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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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CounterWeight
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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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Uggbits
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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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Eddgie
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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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