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Color correction & Valery's thread on the subject

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#51 Gord

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Posted 02 January 2013 - 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!

#52 Kevin Barker

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Posted 03 January 2013 - 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

#53 Gord

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Posted 03 January 2013 - 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,

#54 fred1871

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Posted 03 January 2013 - 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. :grin:

#55 Gord

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Posted 03 January 2013 - 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,

#56 ISTAR Optical

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Posted 05 January 2013 - 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

#57 Gord

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Posted 05 January 2013 - 11:11 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,

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,

#58 Gord

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Posted 05 January 2013 - 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!

#59 Napersky

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Posted 10 January 2013 - 02:41 PM

Gord,

Roland says it best:

http://geogdata.csun...land/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

#60 fred1871

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Posted 01 February 2013 - 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.

#61 Gord

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Posted 02 February 2013 - 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! :lol:

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,

#62 fred1871

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Posted 03 February 2013 - 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. :grin:

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.

#63 Kevin Barker

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Posted 03 February 2013 - 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.istarscop...t#ixzz2Jthass1d

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

Interesting nonetheless.

#64 fred1871

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Posted 03 February 2013 - 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. :)

#65 Kevin Barker

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Posted 04 February 2013 - 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.

#66 Gord

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Posted 05 February 2013 - 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,

#67 Kevin Barker

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Posted 05 February 2013 - 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

#68 ValeryD

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Posted 05 February 2013 - 06:03 PM

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.

#69 Kevin Barker

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Posted 06 February 2013 - 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

#70 Fomalhaut

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Posted 06 February 2013 - 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

#71 ValeryD

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Posted 06 February 2013 - 12:33 PM

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.

#72 Fomalhaut

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Posted 06 February 2013 - 02:08 PM

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? :p
In this case it's just a hint for the others...

Chris

#73 ValeryD

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Posted 06 February 2013 - 02:35 PM


Thanks for the confirmation. But you have not mentioned it here, or did you? :p
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.

#74 Kevin Barker

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Posted 07 February 2013 - 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

#75 Gord

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Posted 07 February 2013 - 09:39 PM

Hey Kevin,

Man, what a crazy week...

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!

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.

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.

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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