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Color correction & Valery's thread on the subject

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#1 Gord

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Posted 28 December 2012 - 10:43 PM

Hi All,

Valery, who many of you already know, from Aries had a thread on here recently regarding color correction in refractors, how it relates to color perception in humans, and how it can affect what we see through telescopes. Here's a link to that thread:

Valery's recent color correction thread

He shared his personal experience with how lens designs can be adjusted to focus on different wavelengths in the color spectrum, thereby changing how things appear. The common example is tuning to focus the blue end of the spectrum better to reduce the infamous "purple haze" around bright objects in a achromat, at the expense of the red end where the human eye is less sensitive. However he points out, this can have a detrimental effect on objects where there is a lot of details in the red end of the spectrum (his example was Jupiter and especially Mars).

I found this to be an interesting subject and one that now pointed out, seems to perhaps have some recent field examples showing this. A couple recent discussions on the forums come to mind:

Jupiter Appearance Thread

In this thread some images of Jupiter were show and the discussion got around to people sharing their experience as to how much/little detail they could see in their telescopes relative to the appearance in the images. One frequent poster here "MikeCee" has a large refractor (10") that is a newer design from IStar Optical in a line they call the R30. This is an achromat that has been designed to show less false color (purple fringe) than a standard achromat by shifting the focus to the blue end.

The interesting thing in the discussion was Mike's experience that he felt he was seeing much less detail than what some others were experiencing with smaller scopes. In fact, he seemed to express that he had not reached the level of detail his previous 8" refractor could show. However, at the same time, he has described in many other discussions being able to see much more detail in the new 10" vs. the 8" on other targets (ex. DSO's, splitting doubles, etc.).

Another one is related to the new APM 6" ED doublet that Markus Ludes is just bringing to market, and some experience with one of the first samples:

APM 6" ED #002 Thread

In this thread, there is some discussion near the end about the first experiences with this scope. There are several comments that the false color is basically nil and that it renders nice sharp images. However there was one comment that I found interesting. It was a comparison to the ES 5" triplet and the observer found that the 6" didn't show any more details than the 5" (again, on Jupiter) other than being brighter, and that the image sounded almost washed out with less contrast.

Again, this is another scope that is tuned (as I understand it) to minimize the false color, but the experience (limited thus far) seems to indicate that there is a loss of contrast. Of course these are only two examples, and there can be many factors involved such as observers, sky conditions, etc. However, it's interesting that these observations seem to be matching up with what Valery has seen, and knows from his experienced back-ground in this area will occur with this such design.

I know from my own experience with my 6" achromat that while there is a very noticeable purple halo around bright objects and to a degree a certain "veil" across the face of thing such as planet disks, there is also a lot of detail present in the planetary disks and my feeling is that in general the contrast is very high (ex. the belts of Jupiter really jump out, surface features on Mars are visible, etc.).

This topic of color correction is a common one here on the refractor forum and has been discussed many times. Some have been very detailed discussions and one I can recall got into the idea of tuning a refractor design (achromat) to be tailored specifically to Jupiter. I've done some searching, but haven't been able to locate the thread. I recall there were comments originally attributed to Roland Christen on the subject and I believe some Aberator simulated images of Jupiter from an achromat of this design. Again, it was the idea of building a specialized objective tailored towards Jupiter based on the color pallet it presents, a sort of "one trick pony".

I did find this thread that seems to have some of the elements around this type of discussion (see around page 7), but it isn't the one I'm thinking of:

Achromat Color Correction Thread

Does anyone here remember this discussion?

Anyway, this leads into a wider discussion around the benefits of designs such as the ones above and how they fit into our selection of instruments. It seems to me that a design with a limitation (or performance limits) on viewing targets where there is a lot of red information (Jupiter, Mars) would not be so ideal. However, it seems to be the market that has decided that this is what it wants, or more so *thinks* it wants.

What I mean by this is that there seems to be a general perception and expression that the classic false color seen in a refractor is bad, and must be avoided, otherwise you won't get good images. We see this in the types of refractors that people are choosing and their comments about being focused on the amount of (or lack of) false color seen. And the vendors of course have responded. The people said this was important to them, so they have come out with designs that address the issue.

But is that what was really an issue (ie. false color) and has it simply been traded off for another issue (lack of red detail)? Has the market gotten so focused on that one (identifiable) issue that it has missed what it was really after (seeing more details)? There's usually no free lunch!

These new designs seem to perform very well on the test of controlling the visible secondary color, so they are working exactly as designed. However I myself did not understand the other side effects that this has and how it can manifest in real world use, as I have a limited understanding of how objective designs work. It took Valery's post explaining things to really make the light bulb go on!

Anyway, just wondering what others think.

Clear skies,

#2 Rutilus

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Posted 29 December 2012 - 05:03 AM

Gord - Very interesting, and thank you for taking the time to put the post together. Having read the "Jupiter Appearance Thread"
I was taken by the image made with the 100ED, for that image is very, very much like the visual views that I get with my
Sky-watcher 150mm f/8 Achromat.

Then if I use my C9.25 SCT on nights of good seeing then the views are better than those seen with
the 6 inch scope. The other night for example the 6 inch scope was giving views that were near identical to the detail in the 100ED
image, while the view with the SCT showed several features that I could not detect(or just hint at seeing) in the Achro.

Maybe I just have very good eyesight, I am in my mid-fifties and stll do not have to wear glasses. At work I
have to do fine visual inspection of products, others around me are having to glasses or even magifying glass
to do the work, but I don't and my work passes the same tests.

I have also modded my 6 inch scope by rotating the lens elements, fitting a new baffle system and
re-painting the entire inner scope with a darker black mat paint.
I also use binoviewers for all my planetary viewing along with a yellow filter.

I have said it before, but my 6 inch achro will be the last scope that I sell, it has given me some wonderful
planetary views over the years and it is a cracking scope for my other interest of double-stars.

#3 Napersky

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Posted 29 December 2012 - 01:30 PM

Gord,

Carl Zeiss Jena seems to have understood this issue and produced it's semi-achromat AS line of lenses. From 60mm to 200mm. All well corrected for the red end of the spectrum. In fact DSOs really need the red to stand out. The AS line corrects in 3 colors: Red, yellow, Green but of course they are not Apochromatic as they lose the blue.

Here is a photo of my lens having been throughly tested by Mr. Rohr. Mine is the lens at the bottom of the page. He mentions astigmatism but he is using a Bath interferometer which is known to introduce astigmatism into its test having a common path. I intend to test it using a Fizeau interferomter.

http://translate.goo...s=n&prev=_t&...

#4 Gord

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Posted 29 December 2012 - 04:01 PM

Hey guys,

It would be nice to see some more real world comparisons between them done. Of course, it's obviously still to early for that with the APM since it was just released.

Now this is Jupiter as a target that was being talked about and there are of course many other targets to consider. The APM has tested really well by Rohr so obviously for some things it's going to do really well. It's just that Jupiter is a pretty common target.

Based on the various designs and how they handle forming images (again Jupiter in this case), I've been thinking of what a theoretical ranking of the designs would be in terms of how much detail they would show. Kind of a straw man to beat around. Since it's a hypothetical comparison, I'm assuming like levels of quality, and more just look at what the design *should* be able to do. I'm also assuming 6" F8's since that seems to be a common design.

Here's how I think they would rank (more detail to less detail):

1. 6" Full triplet apo
2. 6" Fluorite doublet
3. 6" ED doublet normal color correction (tie)
3. 6" Classic achro + Chromacor (tie)
4. New 6" APM Russian ED doublet
5. New 6" APM Chinese ED doublet (tie)
5. ES 5" triplet (tie)
6. 6" Classic achro
7. 6" IStar R-series achro

This is how I think they would flush out based on the comments and discussions I've seen so far. Of course, not all things are equal, and I haven't considered where something like a long focus classic achro would fit. I'm also just guessing at where the Chromacor would fit in since I keep hearing they work like magic. :lol:

What do others think? How would you adjust them based on your thoughts and experience?

Clear skies,

#5 junomike

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Posted 29 December 2012 - 04:58 PM

Gord, I've only owned a handful from you list, but I have a few questions?

How come the Istar R-Series is last when It's supposed to be better than a standard Achro? My understanding is the R30 or R60 is reflected in the Focal Length, so an 6" F8 R30 would perform more like a 6" F10.4.

Is the APM 6" Chinese Doublet the one Markus just released?

I would think the APM 6" Russion Doublet would be closer to the 6" Flourite Doublet, but not sure.

Also, where would you fit in the hugely popular 120ED?

For me personally, CA nullifies most of the detail due to It's presence, but everyone's differnce.
I've also noticed that when some (but not all) ED Doublets are pushed to a high magnification the Image starts to "yellow" in tint. Again, to me this is bothersome, but a personal thing.

Mike

#6 Gord

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Posted 29 December 2012 - 09:41 PM

Gord, I've only owned a handful from you list, but I have a few questions?

How come the Istar R-Series is last when It's supposed to be better than a standard Achro? My understanding is the R30 or R60 is reflected in the Focal Length, so an 6" F8 R30 would perform more like a 6" F10.4.

Mike, this was my understanding as well, at least up until recently when Valery posted the thing above. That's when the wheels started turning. And now it seem obvious looking back at it...

The IStar R-series are achromat's that have been tuned to produce less obvious false color. The tuning is done by adjusting the design to bring the traditional out of focus colors (at the blue end) more into focus. And it works. There is less traditional false color seen, so a shorter FL could look like a longer FL.

I was thinking that's great, what a good idea! But I didn't understand enough about how or what was being done to achieve this. It's done by just shifting the color correction to another spot in the spectrum and just like you had out of focus color before causing purple haze, you now have it somewhere else (red).

In some ways this is all good since the eye is less sensitive to red, so it's less obvious. However, you still need the red in the case of targets that have a lot of red (and mixed) information (ex. Jupiter and Mars). What was pointed out was that when it comes to these targets, you aren't able to get a good of focus, and/or there is a loss of contrast since you are now trying to bring two totally non-focusable colors together.

And that's when the penny dropped (for me)! These alternately tuned designs work just as predicted, and well for those tests/applications, but it comes at a cost.

In the case of the IStar R's, they are just normal achromats, just like my non-R IStar achromat. But since they are not optimized so much on the traditional visual correction like the regular ones, I would expect that they will do more poorly on Jupiter.

Is the APM 6" Chinese Doublet the one Markus just released?

I would think the APM 6" Russion Doublet would be closer to the 6" Flourite Doublet, but not sure.


Yeah, that's the new one's that APM has. In their case, they are a little different still than the IStar's since they both use ED glass as well. The Chinese one uses the older LaF glass, while I think the Russian one is using something like the FPL-53 glass. Because they use ED glass, they are better corrected to start with than an achromat.

However, they apparently are tuned as well to focus on reducing the traditional false color as well. Since they are starting out ahead of the achromat to begin with, they don't have as far to go, so aren't going to be as affected. But it's still not going to be as ideal (again for our Jupiter example) as a normally corrected ED. That's why I think they are likely below an ED doublet like that, although I'm not sure there are any even out there. Perhaps something like a hypothetical Synta FPL-53 ED doublet (ie. a bigger version of the 80/100/120's).

The part that makes me raise an eye-brow is the part about it not looking any better than what the ES 127 would show. To me, a 6" is noticeably different than a 5" and we are talking APO-ish scopes here. The 6" should easily show more details.

I have been seriously watching this new APM with interest, but this has made me think. I can totally understand though the reason for a manufacturer to try to address the perceived false color as it seems to be something that people latch on to. I could already hear people panning the thing if it were to be of more traditional correction and show a slight halo on certain objects, even if it is showing wonderful details.


Also, where would you fit in the hugely popular 120ED?


Good question! Where would it fit? How does it compare to the ES 127 triplet and the like? How about as compared to a 6" F8 achro? I'm guessing in theory it should slide in just behind the ES 127...

For me personally, CA nullifies most of the detail due to It's presence, but everyone's differnce.
I've also noticed that when some (but not all) ED Doublets are pushed to a high magnification the Image starts to "yellow" in tint. Again, to me this is bothersome, but a personal thing.


I understand what you are saying, different people are bothered to different degree's by it. But as others have pointed out, you can still get good detail out of a classic achro on Jupiter, but you do have to learn to look past the purple haze. And of course, when you compare directly to an APO or reflector, you can really see there is some detail that's being masked. And as you say, it turns out to be pretty hard to even get a *perfect* APO performance no matter the glass without going to mirrors.

Clear skies,

#7 Gord

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Posted 29 December 2012 - 09:44 PM

Oh Mike,

Forgot to ask you... what do you feel about the 6" F8 achro with and w/o the Chromacor relative to your triplet in terms of Jupiter performance? Again, more data-points.

Thanks,

#8 microstar

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Posted 29 December 2012 - 10:22 PM

Well, I'll take a chance and leap in here even though I'm more of an astrophotographer and I use a small aperture scope, a Megrez 72FD. What I find interesting is that my 72FD seems to be designed to bring the blue, green and to a lesser extent yellow into closer focus at the expense of the red according to this test ( http://www.astro-for...72FD&p=35217... ) similar to what you are talking about with larger refractors. What is interesting about this test is that inserting an erecting prism into the light path brings the red into much closer correspondence with the other channels. Although this design may be partly the phenomenon you talk about with making it appear better corrected, I think in this case there is also a compromise to appeal to two different markets -- astronomical observers will detect less false color at the expense of loss of red details, but birders, who use an erecting prism, will have the red better corrected. Anybody tried putting a prism in the glass path to see if that helps correct the red in these larger refractor designs?
...Keith

#9 mikey cee

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Posted 29 December 2012 - 11:29 PM

Hi Gord. I believe we have a larger problem here of communicating our anectdotal experiences at the eyepiece. I don't doubt for a second that we have here some trade offs but it's awfully hard to qualify them accurately. Take for instance my comments about comparing my 8" Brandt to my 10" Istar. You must remember that when using my 8" over the last 30 years that the GRS was noticeably a deeper red than it is today. Heck it really faded a few years back as the SEB nearly vanished completely. Also with the exception of that one night of detecting albedo features on Ganymede I haven't really had any knockout good seeing. That night the super good seeing was short lived and the GRS was not visible at that time. Also the GRS now is more of a pinkish orange color. How's that for making up a color shade? :grin: I believe seeing plays a much larger role than optics or a person's individual eyesight compared to another's. You really need to have the scopes side by side to come to any meaningful conclusions here. Otherwise it just makes for fun conversation and speculation and not much else. I'm very satisfied with my lens and I feel that when some good seeing comes it will "knock my socks off" most assuredly. Oh and another thing that in telling these recollections at the eyepiece this lens is still in the process of acclimating to current temps and at any given moment I have no idea how far along it is in the "cooling" process. Most nights I close up shop well before the lens ever reaches total equilibrium I'm sure. ;) Mike

#10 ISTAR Optical

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Posted 30 December 2012 - 05:49 AM

Hello everyone,
Just a quick note to put some things straight. R30 and R35 doublets from Istar are NOT normal standard achromats only with color shift designed to let red hang out.. this is a totally wrong assumption and theory, created by who known whoom... I exaplained true facts about this design on numerous ocassions. The shifting of the color sensitivity is only one of many advantages of R30 and R35 designs. We use a special glass types like short flints to achieve this improved performance which falls within the Halb Apochromat standards. To achieve a greatly enhanced resolution while decreasing the spot size and substantially lowering the level of chromatic aberrationSince can not be done by tweaking the design for one color or another. We use much more expensive glass types compared to what is used in classic achromats, and this is how you get the inhanced performance. So obviously we must charge more for the more expensive lens. I hope this explains a bit.. I suggest that before anyone posts another piece of info about R30 doublet, they should look thru one and compare to any standard achromatic doublet.
Happy New Year to everyone!!!
cheers,
Ales

#11 Mark9473

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Posted 30 December 2012 - 09:39 AM

Ales, do you have a link to the spot diagrams for your R30/35 lenses?

#12 mikey cee

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Posted 30 December 2012 - 11:26 AM

Hello everyone,
Just a quick note to put some things straight. R30 and R35 doublets from Istar are NOT normal standard achromats only with color shift designed to let red hang out.. this is a totally wrong assumption and theory, created by who known whoom... I exaplained true facts about this design on numerous ocassions. The shifting of the color sensitivity is only one of many advantages of R30 and R35 designs. We use a special glass types like short flints to achieve this improved performance which falls within the Halb Apochromat standards. To achieve a greatly enhanced resolution while decreasing the spot size and substantially lowering the level of chromatic aberrationSince can not be done by tweaking the design for one color or another. We use much more expensive glass types compared to what is used in classic achromats, and this is how you get the inhanced performance. So obviously we must charge more for the more expensive lens. I hope this explains a bit.. I suggest that before anyone posts another piece of info about R30 doublet, they should look thru one and compare to any standard achromatic doublet.
Happy New Year to everyone!!!
cheers,
Ales

Yeah! There! :bow: That's the name of that tune. :grin: Mike

#13 Gord

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Posted 30 December 2012 - 01:21 PM

Hello everyone,
Just a quick note to put some things straight. R30 and R35 doublets from Istar are NOT normal standard achromats only with color shift designed to let red hang out.. this is a totally wrong assumption and theory, created by who known whoom... I exaplained true facts about this design on numerous ocassions. The shifting of the color sensitivity is only one of many advantages of R30 and R35 designs. We use a special glass types like short flints to achieve this improved performance which falls within the Halb Apochromat standards. To achieve a greatly enhanced resolution while decreasing the spot size and substantially lowering the level of chromatic aberrationSince can not be done by tweaking the design for one color or another. We use much more expensive glass types compared to what is used in classic achromats, and this is how you get the inhanced performance. So obviously we must charge more for the more expensive lens. I hope this explains a bit.. I suggest that before anyone posts another piece of info about R30 doublet, they should look thru one and compare to any standard achromatic doublet.
Happy New Year to everyone!!!
cheers,
Ales

Hi Ales,

Thanks for taking the time to provide input into this discussion. Some questions:

So the R-series designs are shifting the color correction in order to minimize the visible CA, and this would be to the blue end of focus, correct? If this is the case, then isn't this going to affect the red end of focus as Valery has explained?

I'm not an expert on this but as I understand it in lay-man's terms, a doublet can only focus two colour lines since there are only two elements. Other lines that are close to those that are in focus will be close as well, but the remaining will end up out of focus. You can choose which ones you want to focus on, but something has to be left out. To get more, you have to go to a triplet.

In the case of a classic achro like my IStar 6" F10 Steinheil, it's the violet end that is out of focus. But in the case of the R-series where they have reduced violet, they are focused more on the blue end. Isn't this correct? I understand that different glass types (high index) can improve this, but you are still using just crown/flints so there is a limit to how much things can be reduced as I understand it.

The question though here is a very specific test case of Jupiter and amount of detail visible, and how well. Has anyone done a back to back comparison of your regular achro to the R-series? For example, I know Neil English tested the 6" F10 like mine. Has he tested this new design vs. the old one on Jupiter detail?

On a more general question (to all), Keith raised an interesting point about blue color correction and how it can be affected by a prism. I've heard of prisms affecting the SA correction of refractors. Could this be the case for improving the color correction?

Thanks,

#14 Gord

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Posted 30 December 2012 - 01:29 PM

Hi Gord. I believe we have a larger problem here of communicating our anectdotal experiences at the eyepiece. I don't doubt for a second that we have here some trade offs but it's awfully hard to qualify them accurately. Take for instance my comments about comparing my 8" Brandt to my 10" Istar. You must remember that when using my 8" over the last 30 years that the GRS was noticeably a deeper red than it is today. Heck it really faded a few years back as the SEB nearly vanished completely. Also with the exception of that one night of detecting albedo features on Ganymede I haven't really had any knockout good seeing. That night the super good seeing was short lived and the GRS was not visible at that time. Also the GRS now is more of a pinkish orange color. How's that for making up a color shade? :grin: I believe seeing plays a much larger role than optics or a person's individual eyesight compared to another's. You really need to have the scopes side by side to come to any meaningful conclusions here. Otherwise it just makes for fun conversation and speculation and not much else. I'm very satisfied with my lens and I feel that when some good seeing comes it will "knock my socks off" most assuredly. Oh and another thing that in telling these recollections at the eyepiece this lens is still in the process of acclimating to current temps and at any given moment I have no idea how far along it is in the "cooling" process. Most nights I close up shop well before the lens ever reaches total equilibrium I'm sure. ;) Mike

Mike,

Isn't it the bain of going larger! Diminishing returns. We see it with all types, like my C14 vs. the C8. The refractor design as a whole though is the one that does the best with it, at least as a starting point, it has less battles to fight compared to say the SCT.

That being said, it just seems puzzling in your case that you wouldn't get *some* good performance at this point. Have you had a chance the look at Saturn yet? Valery's comments indicated that this was a target where the effect we are talking about wouldn't have much (if any) impact.

I know you have been raving about the performance in other scenarios, so it's obviously not a dud lens or bad seeing all the time. How is the lunar performance?

Thanks,

-Gord

#15 mikey cee

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Posted 30 December 2012 - 02:33 PM

Hi Gord. All I can honestly say is that in most cases I try to come across more on the conservative side in my eyepiece view evaluations. The case of "seeing" or "detecting" albedo features on Ganymede was probably one of the few times I've actually stated something this "positive" if you will. I know I definitely saw Mars this time much better than in 2003 with my 8" Brandt. It's just that expressing one's self against how other's express themselves on viewing a certain target and therefore at best drawing a meaningful conclusion is utter hogwash. You have to have the scopes and observers together at the same time with the actual scopes being tested. There are just too many variables here. Until this actually happens this all becomes just bar talk. At least that's how I look at it. As long as each of us is happy with what we've got and what we see in the eyepiece is all that really counts. Hell I'm even having trouble right now finding the proper words! But make no mistake this new Istar R30 can really split the doubles no doubt due not only to the aperture and seeing but to the smaller spot sizes.:lol: Mike

#16 RGM

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Posted 30 December 2012 - 03:21 PM

Gord, Your knowledge far exceeds mine, and I enjoy reading your comments and learning from your experience. My only concern is whether you have actually looked through an R30. My 127mm R30 is much better than the 120mm f8.3 achro I used to have. It is not at the same level as my Tak FS78, but the 2 scopes work well together. When I had the 120mm achro, I never used it and only observed with the Tak.

If you have spent time under the stars with an R30, I apologize for jumping to conculsions.

#17 Gord

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Posted 30 December 2012 - 04:00 PM

Hey Bob,

No, I haven't had the opportunity to see one in person. They aren't very common yet.

Interesting comments in the comparison between your 120 achro, although I would expect there to have to be an improvement given the extra aperature, the longer FL (which will improve the color correction), and based on my own experience with the IStar achromat lens, they are fairly well figured compared to some of the earlier Synta offerings.

How is your feeling about the Jupiter images it delivers given that it's well placed right now?

I had to look up where you are located. It's a bit north of here. One thing we could do is try to get together some time and compare them. We could put an aperture mask on my 6" to bring it down to 127mm and it would become an F12 as well. Would be an apples-apples comparison.

But I'd certainly like to hear more details about your comparisons to you other scopes and how/where you feel it is better (or worse). It helps others to get a feel for how things are, and tends to be pretty accurate when you start to get a bunch of detailed inputs.

Clear skies,

#18 Kevin Barker

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Posted 30 December 2012 - 04:01 PM

Hello Gord
I have read your message and the links thoroughly.

In Valery's thread I think he is commenting about istar's R range of Anastigmatic doublets.

I think that to suggest Istar tune the lenses to achieve R 30 etc is a bit of a stretch. I suspect that special glass(short flint perhaps) is probably another factor.

Recently on the Istar scope club Ales K ( an part owner of istar) has released spot diagrams of 6 different 150 mm doublets and triplets. The red spot sizes are larger but not hugely larger than the blue spot sizes. Although the actual wavelength versus focus sketches are missing.

Ales also mentions using short flint and different glass type's several times.

I have yet to read a review of an R30 lens compared to an achromat of similar focal length. Someone needs to set up a fair test with the same observer/aperture/seeing/eyepieces etc

Then we could find out if there is a disadvantage from tuning doublets towards the blue end of the spectrum.

I also read the comparison between the ES 127 and APM's 152 doublet that James Ling mentioned. It is not clear what caused the difference in view. Were the eyepiece's and magnification the same?

I suspect using a zoom eyepiece for planetary observing, the seeing and cool down factors could all contribute to differences.

Personally I own a fine Zeiss APQ 130/1000 telescope which i use with Zeiss 0.96/1.25" orthoscopics, AP SPL's and or Tele vue plossl's. I would never use a zoom for planetary or lunar viewing. I do sometimes use a bino viewer (but only a very good one)

I chiefly observe in seeing that is typically 6-8/10. The suggestion a scope can show the detail of the 10 inch processed pic continuously put forward surprises me.

Occasionally I get close to perfect seeing, once in Missouri using a 20 inch scope i was rewarded with seeing close to 10/10 and when Mars was in opposition in 2003 for half an hour I enjoyed 10/10 seeing with the APQ 130 as fog started to approach.

Is there something in the notion that red tuned refractors are inferior. I am not sure, istar claim to soon have a multi element corrector which will correct for CA.

Kevin Barker
Auckland NZ

#19 ValeryD

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Posted 30 December 2012 - 04:31 PM

Let me explain some things.

1. If the prism diagonal is used, it will shifts the color correction curve towards red. It will not just improve the red, it will also worsens the blue.

2. For planets observing (not just a halo around them) through achromats or semi-APO, the red correction is much more important than correction in blue. In other words planetary achromat or semi-apochromat should have edC correction or at least FeC correction. The FeD correction will show lesser perceived color error, but also the lowest contrast.

3. It is a great mistake to take in to account a Purkynie effect when designing refractor for planetary observing.
Albedoes of Jupiter, Saturn and Mars are high enough and telescope with magnifications about 50x per inch of aperture, create enough illumination on retina that an eye works here with it's day light sensitivity curve.

And I can repeat again: I personally tested 125mm F/8 ED doublet (made in japan) with color correction shifted to blue-violet (for lesser visible color errors on photos) vs 127mm F/7,5 triplet (chinese one) with best correction for blue-green-red (and violet out of control) and two 5" F/9.5 chinese refractors one with classical FeC correction and second one with FeD correction (shifted to blue). In both cases the telescopes with shifted to blue corrections loose in contrast vs telescopes with traditional corrections. I can add one important thing: the optical designers in a past knew the optics and human eye work not any bit worser than todays designers. And I think they knew optics better. ;)
All fundamental investigations in this field were done at that old good times. All classical telescopes were designed by masters and be sure, they experimented with different designs and chooses which work best.

#20 junomike

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Posted 30 December 2012 - 04:42 PM

Oh Mike,

Forgot to ask you... what do you feel about the 6" F8 achro with and w/o the Chromacor relative to your triplet in terms of Jupiter performance? Again, more data-points.



Gord, the Chromacor can be "fiddly", but once adjusted properly It does work!
In my C6R w/o the CC, I would never turn the scope to Jupiter as It's just a purple mess!
With the CC however the purple is gone and the Image is very much Improved, however the AT111EDT is still in a league of It's own. The detail and contrast of the Triplet is just amazing.
I'm not sure If It's the glass used or ??? but the AT111EDT puts up a super rich view (contrast?) of Jupiter that I haven't seen in many other scopes. It's similar to the effect the TV Plossls have on Jupiter as well.

Also, as a side note, This past summer, A friend and I (Tank) were viewing with his RFT 6" F5 Refractor and decided to see how the CC would do with the newer "red shifted glass" (on Vega).
Answer......amazing! w/o the CC, Vega was a mess with the CA extending to about the size of a Full Moon (viewed with naked eye).
With the CC in place (no spacing), The CA was reduced to a size maybe 2X or 3X the size of the Star. An amount similar to what's seen in the older (Semi) Apo Triplets, or ED scopes. And this was a 6" F5 scope, so I'm sure the F5.9 or F6.5 version would fare better.

Mike

#21 Mark9473

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Posted 30 December 2012 - 05:41 PM

the new one's that APM has. In their case, they are a little different still than the IStar's since they both use ED glass as well. The Chinese one uses the older LaF glass, while I think the Russian one is using something like the FPL-53 glass. Because they use ED glass, they are better corrected to start with than an achromat.

However, they apparently are tuned as well to focus on reducing the traditional false color as well. Since they are starting out ahead of the achromat to begin with, they don't have as far to go, so aren't going to be as affected. But it's still not going to be as ideal (again for our Jupiter example) as a normally corrected ED.


Here's a reply from Markus, who as you know is no longer allowed to post in this forum, so he posted this in the vendors forum:

A quick note to the discussion on the refractor forum regards correction of our chinese and LZOS 152 doublet ED. both are not designed to reduce purple and increase red spotts. Both are correct designed, optimized for visual observations, tthe classic tradional metthod !!!!!!!!!!!!!


Comments anybody?

#22 Gord

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Posted 30 December 2012 - 06:40 PM

Here's a reply from Markus, who as you know is no longer allowed to post in this forum, so he posted this in the vendors forum:

A quick note to the discussion on the refractor forum regards correction of our chinese and LZOS 152 doublet ED. both are not designed to reduce purple and increase red spotts. Both are correct designed, optimized for visual observations, tthe classic tradional metthod !!!!!!!!!!!!!


Comments anybody?


That's right, Markus isn't allowed to participate here. I'll include a link to his thread over in Vendors so he can respond and we can relay his inputs. It's important to have his input.

APM 6" ED Thread in Vendors Forum

So I have a question for Markus. I went over the the APM site and looked at the detailed plots for these new doublets. The ones I found very interesting were the longitudinal SA color charts.

As I understand it, these show the relative defocus of a particular colour relative to a common reference (usually green as I understand it). If I look at the LZOS double plots, I see most of the lines stick pretty close together near the centre, but the two red ones hang out to the right side. As I understand it, this means there are a bunch of colors that are in-focus (or nearly so) and the red is out of focus. The opposite is that you could focus the red, but other colors would be out of focus. Is this correct?

If I compare it to the LZOS triplet, there is a point where they all meet.

How close am I on this?

Clear skies,

#23 Gord

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Posted 30 December 2012 - 07:08 PM

Hi Kevin,

Recently on the Istar scope club Ales K ( an part owner of istar) has released spot diagrams of 6 different 150 mm doublets and triplets. The red spot sizes are larger but not hugely larger than the blue spot sizes. Although the actual wavelength versus focus sketches are missing.


Your post reminded me to go over to the IStar scope club to look at the details again. You are right, the longitudinal color plots are missing and would help tell a better picture of things, but I did see some interesting stuff in the spot plots.

You are correct, the red does hang out the farthest with best focus in green. The blue is defocused as well, just not as much. The other chart would be able to tell (as I understand it...) if the blue was brought more into focus, would the red get better or worse?

What is really interesting is how it compares to the plot for the 6" achro. The achro has things much more tightly focused than the R30, except for the purple. And, if you look at the area with the most intensity, it's closer in to the Airy disk than in the R30 and they don't extend as far. If you look at the relative diameters of extending colour, the R30's blur seems to be much wider (5 diameters vs. 3) than the achro, however it would appear to be less intense.

The achro basically has much tighter plots in my opinion with the main colours being tight.

However, one interesting thing that I think could explain some of the comments re: the R30 performance. Mike has commented several times on doubles being split very nicely with this lens. If you look at the plots, I can see how that may be the case compared to the achro. Reason I'm guess is that while the color blur is much wider, it is less intensive especially close in to the airy disk. This would make it easier to spot a companion star than in the achro if it were located in the more intense looking blur of the achro. That's a theory I'm thinking right now.

Again, there is no perfect design, so it makes sense if they aren't as good at planetary, they may be very well suited to something else.

I chiefly observe in seeing that is typically 6-8/10. The suggestion a scope can show the detail of the 10 inch processed pic continuously put forward surprises me.


I think it depends on the how capable the instrument is, the conditions, and an often overlooked factor of spending enough time at the eyepiece. I know I've seen much of the detail posted in that C11 image in the C14, but it was never all at once, never for more then a brief second (or seconds) and it took hours of observing to get it.

Clear skies,

#24 johnnyha

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Posted 30 December 2012 - 07:31 PM

Ales - any idea where the Istar R30 w/ Raycorr might end up in Gord's ranking?

#25 Kevin Barker

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Posted 30 December 2012 - 10:05 PM

Valery
Was it a well controlled fair test ?

Same observer, night, eyepiece, mag, aperture etc on same object.

Or is what you are talking(comparison of four 5 inch refractors) about just an impression.

Kevin






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