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Gord
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Color correction & Valery's thread on the subject
      #5593475 - 12/28/12 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,


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Rutilus
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5593803 - 12/29/12 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.


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Napersky
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Re: Color correction & Valery's thread on the subject new [Re: Rutilus]
      #5594425 - 12/29/12 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.google.com/translate?sl=auto&tl=en&js=n&prev=_t&...


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Napersky]
      #5594659 - 12/29/12 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.

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

Clear skies,


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junomike
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5594720 - 12/29/12 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


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: junomike]
      #5595177 - 12/29/12 09:41 PM

Quote:

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.

Quote:


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.


Quote:


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...

Quote:


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,


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5595183 - 12/29/12 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,


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microstar
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5595235 - 12/29/12 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-foren.de/showthread.php?9190-William-Megrez-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


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mikey cee
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Re: Color correction & Valery's thread on the subject new [Re: microstar]
      #5595324 - 12/29/12 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? 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

Edited by mikey cee (12/30/12 12:46 AM)


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ISTAR Optical
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5595596 - 12/30/12 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

Edited by ISTAR Optical (12/30/12 08:32 AM)


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Mark9473
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Re: Color correction & Valery's thread on the subject new [Re: ISTAR Optical]
      #5595748 - 12/30/12 09:39 AM

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

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mikey cee
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Re: Color correction & Valery's thread on the subject new [Re: ISTAR Optical]
      #5595939 - 12/30/12 11:26 AM

Quote:

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! That's the name of that tune. Mike

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Gord
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Re: Color correction & Valery's thread on the subject new [Re: ISTAR Optical]
      #5596165 - 12/30/12 01:21 PM

Quote:

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,


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: mikey cee]
      #5596176 - 12/30/12 01:29 PM

Quote:

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? 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


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mikey cee
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5596271 - 12/30/12 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. Mike

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RGM
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Re: Color correction & Valery's thread on the subject new [Re: mikey cee]
      #5596356 - 12/30/12 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.


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Gord
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Re: Color correction & Valery's thread on the subject new [Re: RGM]
      #5596433 - 12/30/12 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,


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Kevin Barker
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5596435 - 12/30/12 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


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ValeryD
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Re: Color correction & Valery's thread on the subject new [Re: RGM]
      #5596502 - 12/30/12 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.


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5596524 - 12/30/12 04:42 PM

Quote:

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


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5596625 - 12/30/12 05:41 PM

Quote:

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:

Quote:

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?


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Re: Color correction & Valery's thread on the subject new [Re: Mark9473]
      #5596706 - 12/30/12 06:40 PM

Quote:


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:

Quote:

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,


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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5596763 - 12/30/12 07:08 PM

Hi Kevin,

Quote:


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.

Quote:


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,


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5596797 - 12/30/12 07:31 PM

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

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Kevin Barker
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Re: Color correction & Valery's thread on the subject new [Re: ValeryD]
      #5597009 - 12/30/12 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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ValeryD
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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5597286 - 12/31/12 01:31 AM

Quote:

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




Yes. I have had enough eyepieces (SPL design) to keep magnifications as close as possible. the same diagonals and, of course, the same nights, the same time.

But the difference is obvious. It was especially obvious when observing Mars. I always felt the necessity to refocus the scope with shifted to blue correction curve. There was no solid focus - always something wrong. As high magnification was, as lesser defined focus.


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5597289 - 12/31/12 01:33 AM

Quote:



How close am I on this?






Your understanding of the diagram is correct.


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Re: Color correction & Valery's thread on the subject new [Re: junomike]
      #5597352 - 12/31/12 03:36 AM

Thanks for sharing this piece of info.

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Gord
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Re: Color correction & Valery's thread on the subject new [Re: junomike]
      #5597660 - 12/31/12 10:17 AM

Quote:


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,

Very interesting feedback. In your opinion, where do you think a Chromacor equipped achro would fit? Or, based on what you were describing in terms of different scopes, do you think it could fit at many levels in the list based on how close of "fit" it is to the scope, and/or it's individual setup in the scope?

I've heard the comments about the need to be precise with the setup before and plenty of negative comments if it is off, and positive ones where it is on. So it seems like it's likely something like the latter I describe. I wonder where it could theoretically be at it's best?

It would be good to hear from more users.

On a more general note, and as I indicated before, the list was just a straw-man, a starting point or hypothesis in the scientific method based on the proposal put forward that different tunings will have a different performance, in this case specifically on Jupiter/Mars. What's left is to refine it based on input, and ultimately test it with real world experimentation.

Clear skies,


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Re: Color correction & Valery's thread on the subject new [Re: ISTAR Optical]
      #5597665 - 12/31/12 10:19 AM

Quote:

Thanks for sharing this piece of info.




Yes, thank you. I guess I don't understand all the fuss going on here about a designers choice in balancing out the red and blue spot sizes.

Ales has already explained that his lenses use glasses other than the standard BK7 and F2 or F4 to achieve modest reductions in longitudinal CA. What is unclear about that?

That a designer may choose to leverage or redistribute that reduction between the blue and red is entirely up to him/her. I'm no designer but with the leverage, I'd be sorely tempted to leave the blue spot size alone and reduce that of the red. Even though this may shift the best correction slightly away from green, the overall visual correction (as well as the polychromatic correction from C-F) will be improved over that of the classic Fraunhofer lens of equal focal ratio.

My experience is similar to Valery's though using different tools. Using my D&G 11" F12 achromat, selected aperture stops, various Chromacors (O1, N, U1 Chromacor I and a Chromacor II N) with various spacers, I can manipulate the color and spherochromatic correction almost at will. I have to tell you, everytime I favor the correction towards the blue and sacrifice the red, the image suffers. I have always ended up tweaking the system towards a good balance between blue and red or slightly in favor of the red.

BTW Mike, when you used your Chromacor on the 6" F5, you were probably stopping down the aperture slightly with the nose of the Chromacor. Your color correction improvement may have been, in part, due to the stop down.

Jeff


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5597705 - 12/31/12 10:40 AM

Quote:

In your opinion, where do you think a Chromacor equipped achro would fit? Or, based on what you were describing in terms of different scopes, do you think it could fit at many levels in the list based on how close of "fit" it is to the scope, and/or it's individual setup in the scope?

I've heard the comments about the need to be precise with the setup before and plenty of negative comments if it is off, and positive ones where it is on. So it seems like it's likely something like the latter I describe. I wonder where it could theoretically be at it's best?

It would be good to hear from more users.






I can answer that with a definitive, "It depends".

From my direct experience, I'd say the CR6 with a well matched Chromacor I falls between the old Meade 6" F9 ED and the 6" F8 FPL53 doublet. With a well matched Chromacor II, it's a match for my APM 6" F8 triplet.

On axis.

The trade off when using the Chromacor is an increase in lateral color off axis. The degree and extent of the lateral color depends on the aperture, F stop and optimal spacing but generically, for a given aperture, as the F stop gets slower the spacing of the Chromacor in front of the focal plane increases and lateral color decreases.

But it's not that simple. As you move a given Chromacor forward for a slower F stop for a given lens native correction in green, the system tends to become over corrected in green.

Clear as mud now right?

I do disagree with Ales when he says "results may vary" when using the Raycor for "non-type design" achromats (R30s). Results WILL vary.

But the tweaking is part of the fun of this hobby (just like my audio hobby).

Jeff


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suburbanskies
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Re: Color correction & Valery's thread on the subject new [Re: ValeryD]
      #5597834 - 12/31/12 11:55 AM

Quote:

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.




I'm sure you are correct. And if you and other opticians keep providing us info about optics, eventually consumers will really understand their scopes

Mark


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Re: Color correction & Valery's thread on the subject new [Re: Jeff B]
      #5598019 - 12/31/12 01:49 PM

Quote:

I guess I don't understand all the fuss going on here about a designers choice in balancing out the red and blue spot sizes.




Jeff,

Thanks for weighing in on this as I know you have a lot of experience, and direct experimental hands on experience at that, on this subject. I think you answered the question about what the big deal is in your own post.

If this is a use case where this type of optimization is going to have a negative effect in some cases, it's good to know. It was a surprise to me, but then I didn't understand the interplay between the color blurs and the longitudinal CA before now (as I'm sure was the case for many others).

The examples with the Chromacor were useful too. When used right, it can be up there with the best of them.

On an alternate path on the discussion about blue vs. red color shifting, what cases would it be that the shift to blue is a good thing? I would imagine on something like Uranus/Neptune and we have seen there is an effect on the visibility of the secondary purple blur. What other cases?

Clear skies,


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5598187 - 12/31/12 03:26 PM

This thread a little bit feels like a vendor bashing toward ISTAR R30 telescopes even though the thread has many useful information.

I don't see much detail about the comparison done by Valery. Also I'm not sure Valery is a owner of ISTAR's competitor.

I don't own any ISTAR telescope but this thread seems to be able to really damage business of ISTAR and we need more data to start this type of thread especially against particular brand of scopes.

I looked at spot diagram of TSA-102.

http://www.takahashi-europe.com/en/TSA-102.optics.spots.htm

TSA-102 has much tighter focus on the blue end but FS-102 is much tighter on the red end.
Does it mean FS-102 is really that much better on Jupiter or Mars?
I both owned FS-102 and TSA-102 but the answer was no for my case even though the view was so close.

I'm not an expert on optics and this thread just feels unfair for ISTAR in particular.


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Re: Color correction & Valery's thread on the subject new [Re: Denimsky]
      #5598374 - 12/31/12 05:05 PM

Quote:


I looked at spot diagram of TSA-102.

http://www.takahashi-europe.com/en/TSA-102.optics.spots.htm

TSA-102 has much tighter focus on the blue end but FS-102 is much tighter on the red end.
Does it mean FS-102 is really that much better on Jupiter or Mars?
I both owned FS-102 and TSA-102 but the answer was no for my case even though the view was so close.





Donghun,

Thanks for adding the info about the Takahashi's here. I don't see the longitudinal graphs there, or the airy disk size for reference on the spots. However, it's a comparison against a triplet APO and a Fluorite doublet. The triplet should definitely be better, however the interesting take away is that in the case of the doublet, Takahashi chose to focus the red side more than the blue.

The discussion has been red vs. blue, and this seems to be another data point that the red side of focus is very important. Your experience seems to indicate that both work very well, but the triplet is better. This is what I would expect.

Clear skies,


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Re: Color correction & Valery's thread on the subject new [Re: Jeff B]
      #5598460 - 12/31/12 05:52 PM

Gord, I think Jeff answered more elaborately than I ever possibly could have. I would have guessed at the same placement he did for the C6R/CC, but because I didn't have many of the listed scopes, It would have been just a guess.

Jeff, Do you think (from the Diagrams) that Markus's APM 150mm F8 ED (Chinese) Refractor would be better, equal, or worse than a C6R/CC1?

Mike


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Re: Color correction & Valery's thread on the subject new [Re: Jeff B]
      #5598476 - 12/31/12 06:00 PM

Quote:

BTW Mike, when you used your Chromacor on the 6" F5, you were probably stopping down the aperture slightly with the nose of the Chromacor. Your color correction improvement may have been, in part, due to the stop down.




Jeff, I do realize that the fast F ratio creates a sudden "stop down" in 6" F5 Refractor, but I minimized the spacing by not using spacers on an AP Maxbright Diagonal (104mm) and also using an eyepiece (5mm XO) with a Field Stop close to the Diagonal Mirror.
I'm sure some Aperture was lost which aided in the reduction of CA, but I was still Impressed.

Mike


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Kevin Barker
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5598599 - 12/31/12 07:12 PM

Gord you say
"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."

It seems you have convinced yourself to this end. (the istar R30 design does not perform as well as an C F corrected achromat on planets) based on spot diagrams.

You have however not seen through an R30 scope with your own eyes and in fact are making assumptions based on what a vendor has said before about other scopes.

This (Valery's insinuation directed at the R30 Istar scopes) reminds me a bit of Newton dismissing Cassegrain's scope design a few centuries ago.

I think his thread is unfair in this respect to 2 vendors (Istar and APM). They are being dismissed theoretically without any concrete evidence.

No one has done a comparison with an istar R30 and a similar C F achromat.

I guess at least folks are not still claiming they use the same glass as normal F2 BAK 7 fraunhofer achromats.

Casting dispersions can be unfair when they are not supported by hard facts.



Kevin Barker


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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5598635 - 12/31/12 07:41 PM

Quote:

Gord you say
"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."

It seems you have convinced yourself to this end. (the istar R30 design does not perform as well as an C F corrected achromat on planets) based on spot diagrams.

You have however not seen through an R30 scope with your own eyes and in fact are making assumptions based on what a vendor has said before about other scopes.

This (Valery's insinuation directed at the R30 Istar scopes) reminds me a bit of Newton dismissing Cassegrain's scope design a few centuries ago.

I think his thread is unfair in this respect to 2 vendors (Istar and APM). They are being dismissed theoretically without any concrete evidence.

No one has done a comparison with an istar R30 and a similar C F achromat.

I guess at least folks are not still claiming they use the same glass as normal F2 BAK 7 fraunhofer achromats.

Casting dispersions can be unfair when they are not supported by hard facts.



Kevin Barker




I think you hit the nail on the head here Kevin. A lot of discussion based on numbers on paper without actually looking through the scope seems like a waste of bandwidth.


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Jeff B
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Re: Color correction & Valery's thread on the subject new [Re: junomike]
      #5598811 - 12/31/12 09:58 PM

Quote:

Jeff, Do you think (from the Diagrams) that Markus's APM 150mm F8 ED (Chinese) Refractor would be better, equal, or worse than a C6R/CC1?

Mike




On axis, perfectly matched and installed, slightly better visually.

As I recall the CR6/Chromacor I combo crossed the .8 Strehl line at ~480 in the blue and ~640 in the red on axis but remember when you get more than a couple of arc minutes off axis, the lateral color spread knocks those values down considerably. To me, the Chromacor really comes into its own for large aperture(8" and above)achromats.

Personally I'm really excited by this APM doublet. I've an old AP 6" F9 "blue tube" triplet that I just love and I paid for it right around what the APM is going for. It performs wonderfully and the differences between it and my APM 6" F8 triplet are rather subtle at focus. If I can get better performance than the old AP in a cheaper, lighter weight, shorter doublet, well....duh.

Jeff


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Re: Color correction & Valery's thread on the subject new [Re: Jeff B]
      #5598878 - 12/31/12 10:59 PM

Quote:


Personally I'm really excited by this APM doublet. I've an old AP 6" F9 "blue tube" triplet that I just love and I paid for it right around what the APM is going for. It performs wonderfully and the differences between it and my APM 6" F8 triplet are rather subtle at focus. If I can get better performance than the old AP in a cheaper, lighter weight, shorter doublet, well....duh.





Jeff, you aren't the only one who's interested in the APM Chinese ED. I was very close to breaking out the plastic, the heck with the CFO's wrath!

I've been digging around doing some reading and found some interesting numbers to help put things in context a little. I think it is important to point out how much better these APM doublets both are as compared to the achromat's. These are from the longitudinal SA numbers, and again, just to put things in context.


  • Good APO triplet(ex. APM LZOS triplet) has focus shift in the range of 0.1mm.
  • These new APM doublets appear to be in the range of 0.3-0.4mm.
  • A 6" F15 Fraunhofer his a shift of about 1.13mm according to what I was reading about at the link below by Bob Royce. I couldn't find a 6" F8 to make a good comparison, but it's my understanding that the F8 will be worse still. Maybe someone here can give the value.


The APM's are much closer to the true APO's than they are the achro's. I'm just how much they could be giving up with this correction design?

As I mentioned above, I found a very interesting read about achro color correction that Robert Royce has put together. Have a look here:

Robert Royce on achromat objectives

It gives a really good intro into a bunch of the things being talked about in this thread. An interesting factoid he mentions about a large achro (the 36" Lick) is that the logitudinal shift between the main colors is not millimeters like above, it's inches! 1.5" he states. Wow, can you imagine having to rack the focuser 1.5" to hit the different color focus points?!

Clear skies,


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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5598901 - 12/31/12 11:22 PM

Quote:


No one has done a comparison with an istar R30 and a similar C F achromat.




I agree, you are right. It's something that would be very good to see. I suggested it to Ales earlier in the thread.

Quote:


I guess at least folks are not still claiming they use the same glass as normal F2 BAK 7 fraunhofer achromats.




I don't think anyone has claimed the R30 were the same as F2/BK7 Fraunhofers. I stated at one point that they are achromats (modified ones) and you know, I'll stand by that. Here's why (although this is a poor horse that has been dug up and beaten to death so many times...).

Achromat does not mean a Fraunhofer. Achromat is just a general label regarding color correction performance. There can be Fraunhofer's, Steinheil's, Littrow's, Clarks, etc.

Not all achromats are using F2 and BK7. There are other ways to do it (see the Bob Royce link above for an example). And it's unlikely they are using ED glasses otherwise the cost would be higher and when ED glasses are used, it's almost certainly mentioned for marketing purposes.

Given the color correction of the R30's (they're still colorful), and that achromat's can come in various flavors of glass combinations, they're achromats. And they have shifted color correction to De-emphasize the traditional purple halo.

Clear skies,


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Re: Color correction & Valery's thread on the subject *DELETED* *DELETED* new [Re: Gord]
      #5599089 - 01/01/13 03:34 AM

... looking forward to your comments once you have seen through the R30 Scope ....

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Re: Color correction & Valery's thread on the subject new [Re: Kunama]
      #5599374 - 01/01/13 11:00 AM

Happy New Year all! Here's to a great 2013!

Well, it seems my integrity is now being questioned, but this does bring up an important point. I am no longer in a position to offer my personal experience in trying to verify this theory as if I report anything other than a positive result to the the product, my results will be considered biased.

I have made an additional suggestion to Ales at IStar that he have one of the 6" F8 R30's reviewed by an experienced observer. I have suggested Jeff (JeffB) would be an ideal candidate because of his extensive knowledge of the theory, observing, testing, and experimentation. He is very respected and his knowledge outstrips most here. He is also one of the earliest IStar customers and has the 6" F10 like mine which would be a good candidate to test against for the 30% improvement benefit of the R30's. I believe he also has access to other 6" F8 achromats and has (or has had) many other scopes of this nature.

At this point we need to see some real use results reported. Looks like James Ling and his friends are set to do some testing with the new APM, and I'm sure there are many looking forward to hearing their reports.

Clear skies!


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Re: Color correction & Valery's thread on the subject new [Re: Denimsky]
      #5599443 - 01/01/13 11:51 AM

Quote:

This thread a little bit feels like a vendor bashing toward ISTAR R30 telescopes even though the thread has many useful information.

I don't see much detail about the comparison done by Valery. Also I'm not sure Valery is a owner of ISTAR's competitor.

I don't own any ISTAR telescope but this thread seems to be able to really damage business of ISTAR and we need more data to start this type of thread especially against particular brand of scopes.

I looked at spot diagram of TSA-102.

http://www.takahashi-europe.com/en/TSA-102.optics.spots.htm

TSA-102 has much tighter focus on the blue end but FS-102 is much tighter on the red end.
Does it mean FS-102 is really that much better on Jupiter or Mars?
I both owned FS-102 and TSA-102 but the answer was no for my case even though the view was so close.

I'm not an expert on optics and this thread just feels unfair for ISTAR in particular.




Keep in mind that the Airy disk size for a 4" f/8 scope is just under 11 microns. The "boxes" on the Takahashi website are 100 microns across. So, any spots that are 1/9th the width of the box or less are essentially perfect. So, while it may appear that the FS is much better corrected in the red, for all practical purposes the triplet is also perfect (at least on paper--these are theoretical spots, not actual measurements). The TSA spot size stays smaller than the Airy disk size up to roughly 650nm.


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5599639 - 01/01/13 01:33 PM

Gord, I think this is a fantastic thread and It does answer some question while creating others. For me though, It's timing is perfect as I'm looking to upgrade to a larger Apo in the very near future. I just haven't decided on how or what yet?

So If the newer APM 6" F8 is going to perform (CA wise) similar to mt C6R/CC, then I see no point. If It's going to be similar to the color correction of the Synta 120ED's them I'm in and will order the lens cell replacement!

Inevitably this thread (and James Ling's as well) will most likely help me decide, but I think I'll be looking for a Triplet 130mm FPL 53 or TEC 140.

Mike


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Re: Color correction & Valery's thread on the subject new [Re: junomike]
      #5599861 - 01/01/13 03:48 PM

My guess is and especially after reading James Ling's latest update, that the new $4K APM 6" APO doublet will *handily* beat the C6R/CC image.

Happy New Year Everybody!


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Kunama
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5600134 - 01/01/13 06:48 PM

Quote:

....... I am no longer in a position to offer my personal experience in trying to verify this theory as if I report anything other than a positive result to the the product, my results will be considered biased.........




On the contrary Gord, I for one would love to hear your opinion of the R30 performance once you have had the opportunity to use it and compare it, whether your conclusions and opinions of the R30 compared to the standard achro are positive or negative matters not, but I would prefer to hear them once you have used the scope.
You may well be correct in what you advocate.


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5600189 - 01/01/13 07:26 PM

Gord
I am well aware of what an achromatic doublet is but thank you for the explanation.

My comment comes from reading the link in the material you originally posted which insinuated that normal achromats have been blue shifted to create R 30 R 50 etc

I enjoyed last night great seeing and using an old 5 inch f-6.5 lens(ex C5R??) and a home made alt-az set up and surprise surprise Jupiter had a wealth of detail I was away camping and did not have room for my Zeiss APQ130/1000 but reckon it would have been even better. Very qualitative comment really most folks just enjoy using a scope and observing the night sky. I have no idea if the 5 inch f-6.5 is CF corrected??

It is amazing what a cheap lens can deliver.

Kevin Barker


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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5602040 - 01/02/13 10:04 PM

Matt, Kevin,

I do want to say thank you for your contributions in the thread. Points, counter-points, and different voices are important to a healthy discussion. If I am provided with the opportunity to view through or test these telescopes (any telescope!), of course I take the opportunity.

You have to remember that for many of us here (at this time of year...), there aren't many chances to even see a clear sky. It's a good chance to do some reading, research, and discussion.

And speaking of simple scopes, I have a nice Synta 4" F10 achromat that turns out surprisingly nice images and a very nice amount of detail on Jupiter. I believe I commented on it in the Jupiter comparison thread above.

Clear skies,


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5602095 - 01/02/13 10:49 PM

So, the latest test information posted by James and Richard on the new APM in the other thread seems to match Valery's predictions quite well, right down to the red halo and softness. I've been doing some more reading on the APM doublets and some older discussions regarding them that have occurred leading up to now. There were some color correction discussions in this thread (page 6):

Interesting new APM 6" F8

Here, Vla (wh48gs) has posted some LA graphs for a design using OK4/OF1 vs. a fluorite doublet, both with balanced red/blue correction. I believe the OK4/OF1 on the left is supposed to represent the new LZOS 6" F8 doublet, or something like this design could be.

From the diagram, I see the de-focus distance between green and blue/red appears to be about 0.2mm (since blue/red are together). From the LA graph we mentioned above for the actual new APM doublets, the de-focus covering green/blue/red was about 0.3-0.4mm.

So, would the following be true:

1. By pulling the blue line in as in the APM design vs. the one shown by Vla above, the red line ends up pushed out more so the overall amount of de-focus is worse
2. In the balanced design shown above, there will be a slight traditional purple halo (where the red/blue overlap)
3. The larger de-focus design will have a softer image
4. The less de-focus design will be sharper, but display a bit of traditional false color

I just want to know if I'm interpreting and understanding the designs correctly.

Thanks!


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Kevin Barker
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5602311 - 01/03/13 02:44 AM

Gord
The forum's name is after all "cloudy nights", I think you could do well to venture to some clear skies and look through some scopes. Perhaps venture to the Southern hemisphere or close to the tropics!!!!

In the Istar proboard forum you have mentioned wavelength versus longitudinal focus graphs. I am not surprised they are not displayed. I have rarely seen these from manufacturers.

This information would likely be more commercially sensitive and could well be misrepresented. Although it is an easy way to see what is going on with respect to which wavelengths focus at the same point as you have stated.

I am also not surprised Istar does not send an expensive scope to your mate Jeff to check out either as they are a relatively young company and sales may not be that high to date.

I like the idea of their flexibility with respect to providing large lenses for amateurs at relatively good prices.

I also question why another vendor is sniping at another vendor.

Surely he is above this sort of caper. It is not a good look and one wonders why bother.

I am not convinced that there will be any appreciable difference between a scope with a slightly blue leaning correction as opposed to a C F correction. But maybe there is a slight difference??

I suspect any difference might be overruled by other factors.

I do not think there is a spot diagram anomaly either, there will be variation in the stated f ratio, I calculated green airy disc sizes similar to what was stated but for example I reckon the 180 mm spot diagram is for an f 8.45 scope. ( from the degree measurements in mm)

Clear skies and good seeing (hopefully through a nice refractor)

Kevin


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Re: Color correction & Valery's thread on the subject new [Re: Kevin Barker]
      #5603346 - 01/03/13 04:42 PM

Hi Kevin,

On the blue shift question, it's obvious from the testing so far with the new APM that this small amount of extra red de-focus is visible at the eyepiece (the notes about the red halos). But as you say, the real question is how much of a difference would there be to the FeC correction.

Looking back over the thread, Valery made an interesting comment that would make for a good test. He said a prism diagonal would improve the red correction, but worsen the blue. That would seem to be opposite of what this FeD corrected design is and should be closer to the normal FeC. So a good test would be to use a prism diagonal and see what effect if has on the image (better/worse/same).

On the IStar spot diagrams, I agree the calculated airy disk sizes look right to me as well. What doesn't look right is that they are displayed differently in each diagram, and the color blurs are very large, and bigger in the R30 than the achromat! The 6" F8 R30 is supposed to have a color blur equivalent to a 6" F10.4. The F10.4 is already going to have a larger airy disk than the F8, so the blur is going to have to be much smaller on the R30 relative to the F8 achromat.

But as I indicated in the IStar forum thread, it now occurs to me that it's not clear how the 30% value is measured. The sample spots shown just don't illustrate to me as clearly what's going on as the ones on Robert Royce's page I linked to above do.

Clear skies,


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5603583 - 01/03/13 07:35 PM

This is a fascinating thread and I'm hoping we'll get some genuine light, and not just heat, from it, despite notions about makers trying to sabotage their rivals. Usefully, one would address the ideas presented, as Gord and some others are doing, rather than engage in ad hominem comments. The issue is different types of colour correction, not "who" said something.

That said - and addressing the matter of looking through telescopes - Gord has already pointed to some observing experience by others that has been described on another thread here.

I'd add the discussion a couple of years ago, when Clive Gibbons detailed his experience of getting superior performance with his apo refractor by using a prism diagonal. He had previously suggested this, and provided some further details on the improvement it made. Fits Gord's latest comment re Valery's suggestion in this regard.

This was unsurprising to some of us because we'd heard details of colour correction in a lot of apo refractors being optimised for CCD imaging, not for visual use. Which meant there was a difference. So it was interesting to see Dick Suiter, in the 2nd edition of his "Star Testing" book, describing different forms of colour correction in section 12.4. THere he briefly describes,and gives "color focal-shift curves" in a diagram, for a number of 6-inch f/10 instruments.

As he remarks, there are differences to the range of wavelengths within the apochromatic range, according to design. And preferences for which design will work best will vary with the intended use.

Locally, in the Southern Hemisphere, I spend a fair bit of time looking through telescopes. Regrettably, the only Istar R30 I know of in my region is not yet on a mounting (that's being built). I'll look forward to seeing through it when it is set up, and discover whether it's better for doubles and less good for Jupiter, or great for both but with reduced blue halo, or needs a chromacorr (or equivalent) to give of its best.

Meanwhile, I think Gord's most recent note raises some good questions, which should be treated as questions, not answers (the latter approach leads to dismissive comments). I'll now put on my flame-proof jacket.


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Re: Color correction & Valery's thread on the subject new [Re: fred1871]
      #5603643 - 01/03/13 08:13 PM

Thanks Fred! I had forgot about Clive's investigations. I'll have to do some searching, I now do recall a lot of discussions about prisms.

I really need to get that Suiter book...

Clear skies,


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5606033 - 01/05/13 05:07 AM

Dear Gord,
size of the Classic Achro blur is approx. 100 while size of the R30 is approx. 40. Im no longer sure if we are looking at same diagrams here. Im leaving Europe next week, very busy, but once I have more time upon my arrival from US and Asia I will post some detailed info on this subject. Thank you for your patience,
cheers,
Ales


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Re: Color correction & Valery's thread on the subject new [Re: ISTAR Optical]
      #5606346 - 01/05/13 11:11 AM

Quote:

Dear Gord,
size of the Classic Achro blur is approx. 100 while size of the R30 is approx. 40. Im no longer sure if we are looking at same diagrams here. Im leaving Europe next week, very busy, but once I have more time upon my arrival from US and Asia I will post some detailed info on this subject. Thank you for your patience,




Hi Ales,

Thank you for taking the time to answer questions here. I've gone back and looked at the images some more and I think I understand the numbers you are referring to (micron size), but they don't seem to agree with the diagrams. I wanted to compare the regular achromat to the R30, and the only one I could find where both were shown is the 6" F8's shown here:

IStar forum 6" F8 spot comparisons

So, there are a few things that don't match between the diagrams and the numbers you quote above. For example, the airy disk size shown on each graph is different, but the scale ruler is shown as 40micron on each graph. The airy disks will be the same for each scope since they are both 6" F8, so one of the above numbers is incorrect (should be either 50 for the achro, or 80 for the R30. Or both could be off and the ruler just inaccurate.

Either way though, the airy disk on the R30 diagram is half the size of the one for the achro, so the scale of that blur diagram is actually twice the size of what it is showing relative to the achro diagram. Both have airy disks of around 10 micron.

The other part that I don't understand is the diagram for the achro. It appears to be showing the red extremely tightly focused and the blue extremely defocused. In fact, the red is as tight at the green! According the the discussion we have been having, if the red/blue focus points are left to be that far apart, then the blue is being let go farther than it should be relative to red. In this case, the overall blue defocus is what is making up the blur you are quoting as the blur size for the achro.

If I compare it to a more balanced spot diagram as shown in the link earlier by Robert Royce, I see the spot diagram is much tighter for the achro he is showing, and actually about the same amount as you show for the R30.

It appears you are considering the blur size measurement to be the maximum defocus value in microns for either the red or blue defocus, whichever is worse. Is that correct?

I think it's a good measure from what I have seen based on the discussions here.

Looking forward to your clarifications.

Thanks,

Edited by Gord (01/05/13 11:18 AM)


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5606360 - 01/05/13 11:16 AM

Hi Ales,

Forgot to add; the easiest way to show what you are trying to describe is the LA graphs and matrix spot diagrams for the corresponding designs similar to what Markus has done for this APM offerings. These show exactly the performance differences between the designs.

Could you provide this information?

Thanks!


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5615659 - 01/10/13 02:41 PM

Gord,

Roland says it best:

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

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.


I noticed your list. Find a way to test the telescopes and find their spectrum.Then you can pigeon hole it in Roland's list.

2 Cents


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fred1871
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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5657060 - 02/01/13 08:03 PM

Gord, did we ever get a reply/response from Ales about the particulars of the Istar optical designs, that would match the kind of information you mention Markus providing for APM designs?

I and I'm sure a lot of others would like to know more about the Istar scopes and how their optical design differs/compares with other offerings. I don't think I've bought my last refractor yet.


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Re: Color correction & Valery's thread on the subject new [Re: fred1871]
      #5658500 - 02/02/13 04:54 PM

Hi Fred,

No, I haven't heard any more from Ales other than what he had indicated to me and that I had mentioned above. As you say, I think there is lots of interest in seeing more info.

I did have a little info sent to me on a test on an IStar lens, but it was one of the classic achro's, not the R-series. Quite a detailed analysis, see here:

IStar 6" F15 classic achro test

I've also been doing more reading over on the Astro-Foren site and the tests that Rohr has been doing. Now that I've got a bit better understanding of what some of these things mean, the analysis posted there makes a lot of sense. Unfortunately I still don't understand German, so a lot of it is, well, still German to me!

Here's the ones for the two new APM doublets that we had been discussing here:

APM LZOS 6" F8 doublet ED
APM Chinese 6" F8 doublet ED

It seems that Rohr was making note about the different focus point for the red as compared to the others. You can see in the images how this affects the image by showing the red halo's we've been talking about.

They don't have any tests for IStar's, but they do for some other Chinese sourced achromats (DKD, Intane, etc.). It's long been suspected that the IStar's are actually coming from one of these sources. In fact, there were ad's from one of these (I can't recall who) saying they could make anything you want to order, and print whatever you want on the front lens ring. Certainly seems plausible.

Related to this, I saw one test on Astro-Foren for an un-named Chinese sourced achromat that was not a traditional C-F corrected achro, but one with the blue more tightly focused, and the red hanging out. That sounds quite similar to the IStar R-series. This sample didn't look particularly good and unfortunately they didn't show the simulated star images in their full color form, only the green (where they are best corrected). Compared to the APM lenses above and the amount of red defocus that they have to this Chinese lens, I can only imagine how poor things would look! Have a look here:

Chinese 180 F9 achro with Fed color correction

I also still have yet to find any reports for these R-series lenses other than the one that Mike C. has. I consider his to be an outlier in a sense that it is so large and there are so few other large refractors, it's hard to get a really good comparison. What's really needed are some reports in the 5-6" range. These have been out for some time now and yet there isn't any info and I seem to see a number of them for sale used.

Anyway, I don't think I've built my last refractor yet either! I'm looking at something in a completely different class at the moment. I'll start a new thread for that once it gets going.

Clear skies,


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Re: Color correction & Valery's thread on the subject new [Re: Gord]
      #5660576 - 02/03/13 06:58 PM

Gord, thanks for the links, they make interesting reading (and viewing). The two APM scopes, different designs and designers, both appear to be decently designed and figured.

The same can't be said for the anonymous E-bay achromat. The intra- and extra-focal images, compared to the Zeiss, are particularly revealing. OMG!! in the case of "anonymous". And the mid-range sag in the MTF implies poor planetary images, as noted.

So, yes. cleaning up the visible blue haze of an achromat sounds good if it can be done without getting the red into a problem. If you fix both, I think it's called an apochromat.

Whether, with an achromat, the blue can be reduced without creating other problems I don't know. It's a lovely idea. If Istar have managed it they deserve to sell a lot of such scopes. But we're still waiting for the extra information that would tell us how the R-series are chromatically corrected - in detail, similar to Rohr's reports.

Perhaps someone will send their Istar R off to be fully tested? - or there are surely some owners of these already who can comment in detail on what they see through their telescopes - planetary views, double stars, etc. Because Istar's claims suggest a useful step forwards, those of us who love refractors would like to know if that's truly the case.

Edited by fred1871 (02/03/13 06:59 PM)


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Kevin Barker
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Color Correction Istar R30 Scopes new [Re: fred1871]
      #5660822 - 02/03/13 10:18 PM

I note a report from Yukon where an Istar R30 scope is compared to a meade achromat.... I will copy it below

"I can just say: Oh boy, oh boy it is amazing view that I saw couple of nights ago through my Istar 6" R30! Finally the Yukon had a warmer weather, just about -4 Celsius. I took the scope outside well before dusk. A little ice-dew shattered on the tube and the 2" diagonal. but when it became dark, I was ready for observing my favorite Jupiter. I can tell that the planet's stripes were amazing in my 16mm Zeiss eyepiece (75x)! I was so excited about the view that I started to push the magnification quickly and gradually to 120x, then to 200x, then even to 300x! Well, far better images than in my meade. In other words, they are not even comparable on planets. I have actually never seen the Jupiter so crisp and detail in my meade refractor compared to my new r30 anastogmatic lens! In addition, the sky was partially cloudy, so that the Jupiter got faint frequently because of the high atmosphere cirrus clouds passing by! Can you imagine what this scope can do in an actually excellent night? The same night I tried to put in my 5x powermate in the focuser then I slid my 25 mm orthoscopic eyepiece in it. The image of the planet did not fell apart, however the sky conditions limited my useful magnification to 250x. I am very satisfied with this lens. In addition, using the scope in warmer temperature yielded way less colouring than in minus 40 degrees temperature. I did not see ANY false colour using the 16 mm eyepiece. When I pushed the magnification higher, I started to notice some faint violet colour around Jupiter but it was so insignificant and faint that it did not even bother me at all."

Read more: http://www.istarscopeclub.proboards.com/index.cgi?action=recent#ixzz2Jthass1d

Hardly a fair test and probably a fair bit of "CONFIRMATION BIAS" involved as it is a newer scope.

Interesting nonetheless.


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fred1871
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Re: Color Correction Istar R30 Scopes new [Re: Kevin Barker]
      #5660929 - 02/03/13 11:47 PM

Thanks, Kevin. Sounds positive, though I expect we'd need to know how good or bad the Meade objective was to get a full picture - some Meade doublets were good, some pretty bad : lots of problems with lens cells.

I had a look at some of the threads on the Istar board when I looked up the original version of what you've quoted. Interesting stuff, though I see a lot of defensiveness there including the "who cares about theory" style that I find troubling (anywhere it occurs). But there were some useful and positive comments from observers too, which helps fill in the picture. And I'd already seen Mikey Cee's favourable remarks about double stars through his Istar.

As Ales indicated in one thread that he was planning to provide more data early this year I'll look forward to that happening. Meanwhile, I'm on the hunt for an Istar R30/35 scope to look through in my part of the country.


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Kevin Barker
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Re: Color Correction Istar R30 Scopes new [Re: fred1871]
      #5660990 - 02/04/13 12:28 AM

Hi Fred
I have ordered a large R30/R35 lens for an ATM project as you may know from the Istar proboard site.

I have to admit I own a pretty good planetary scope so i am not expecting a miracle with respect to planetary performance.

I suspect aside from the talk of CEF vc CdE correction etc some may overlooked the fact that the R30/35 lenses use slightly special glasses to achieve the stated improved correction.

Trouble is without the dispersion info for the glass used it is impossible to calculate.

For example using the glass types quoted by Zeiss for the AS100/1000 scope I calculated the secondary spectrum to be approximately 1/2450 f. This is about 40-45 % better than a standard F2 bak 7 doublet,

I own a superb wee Zeiss AS80/840 and yes it performs a lot like a f- 15 3 inch achromat. And yes as Valery and others have stated it does produce a bluish secondary spectrum. Traditional achromats tend to show a magenta sec spec which is combinaton of colours.

I do not know if having a correction towards the red or blue end is better for planetary??

I am sure in time we will know.

Edited by Kevin Barker (02/04/13 01:45 AM)


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Gord
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Re: Color correction & Valery's thread on the subj new [Re: Kevin Barker]
      #5663211 - 02/05/13 09:58 AM

Hi Kevin, Fred, everyone,

I decided to reach out directly to Valery to ask for his input on this and to clarify some of his previous comments. I know the TOS limit's what vendors are allowed to contribute so this way I could get a bit more info. I'm happy he took the time to respond (with such detail), but he didn't comment directly on some things (seems to be treading carefully...). He did provide a lot of information on the background of many things (much of it I didn't know...) and since I didn't ask for explicit permission to post verbatim, I'll paraphrase and organize his many comments (along with some of my own commentary).

1. The types of glass being used will determine the color correction abilities for the design separate from anything that is done specially with adjusting color correction balance.

2. There are many kinds of glass available from different glass catalogs. Not all are suitable for use even though some with higher indexes would seem to be better from a simplistic point of view. Also, not all glasses are available in all catalogs (ex. some are only from Japan, Russia, etc.) and various glasses have different associated costs.

3. He made an indirect comment about short flints being used (for these new achromat designs in question I'd say) and that there are some specific reasons why this wouldn't be the case;
a). short flints can be used in an achro design to achieve better color correction, however it will only work in slow designs (F12 and slower). He made a reference to the Zeiss AS (see point below)
b). there are no short flints that exist that can provide improvements for faster designs (< F12) that exist in *any* glass catalogs
c). there was one short flint that he indicated that could be used in a slower design that is available in the Chinese catalog, but it's cost is very high (as high as the ED glasses). Apparently so high that the cost of the glass alone would be more than the cost that these objectives are selling for. I know these lenses are sourced from China, so I'd say it's likely they are using that glass source and whatever is available from there. We do see some that use other sources (ex. some of the Synta's using FPL-53), but the prices of those do see to reflect the higher cost of materials

4. He talked a bit about the Zeiss AS design and that it did have a meaningful improvement in color correction, but it comes with some conditions;
a). the design only works at slower speeds
b). the glass type needed for them is not available in China and must come from Schott. The cost of such objective would apparently be 4-5x more than what these Chinese achromats are going for
c). the glass is no longer made in larger sizes (strips available now are only 160mm wide) and it's not possible to get even a useable 150mm blank out of them (there would be severe astigmatism)

5. He said the absolute proof is in the spot diagrams and LA graphs for how a design is doing for color correction. Any design with improved color correction will absolutely show a clear reduction in the total spot size. If there isn't this absolute reduction, then there isn't an improvement in correction and any changes in appearance are just then changes to balances in correction or which colors to favor.

He re-iterated that this choice of balancing is up to the designer, but that historically what has been favored has been what he had indicated in a post already (and used by the Clarks, etc.).

Very interesting background to all this I found. In my position as a lay-person, I could never know what does or doesn't exist, what is or isn't possible. Unrelated to this discussion, I found the remarks about the Zeiss AS glass interesting (the part about problems with availability). We have been hearing this quite often it seems about no more suitable glass being available in larger sizes for all different kinds of scopes. This is just another case in point and an unfortunate one to seeing bigger/better (and affordable) optics produced.

What I take from all his comments and how they fit into the discussion is that it's really coming down to a few simple things:
  • money - if it costs more to produce than is being charged for it, it can't work and isn't being done
  • possibility - if it just can't be done, then it isn't being done
  • alternate explanations - if there is an alternate explanation that satisfies the above conditions and explains what is observed, that is likely the answer


To me this information combined with what I have seen shared by IStar (spot diagrams, etc.) makes me question this design. There needs to be some real hard details to show that it is different than just an alternate correction design of a normal achromat. Hopefully we'll see some details posted by Ales soon.

Clear skies,


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Kevin Barker
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Re: Color correction & Valery's thread on the subj new [Re: Gord]
      #5663601 - 02/05/13 01:35 PM

Hi Gord

Thanks for your reply, I am a layman too.
I am not an optician but I am educated to the degree grad level in Physics and post grad education related to Physics. I have read various texts and web resources on doublet design, and from what i have it is possible to improve the colour correction with short flint.
Zeiss made two several short flint scopes with f- ratios under f-12. AS80/840 and AS100/1000?? Both work superbly. I own a AS80/840 and a friend has a 100/1000. Yes they are small apertures.

That appears to contradict Mr Valery's statement on short flint lenses !!!

The prices and costs you quote are very vague.

If Istar are making lenses of the quality they claim, they must have glass types which work as per standard doublet design.

I imagine glass costs etc are dependent on quantity and business relationships. It does appear however the R series lenses are made in low quantities. However he should know this if he is making refractor objectives. I also agree there are limited suppiers, Schott, Ohara and Hoya being the big three i am aware of.

I suspect he is still angling at "having a go" at the other vendor, Gord. Perhaps he is worried about losing business or market share, perhaps he is genuinely concerned for customers/astronomers?? We do not know.

There does not appear to be much "love lost" between the vendors reading other forums etc

In the mean time I think we should judge companies from personal observations and facts, not assertions not based on hard evidence,

I reckon we will know soon, then Mr Valery can be vindicated for his concerns or questioned for his tactics.

Kevin


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ValeryD
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Re: Color correction & Valery's thread on the subj new [Re: Kevin Barker]
      #5664084 - 02/05/13 06:03 PM

Quote:


Zeiss made two several short flint scopes with f- ratios under f-12. AS80/840 and AS100/1000?? Both work superbly. I own a AS80/840 and a friend has a 100/1000. Yes they are small apertures.

That appears to contradict Mr Valery's statement on short flint lenses !!!






Kevin, you obviously have not enough knowledges in optics and in astro objectives designs in particular. You also don't know what is going on in this business.
So, let me tell you some basic things:

1. AS type objectives can't be scaled up without severe worsening of aberrations correction. You can scale such objectives down, but not up, at least not in 1.5x or even 1.8x and 2x in aperture without increasing F/D ratio.

2. Again the key short flint glass in the AS design is not available from China. Schott glass costs more at least 4-5x then both chinese glasses in their achromats.
Also, the limitation of maximal diameter.

3. The worst info for you: on the spot diagram shown for standard achromats and for objectives with reduced spots the effective RMS spot size is smaller in standard achromats. This one fact is worth of all other hundreds of words. If the secondary spectrum is reduced really (in other words with special glasses), then reduction of spot sizes results also with reducing of effective RMS spot size - in other words, energy concentration becomes better.
But spot diagrams shown, show us opposite - RMS spot size increasing = energy concentration reduced and = contrast reduced.

4. Countless time, since 2001, I was asked by amateurs to start to produce achromats of better quality with Chromacors pre-installed even at higher price than chinese OTAs + Chromacor. I was also countless times asked for starting to offer another service - to buy chinese 6" achromats, install matched Chromacors in them and then sell at a price. This is NOT MY BUSINESS. This is not the level of work for my company. We will never produce any kind of achromats - too low level of optics, too cheap optics = too low profit.

If you don't believe - your problem and not mine. If you don't believe in what I say in optics - your problem, not mine. I am not against that you will buy even singlet chromate and build even air telescope and enjoy it = your deal, your problem.


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Kevin Barker
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Re: Color correction & Valery's thread on the subj new [Re: ValeryD]
      #5664617 - 02/06/13 02:56 AM



Thank you Valery for your response and your advice...

It does make sense re the longer f ratios required for larger apertures with AS scopes.

One part I am confused by is when you say the following...

" ...... on the spot diagram shown for standard achromats and for objectives with reduced spots the effective RMS spot size is smaller in standard achromats..... "

Are you referring to the spot diagrams for Istar objectives ?? If not which specific objectives are you referring to.



Kevin


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Fomalhaut
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Re: Color correction & Valery's thread on the subj new [Re: Kevin Barker]
      #5664659 - 02/06/13 05:10 AM

Kevin, Valery,

The main reasons of the superiority of the original Zeiss-AS (means "Astro-Special") design over standard Fraunhofer designs is based on...

a) ...better correction in the red part of the spectrum (where the relative area is biggest). Actually, the red curve bends back to almost zero at the edge of the objective. The blue part of the spectrum is less important for planetary than the red part.

b) ...the asphericization (by hand of a master-optician) of one of the four lens-surfaces!!! Only this way the AS' BK7/KzF2-glass-combination achieves its superiority over standard achromats of the same f-ratio, especially on planetary. This certainly is another reason for the Chinese refraining from trying to copy AS-objectives as they're doing everything else...
(Details see in "Astrooptik" by Uwe Laux, former Zeiss optician).

Chris


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ValeryD
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Re: Color correction & Valery's thread on the subj new [Re: Fomalhaut]
      #5665158 - 02/06/13 12:33 PM

Quote:

Kevin, Valery,

The main reasons of the superiority of the original Zeiss-AS (means "Astro-Special") design over standard Fraunhofer designs is based on...

a) ...better correction in the red part of the spectrum (where the relative area is biggest). Actually, the red curve bends back to almost zero at the edge of the objective. The blue part of the spectrum is less important for planetary than the red part.

b) ...the asphericization (by hand of a master-optician) of one of the four lens-surfaces!!! Only this way the AS' BK7/KzF2-glass-combination achieves its superiority over standard achromats of the same f-ratio, especially on planetary. This certainly is another reason for the Chinese refraining from trying to copy AS-objectives as they're doing everything else...
(Details see in "Astrooptik" by Uwe Laux, former Zeiss optician).

Chris




Chris,

I know this since 1985.


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Fomalhaut
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Re: Color correction & Valery's thread on the subj new [Re: ValeryD]
      #5665352 - 02/06/13 02:08 PM

Quote:

Quote:

Kevin, Valery,

The main reasons of the superiority of the original Zeiss-AS (means "Astro-Special") design over standard Fraunhofer designs is based on...

a) ...better correction in the red part of the spectrum (where the relative area is biggest). Actually, the red curve bends back to almost zero at the edge of the objective. The blue part of the spectrum is less important for planetary than the red part.

b) ...the asphericization (by hand of a master-optician) of one of the four lens-surfaces!!! Only this way the AS' BK7/KzF2-glass-combination achieves its superiority over standard achromats of the same f-ratio, especially on planetary. This certainly is another reason for the Chinese refraining from trying to copy AS-objectives as they're doing everything else...
(Details see in "Astrooptik" by Uwe Laux, former Zeiss optician).

Chris




Chris,

I know this since 1985.




Thanks for the confirmation. But you have not mentioned it here, or have you?
In this case it's just a hint for the others...

Chris


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ValeryD
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Re: Color correction & Valery's thread on the subj new [Re: Fomalhaut]
      #5665405 - 02/06/13 02:35 PM

Quote:



Thanks for the confirmation. But you have not mentioned it here, or did you?
In this case it's just a hint for the others...

Chris




Such details are not very useful for most, especially if they don't have special knoweleges.
However, thanks for posting these details here.

Valery.


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Kevin Barker
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Re: Color correction & Valery's thread on the subj new [Re: ValeryD]
      #5666292 - 02/07/13 12:10 AM

Valery, Gord, Chris

I note when checking Istar 's website today there are new spot diagrams posted for some of the R lenses(loaded 5th Feb)

I have had a quick look at them. It is not clear from my initial look as to whether the ink colours correspond to wavelengths. The numbers above 0.47 up to O.62 may well be microns/micrometres.

If so then the red does for example hang out a fair bit. 5 or so times the diameter of the airy disc for the 150 mm f-8. It is difficult to tell which wavelength has the smallest spot size relative to the airy disc (green?).

I'd love to see the equivalent spot diagram for a semi apochromat like the AS lenses.( I have seen a f-10 AS100/1000 spot diagram on a Zeiss pamphlet)

There is also data re spatial frequency and other info but it is not easy to read on my old apple screen.

I would be interested in what you guys make of it them and also what you would think a lens like this would perform like.

I suspect there is not enough information to really tell.

I guess also is this a product of using a special glass?e.g. short flint or simply a design to make the lens appear to have less CA in focus as has been insinuated and suggested. I say this irrespective of whether the glass is expensive or beyond the manufacturing capability of the Chinese.

Further am I right in thinking if the lens design is to scale as shown in the diagram(from a quick glance) there seems to be a large air gap and the inner surface seems to have a long radius of curvature. The second and third radii look to have similar but opposite radii ??

It looks like a design i have seen before, perhaps Littrow or Clark. It could well not be to scale.

Am i right in suspecting a hint of (skepticism about Chinese manufacturing) in this thread. ? I guess we all hear about and have seen low quality mass produced products with poor quality control from some China. I would hasten to add I personally think under estimating Chinese manufacturing is at your peril.

I recall similar sentiment to things made in Japan 30 or more years ago. This was based initially on fact but products soon improved.

The world is changing. I say this irrespective of whether or not Istar or any other Chinese manufactured optics are good, bad or otherwise.

Kevin

Edited by Kevin Barker (02/07/13 12:37 AM)


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Gord
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Re: Color correction & Valery's thread on the subj new [Re: Kevin Barker]
      #5667908 - 02/07/13 09:39 PM

Hey Kevin,

Man, what a crazy week...
Quote:


Thanks for your reply, I am a layman too.
I am not an optician but I am educated to the degree grad level in Physics and post grad education related to Physics.




Yep I know what you mean. I can't claim to be a layman in other areas, but it certainly fits in this subject!

Quote:


I have read various texts and web resources on doublet design, and from what i have it is possible to improve the colour correction with short flint.
Zeiss made two several short flint scopes with f- ratios under f-12. AS80/840 and AS100/1000?? Both work superbly. I own a AS80/840 and a friend has a 100/1000. Yes they are small apertures.




I think that's one of the keys there. Those two are getting down into smaller sizes, and neither is particularly "fast", certainly not like F8 or F5. Refractors don't scale well and the bigger Zeiss lenses quickly get to longer F-ratios. I think there is a ~5" in there as well (F12 or 15?)?

Since the subject has been on the 6" sort of range, I took Valery's comments to be related to that where the Zeiss is a 6" F15.

Quote:


I suspect he is still angling at "having a go" at the other vendor, Gord. Perhaps he is worried about losing business or market share, perhaps he is genuinely concerned for customers/astronomers?? We do not know.




I think that comment has been brought up in this thread already before, but I just can't see it. The two are in totally different leagues and don't compete directly. Not even close! I see Valery has already made a comment to that effect, and personally I can't see him being affected in a negative way by IStar's business. I can only take that he is interested in the subject and wants to help further peoples understanding.

Quote:


In the mean time I think we should judge companies from personal observations and facts, not assertions not based on hard evidence,




Yep, it will be good to hear some details from the observing field. There are now a couple of new reports out I see which is good to see coming. Both were enthusiastic, but light on details and direct comparison. The one for the 6" could have been written to describe the views through either my 60mm A-P, or my C14! Hopefully more to come.

On your note about the new info on IStar's site, good catch! I hadn't seen this before. I don't think there is anything wrong with your screen though. The diagrams are small, kind of fuzzy and hard to read. And most importantly lacking the real comparison info. There is a bunch of things shown, but nothing about the LA characteristics, the spots at various wavelengths, etc.

One last point that you brought up re: Chinese sourcing. I think at one time there was certainly a strong sense of that sentiment when it was earlier days and the quality wasn't as good/consistent. Personally, I don't get that feeling so much these days and it's been pretty aptly demonstrated that the Chinese are capable of delivering the goods and are continuing the improve, much like the Japan of the past reference has shown. We're also see that as things do improve, there is a difference in cost as well. But, they can still source pretty much whatever the buyer asks for (from great to, well, ...), reflected in the cost of course.

Clear skies,


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Re: Color correction & Valery's thread on the subj new [Re: Fomalhaut]
      #5667947 - 02/07/13 10:06 PM

Quote:


a) ...better correction in the red part of the spectrum (where the relative area is biggest). Actually, the red curve bends back to almost zero at the edge of the objective. The blue part of the spectrum is less important for planetary than the red part.

b) ...the asphericization (by hand of a master-optician) of one of the four lens-surfaces!!! Only this way the AS' BK7/KzF2-glass-combination achieves its superiority over standard achromats of the same f-ratio, especially on planetary. This certainly is another reason for the Chinese refraining from trying to copy AS-objectives as they're doing everything else...
(Details see in "Astrooptik" by Uwe Laux, former Zeiss optician).



Hey Chris,

Thanks for the input on the Zeiss. The first point is interesting and seems to be another data point in favor of the red channel being extremely important to planetary viewing as we have been discussing in this thread.

Does that info come from the source you cite? Is that a book?

I was doing some more reading online and came across another reference to the importance of the color correction and specifically the red spectrum from a historical perspective. See this link on making a refractor by Roger Ceragioli:

Color Correction and Spherochomatism

See the 5th & 6th paragraph where he talks about the traditional balance and questioning if the FeD is actually a mistake. He leaves it as an open question and that the real answer is in how it performs on star test and planetary detail. He makes a reference to a "well known contemporary designer". I wonder who that might be?

There was another reference I had found too, but can't seem to locate it at the moment.

Clear skies,


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Kevin Barker
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Re: Color correction & Valery's thread on the subj new [Re: Gord]
      #5668124 - 02/07/13 11:38 PM

Gord
I remember reading a reference re D&G a few years ago when I think the owner talked about the complexities of scaling up aperture with respect to going from 5/6 inches up to 8 with an achromat.

It involved a lot of hard work.

In 2011 I had the opportunity to look through a friend's Zeiss AS150/2250 alongside his APQ 130 and AP 180. Seeing fairly good and viewing the same objects. Saturn was poorly placed but one target. Orion et al and various doubles were viewed.

The big AP was the star really as the increase from 5 to 7 inches is quite big. The AP180 had real punch on deep sky. The 130 mm Zeiss gave the sharpest views as dictated by seeing and the smaller aperture.

The big AS was not an apo by any means compared to the 130 Zeiss and 180 AP it threw up a fair bit of blue. If the other scopes were not there I would have been happy to use the AS though.
I think however the old Zeiss AS scopes are probably a step above the fast and large achromats that Istar are selling as anastigmatic. They were also quite expensive and the lenses alone were available up to the early 1990's.

I agree with your comments re fast refractors in larger apertures.

If I had a decent sized observatory I'd love to have one of to get my hands on an old AS 150/2250 or 200/3000 scopes.

The big 7 inch f-8 Istar lens I have ordered might well end up in an ota similar to the one you made. Made to be portable and hopefully not too heavy. I am not anticipating it to be even a semi apo, hopefully just a good fast achromat. I certainly hope it has better optics than the 180 mm 1600 mm Chinese lens that was tested in Germany(from your link). It was not even 1/4 wave PV and had a poor Strehl in green etc

I will endeavour to read your color correction link. I am about to be snowed in with work to as the academic year starts to wind up.

Clear skies

Kevin


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Fomalhaut
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Re: Color correction & Valery's thread on the subj new [Re: Gord]
      #5668347 - 02/08/13 05:09 AM

Hi Gord,

Yes, "Astrooptik" by Uwe Laux is a book written in German.
It provides 250 graphical and 167 tabular charts which might be useful even if one doesn't understand German - all depending on the reader's personal level of education in optics, of course.
I have the 2nd edition from 1999. The ISBN is 3-87973-928-5.

Chris


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