apo's" ?

http://www.reference.com/browse/Apochromat?s=t




Basically there are two ways to correct for 3 colors. One is to use 3 lenses to bring 3 wavelengths to a common focus. The other is to use low dispersion glass that does not separate the colors so much. So the ED doublet brings 2 colors to a common focus and relies on the low dispersion ED element to minimimze the out of focusness of the remaining colors.

Gale


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Astropin
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Re: What an ED refractor can and cannot do new [Re: gdd]
      #5317699 - 07/14/12 03:46 PM

Or three ways if you take into consideration the focal length. Focal length can also be used to correct (to a degree) CA.

Apo does not mean "triplet" Apochromatic is simply a description of high performance (CA correction). A very well made doublet using ED glass could certainly qualify as an APO.....at least visually.


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Napersky
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Re: What an ED refractor can and cannot do new [Re: bratislav]
      #5598287 - 12/31/12 04:14 PM

Quote:

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.




Lots of misconceptions in this thread...

Lets sort out some simple truths first. Two element objective (so called ED doublet) CAN bring three colors to focus. Here's one I've just cooked up in Zemax. FPL52+ZKN7. Three crossings. Triplets can be designed to bring four colors to focus (in some cases even FIVE).

BTW, I would not take the number of wavelengths that are simultaneously focused too seriously as a quality measure. A doublet with two color crossing and tight chromatic focal shift where it counts is vastly preferred than three color (but at wrong wavelengths and huge CFS).





Does Yellow Count as a Color? It's not a primary color such as Blue, Green, or Red?


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Napersky
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Re: What an ED refractor can and cannot do new [Re: Napersky]
      #5598289 - 12/31/12 04:16 PM

Obviously Yellow is not a color.

Only Primary colors count in Tom Back's definition, Although it's lacking in his definition of an APO.


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Pete-LH
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Re: What an ED refractor can and cannot do new [Re: Napersky]
      #5598365 - 12/31/12 05:01 PM

I responded to this and deleted it because my answer was somewhat incorrect. I am embarassed because in school I was heavily involved in spectroscopy but with time I have evloved/devolved into a paint chemist (ironic because I am red-green colour blind).
So as a paint chemist I responded that red, blue and yellow are the primary colours(reflective or subtractive).
However here we are talking about emission colours and here the primary colours are red, green and blue(or addtive primaries).
However, these are colour terminologies related to how we combine these for produced images: red, green, blue for Television or CRT's and red, blue, yellow for paint (or cyan, magenta, yellow for printing processes).
But in spectroscopic terms I believe yellow light emitted from a star is at the wavelenth of 570nm (or 589 nm from those damned Low-pressure sodium lamps used in parking lots).
This response is probably out of context since I came to this thread late and see it is many pages long(and I have just scanned it quickly).
Still I am grateful because it gets me back to thnking about the basics of what we visualize.
As for What an ED refractor can or can't do, if it is less than 5" it cannot resolve a globular star cluster to my satisfaction. But otherwise I find the visual experience to be just fine.


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Napersky
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Re: What an ED refractor can and cannot do new [Re: Pete-LH]
      #5598448 - 12/31/12 05:45 PM

I emphasise Yellow because Zeiss AS lenses correct in 3 colors, all which come to a common focus: Green and Yellow as all achromats do plus red. However they are achromats and not APOs because they miss the Blue.

So here you have a doublet which doesn't use ED glass at all and has 3 color crossings but is certainly not an APO.

ED objectives I think do bring 4 colors into focus: Blue, Green, Yellow and Red. The focus of those colors may be very good and put them solidly into the APO catagory or they might correct poorly yet much better than a simple achromat which only has a common focus of the yellow and green but loses the blue and red entirely.

So I would believe ED objectives might range somewhere in between a semi-achromat to an full APO depending on the quality of build but they certainly do not fall into the achromat catagory.

Mark


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Peter Natscher
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Re: What an ED refractor can and cannot do new [Re: Napersky]
      #5598736 - 12/31/12 08:55 PM Attachment (24 downloads)

Astro-Physics shows three color crossings in the 160EDF specs. -- Red, Green, Blue.


Quote:

I emphasise Yellow because Zeiss AS lenses correct in 3 colors, all which come to a common focus: Green and Yellow as all achromats do plus red. However they are achromats and not APOs because they miss the Blue.

So here you have a doublet which doesn't use ED glass at all and has 3 color crossings but is certainly not an APO.

ED objectives I think do bring 4 colors into focus: Blue, Green, Yellow and Red. The focus of those colors may be very good and put them solidly into the APO catagory or they might correct poorly yet much better than a simple achromat which only has a common focus of the yellow and green but loses the blue and red entirely.

So I would believe ED objectives might range somewhere in between a semi-achromat to an full APO depending on the quality of build but they certainly do not fall into the achromat catagory.

Mark




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Julio
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Re: What an ED refractor can and cannot do new [Re: Peter Natscher]
      #5599017 - 01/01/13 01:17 AM

Well i like my Orion 100ED achromat just fine.

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orion61

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Re: What an ED refractor can and cannot do new [Re: Julio]
      #5599586 - 01/01/13 01:05 PM

Amen My. Meade 102 ED gives Planetary detail invisable to my 8" SCT and for the live of me cant get color from it.

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kevint1
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Re: What an ED refractor can and cannot do new [Re: orion61]
      #5600114 - 01/01/13 06:38 PM

Quote:

Amen My. Meade 102 ED gives Planetary detail invisable to my 8" SCT and for the live of me cant get color from it.




+1 on both counts for my ES 102 ED.


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Jon Isaacs
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Re: What an ED refractor can and cannot do new [Re: orion61]
      #5600330 - 01/01/13 09:02 PM

Quote:

Amen My. Meade 102 ED gives Planetary detail invisable to my 8" SCT and for the live of me cant get color from it.




How well collimated is your 8 inch? Are you "critically collimating" it? How about thermal equilibrium?

Jon


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mgwhittle
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Re: What an ED refractor can and cannot do new [Re: orion61]
      #5600337 - 01/01/13 09:08 PM

Quote:

Amen My. Meade 102 ED gives Planetary detail invisable to my 8" SCT and for the live of me cant get color from it.




Gosh, that just doesn't sound right!


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Napersky
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Re: What an ED refractor can and cannot do new [Re: mgwhittle]
      #5600825 - 01/02/13 08:02 AM

Wolfgang Rohr on ED correction

http://translate.googleusercontent.com/translate_c?depth=1&rurl=translate...


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Napersky
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Re: What an ED refractor can and cannot do new [Re: Napersky]
      #5615614 - 01/10/13 02:11 PM

The MASTER says it best:

Fast 6"F8 Cde achromat: 550 - 650 nm
Long 6"F15 CeF achomat: 480 - 650 nm
Fast 6"F9 ED doublet: 450 - 650 nm
Fast 6" fluorite doublet: 420 - 1000 nm
Fast 6" FPL52/53 triplet: 380 - 1000 nm
Fast 6" fluorite triplet: 360 - 1000nm

It would be interesting then to divide the cost of each lens by its
useful wavelength range. For instance, a 6"F8 Cde achromat selling
for around $800 today would come in at $10 per nanometer. (our 6"
EDFS at $4900 comes in at $7.90 per nanometer). Interestingly, an 8"
SCT selling for around $900 comes in at $3.81 per nanometer. No fair
asking how a Newtonian would fare!

Seriously, why would you need correction well into the blue-violet
past 480nm? With black and white emulsions, this was necessary
because they have considerable sensitivity down to 380nm. Today's new
blue sensitive CCD cameras also need good correction in the violet.
Also, CCD cameras pick up lots of IR light below 650nm, so correction
to 1000nm is a distinct advantage. For pure visual use, it would be
quite sufficient if the useable range extended only from 440 to 650
nm. So, check the above table for your particular needs and make your
choice.


http://geogdata.csun.edu/~voltaire/roland/color.html


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Gem1021
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Re: What an ED refractor can and cannot do new [Re: jmiele]
      #5616168 - 01/10/13 07:53 PM

Guys I was lead to believe ED glass in a refractor is to improve the light passing through the glass. Makes the image brighter

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Jared
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Re: What an ED refractor can and cannot do new [Re: Gem1021]
      #5616177 - 01/10/13 07:58 PM

Welcome to Cloudy Nights!

Unfortunately, whoever gave you that information was incorrect. The amount of absorption through any quality optical glass in a small refractor is trivial--on the order of 1% or less. That's true whether you are talking about ED glass or normal crown/flint achromats. Fluorite crystal (used on some really high-end refractors) is even lower.

The purpose of ED glass in a refractor (or a telephoto lens, for that matter) is to reduce chromatic aberration--to improve color correction. It gets rid of or at least reduces the "purple halos" around the moon, planets, and bright stars. It also improves telescope sharpness by properly focusing a larger range of frequencies of light than an achromat of the same aperture and focal length (though the differences are smaller than you might expect since the eye's sensitivity drops off as you move away from the green portion of the spectrum). It does all this at the expense of, well, expense. ED glass and the appropriate mating glass cost more than their traditional counterparts.


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