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Equipment Discussions >> Refractors

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olivdeso
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Reged: 02/20/11

Loc: Paris FR
Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5873339 - 05/20/13 09:42 AM

In this other report from AiryLab / France you can see on p12, that the focus shift between the 3 colors is 60µ. Very similar to concurent products. The TOA has a little bt longer Fl ratio which helps in containing the longitudinal chromatic error.

http://airylab.net/contenu/mesures/astro/rapport%202011-40001-a.pdf


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5873407 - 05/20/13 10:22 AM

Well, what I see is that Rohr gives the relative defocus vs. e-line going from -68 microns in F to +32 microns in the C line. That's 100 microns, or 0.1mm, at least five, according to the graph, or ten, according to Takahashi China, times the TOA's advertised shift. If we look at the graph, the entire range is within less than 0.02mm. Can Rohr control measurements to that level of precision? He doesn't specify how does he determine best focus position. What I've seen of his site - admittedly, not much - doesn't make me confident. Takahashi Europe shows this same TOA130 chromatic shift graph. Chromatic correction is not subjected to significant changes vs. design, as long as it uses design glasses. That includes spherochromatism. E-line correction will be less good, but it is not a factor affecting secondary spectrum. Rohr's test is in disagreement with the official data, and if I am to take my pick, it's Takahashi. Still remember the mess that Rohr made of testing a simple system such as Dall-Kirkham (Tak Mewlon). The other testing site, the French one, didn't impress me either. Someone claiming he's from the site didn't show he knows why the Zernikes weren't used properly, or how their longitudinal aberration graph was obtained (CN thread). These days, sophisticated optics software is readily available, and may get used by those not quite up to the task.

Vla


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5874264 - 05/20/13 05:18 PM

Quote:

Chromatic correction is not subjected to significant changes vs. design, as long as it uses design glasses.




I should have said as long as it uses design glasses and have power components of the objective near optimally balanced. If, for instance, the positive component is weaker, the blue end shifts away, and the red end comes closer. Still, this means that the error is induced to both ends, if starting from near-optimum balance (design).

On the other hand, the info on the Tak is not consistent as well, although not nearly as much. Confusion seems to be coming from vendors' interpretations and assumptions, but part of it is that the official info from Takahashi is scarce and incomplete. I am not aware of any glass combinations that would produce nothing even close to such a tight chromatic shift curve in a this large/fast triplet. The possibility is that it shows chromatic shift based on best, not paraxial (usual) foci. But one can only guess. It's kind of strange that people do not get specific info on anything that they are paying good money for.

Vla


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olivdeso
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Reged: 02/20/11

Loc: Paris FR
Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5874527 - 05/20/13 07:49 PM

Quote:

The other testing site, the French one, didn't impress me either. Someone claiming he's from the site didn't show he knows why the Zernikes weren't used properly, or how their longitudinal aberration graph was obtained (CN thread). These days, sophisticated optics software is readily available, and may get used by those not quite up to the task.

Vla




Stange... the guy is a professional guy with profesionnal tools, he knows his job.
Whatever the risk of error on the focus plane measurement is very low.

But to me the most intresting in these test reports are not the absolute values, but the relative values from one scope to another tested in the same conditions by the same operator.


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5874974 - 05/20/13 11:43 PM

Quote:

Stange... the guy is a professional guy with profesionnal tools, he knows his job.




Well, take a look.

Quote:

Whatever the risk of error on the focus plane measurement is very low.




Anything specific?

Vla


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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5875116 - 05/21/13 03:30 AM

Quote:

But to me the most intresting in these test reports are not the absolute values, but the relative values from one scope to another tested in the same conditions by the same operator.




So, here another TOA (this time a TOA-150) tested by the same operator:

http://www.astro-foren.de/showthread.php?12633-TOA-150-1100-ein-quot-Sahnestü...

Chris


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olivdeso
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Reged: 02/20/11

Loc: Paris FR
Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5875332 - 05/21/13 08:43 AM

Quote:

Quote:

Stange... the guy is a professional guy with profesionnal tools, he knows his job.




Well, take a look.





You mean the spherochromatism graph calculated from 3,5 and 7...The graps seems a little bit strange to me at higher values than 70/80%, especially if we compare to the front wave map, but however, this graph is not so far from what is expected : for instance we can see a 70/80% crossing and the graph is roughly OK.
I don't know whether the "difference" at high values comes from the calculation hypothesis or from the real world difference to ideal optics/specs, but at the end this gives a good idea of how the optics work.
To me the most interesting is comparing the graphs from a refractor to another.


The second test from Rohr gives 70µ offset between bue and yellow which seem to be confirmed by the color foucault, no?


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5875701 - 05/21/13 12:08 PM

Quote:

You mean spherochromatism graph calculated from 3,5 and 7...




No. What I mean is:
1-arbitrarily discarding Zernike terms, which shows no knowledge of how they work
2-mixing pupil size with the value of Zernikes, which is a factor in ophthalmology, not in telescope optics
3-evaluating primary spherical based on its Zernike term, which has little sense with apos where it is only a component in modeling the wavefront, routinely affected by the higher order terms, as well as other aberration terms
4-saying that the wavefront *phase* error is given in microns (that's zero level error)
5-saying that TOA and Maks are the only systems *not showing any spherochromatism* (another zero-level error)
6-not answering the obvious inconsistency between his explanation of how the LA graph was obtained and their wavefront maps
7- saying that in the AP155 5th and 7th order spherical (i.e. 6th and 8th in the modern nomenclature) are "quite the same over the spectrum" (take a look of the graph)

Quote:

The second test from Rohr gives 70µ offset between bue and yellow which seem to be confirmed by the color foucault, no?





Not really. The second test is for the 150/1100 TOA, with 10% longer f.l. which means that the secondary spectrum should be 10% greater only based on the f.l. And somewhat more than that due to secondary spectrum in apos being affected by the magnitude of spherochromatism, which increases exponentially with the f-ratio (f/7.3 vs. f/7.7). With the TOA130 tested at 100 microns F-C, this one should be more than 110 microns. Instead, it is 68 microns, 2/3 of the smaller/slower TOA's. Doesn't add up.

Vla


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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5877855 - 05/22/13 11:49 AM

Okay you have issues with some of the testing Rohr does. But as far as I can tell he is the only source for in-depth testing of astronomical optics on the net. I am interested in what he does because when I want to buy a telescope (new or used) I can go to his website and find out how well the optic was made. And if I remember he also gives the polychromatic strehl for refractors which I don't see very often. No one, and I mean no one, does more exhaustive and thorough testing of telescope optics. Maybe you should give him some competition?

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Jon Isaacs
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Reged: 06/16/04

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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5878091 - 05/22/13 01:46 PM

Quote:

Okay you have issues with some of the testing Rohr does. But as far as I can tell he is the only source for in-depth testing of astronomical optics on the net. I am interested in what he does because when I want to buy a telescope (new or used) I can go to his website and find out how well the optic was made. And if I remember he also gives the polychromatic strehl for refractors which I don't see very often. No one, and I mean no one, does more exhaustive and thorough testing of telescope optics. Maybe you should give him some competition?




Or maybe some advice...

Testing is one thing but it must be done properly and with an understanding of the analysis.

Jon


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5878422 - 05/22/13 03:50 PM Attachment (24 downloads)

I'm not sure what is "in-depth" worth, if it is not reliable. Also, "exhaustive" is not necessarily a quality. To me, the test reports I saw are way to overcrowded with all kinds of software interfaces and numbers. All it takes is a few numbers that specify optimized line correction, chromatic defocus i spherochromatism for several selected wavelengths. Wavefront maps for those won't hurt.

Going back to the specifics, here's a 130mm f/7/7 triplet made of Ohara FPL53 and Schott ZKN7. The latter is available, inexpensive crown that happened to be the best match for FPL53 with respect to secondary spectrum reduction. The only significant chromatism comes from the spherochromatism (partial dispesion diagram indicates that Ohara BSM81 could be a bit better with respect to minimizing spherochromatism). But it is also negligible. The polychromatic Strehl (photopic, 430-670nm) is 0.983, and with the e-line correction error taken out, it degrades the Strehl by no more than 0.99 factor (this is not strictly optimized lens, but it can't be significantly better).

The paraxial foci from 400-800nm (and wider) are within 0.2mm. Best F/C foci, which nearly coincide, are at about 0.018mm, or 18 microns from the best e-line focus. That's 1/13 wave P-V of defocus in the blue, and 1/17 wave in the red.

At the location of best e-line focus, the F-line error is still only 1/9 wave P-V. According to Rohr's test, this error is four times larger longitudinally and nearly 1/3 wave P-V.

Why would Takahashi, going for a top apo make something that is significantly worse than readily available, relatively inexpensive and simple triplet? Of course, the actual objective will likely deviate from the design optimum. In fact, by slightly weakening its positive component (2.3% longer rear surface radius), best F and C foci come at nearly the exact distance from the e-line focus as in Rohr's measurement, with the e-line correction still better than 1/10 wave P-V. But the polychromatic Strehl is still just short of 0.96 with the degradation factor due to spherochromatism still over 0.98.

And Rohr gets 0.909 polychromatic Strehl, with 0.998 e-line Strehl, which means that it is chromatism (spherochromatism+secondary spectrum) accounting for all that degradation.

Just doesn't add up. Again, this triplet is supposedly not as good a design as TOA. Either is Takahashi running a big conspiracy scheme to purposely cheat on its customers, or Rohr's test results are a suspect. I opt for the latter.

Vla


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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5879595 - 05/23/13 05:32 AM

Quote:

Quote:

The second test from Rohr gives 70µ offset between bue and yellow which seem to be confirmed by the color foucault, no?




Not really. The second test is for the 150/1100 TOA, with 10% longer f.l. which means that the secondary spectrum should be 10% greater only based on the f.l. And somewhat more than that due to secondary spectrum in apos being affected by the magnitude of spherochromatism, which increases exponentially with the f-ratio (f/7.3 vs. f/7.7). With the TOA130 tested at 100 microns F-C, this one should be more than 110 microns. Instead, it is 68 microns, 2/3 of the smaller/slower TOA's. Doesn't add up.
Vla




Vla,
Intercon-Spacetec, the German distributor of Takahashi, writes the following:
"Der TOA-150 ist nicht nur einfach der größere Bruder des TOA-130, sondern das Linsendesign ist völlig neu konzipiert und berechnet worden, um ein Höchstmaß an Korrektur der chromatischen Aberration zu gewährleisten."
Translation: "The TOA-150 isn't just the TOA-130's big brother but its lens design has been completely newly conceived and figured from scratch in order to guarantee the best possible correction of chromatic aberration."

=> So please do not expect the TOA-150 to be just an upscaled version of the TOA-130!

Quote:

...And Rohr gets 0.909 polychromatic Strehl, with 0.998 e-line Strehl, which means that it is chromatism (spherochromatism+secondary spectrum)...



Mr. Rohr doesn't think much of caring about the polychromatic Strehl. Instead, he prefers to measure the monochromatic Strehls in five or six different colors and (as in the case of the TOA-150) then demonstrates the human eyes' differing sensitivity for different colors by the width of the Strehl-bars in a chart.
There is another German called Kurt Schreckling who is after the polystrehl and has (interferometrically) measured and calculated it for about a dozen well-known apos. He did so by measuring the monostrehls in 11 colors and then weighting these results according to the human eyes' sensitivity for each of the measured wavelengths.

Mr. Rohr also uses a (Bath-) interferometer and if you want to gather how exactly he does so then please study
the first ~twenty entries here:
http://www.astro-foren.de/showthread.php?6084-Verzeichnis-optischer-Beroichte...
because of course he doesn't explain it all over again with each new telescope...

Vla, you certainly are a competent expert in optical theory... But did you ever personally measure and interpret real-world (!) instruments? If so, then pleeease give us not hundreds (as Mr. Rohr did) but at least half a dozen examples of what for us laymen is at least as, if not more important than bloodless theory.

Chris


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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5879992 - 05/23/13 11:03 AM

I stand by my comments. Mr. Rohr is "the best source" for optical testing of amatuer telescope optics for those of us who don't have the resources or time to do what he does. It is always easy to criticize another's work. That was why I suggested that maybe Vla could start performing a similar service.

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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5880000 - 05/23/13 11:12 AM

Quote:

"The TOA-150 isn't just the TOA-130's big brother but its lens design has been completely newly conceived and figured from scratch in order to guarantee the best possible correction of chromatic aberration."

=> So please do not expect the TOA-150 to be just an upscaled version of the TOA-130!




This is what they say, completely:

"The TOA-150 is not just the big brother of the TOA-130, but the lens design has been completely re-designed and calculated to provide maximum correction of chromatic aberration. In this case, not only visible wavelengths have been included, but also the near ultraviolet and the infrared. This provides a perfect platform for CCD applications. The large back focus of 214mm allows the connection of diverse accessory from the CCD camera with Robofokus to the dissecting microscope."

Which is simply that TOA130 is visually optimized, and andn TOA150 CCD-optimized. All it takes is a slight adjustment in radii (even single radius), in order to pull violet and deep red closer. This typically comes at the price of increasing the error in F and C, so the bigger TOA would have it larger than what would come from its longer f.l. and faster f-ratio alone. That's what is likely. I wouldn't pay much attention to the marketing talk, "completely new", "from scratch" etc. It uses the same FPL-53 between two crown elements. It cannot have significantly better correction than another top apo using same glasses and configuration. It can be optimized for CCD, and that's all.

Quote:

Mr. Rohr doesn't think much of caring about the polychromatic Strehl.




Is that explaining why its polychromatic Strehl puts Tak TOA130 at the level of semi-apo? BTW, plychromatic Strehl is the single most important and most informative number with respect to the level of chromatic correction. It also cannot be in conflict with the line Strehls, simply because it is obtained from them.

Quote:

There is another German called Kurt Schreckling who is after the polystrehl and has (interferometrically) measured and calculated it for about a dozen well-known apos.




I've seen it a few years ago. Wasn't impressed.

Quote:

Mr. Rohr also uses a (Bath-) interferometer and if you want to gather how exactly he does so then please study
the first ~twenty entries here:
http://www.astro-foren.de/showthread.php?6084-Verzeichnis-optischer-Beroichte...
because of course he doesn't explain it all over again with each new telescope...




The basic technique is using color filter to measure best focus for the bandwidth. The question is how precise it is. Another question is how appropriate is the use of software, e.g. Zernikes. Bath has nothing to do with determining best focus location for a wavelength - it measures wavefront for a given focus point. And, yes, one can err with the Bath as well.

Quote:

Vla, you certainly are a competent expert in optical theory... But did you ever personally measure and interpret real-world (!) instruments? If so, then pleeease give us not hundreds (as Mr. Rohr did) but at least half a dozen examples of what for us laymen is at least as, if not more important than bloodless theory.




Well, that "bloodless theory" actually enables you to see inconsistencies and errors in tests results, instead of merely believing it, or not. Like, for instance, that doesn't make any sense that the TOA130 is - according to Rohr's test - semiapo. That a plain FPL53 triplet beats it hands down. That it claims measuring e-line for it at 0.998 Strehl (comparable to less than 1/40 wave p-v of primary spherical). That has the error in F line 4.5 times the error in C line (very unlikely to impossible). And so on.

Anyone can believe in anything. But if someone with a different view comes up with specific objections, the believer should face them and respond directly instead of responding with general unverified and unverifiable assumptions. It is really irrelevant whether I ever did any actual measurements or not. The subject here are the measurements itself.

Vla


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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5880084 - 05/23/13 12:02 PM

Quote:

Quote:

"The TOA-150 isn't just the TOA-130's big brother but its lens design has been completely newly conceived and figured from scratch in order to guarantee the best possible correction of chromatic aberration."

=> So please do not expect the TOA-150 to be just an upscaled version of the TOA-130!




... so the bigger TOA would have it larger than what would come from its longer f.l. and faster f-ratio alone. That's what is likely. I wouldn't pay much attention to the marketing talk, "completely new", "from scratch" etc. It uses the same FPL-53 between two crown elements. It cannot have significantly better correction than another top apo using same glasses and configuration....




Citation Takahashi Europe, and Takahashi China as well:
"The objective of TOA-150 (D=150mm, F=1100mm, F/D 7,3) is an air-spaced triplet of which two of the lenses consist of ED glass with very weak chromatic dispersion."

=> The TOA-150 is obviously said (by people who should know) to be a pnp-objective and not an npn such as the TSA. (p = positive and n = negative lens)
=> Are you really sure you know everything already???

Chris


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roadi
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Reged: 08/18/07

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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5880200 - 05/23/13 01:06 PM

Chris, that last sentense wasn't neccesary! Vla's arguments are oviously backed up on knowledge and insight. Always interesting readings vla!!

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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5880400 - 05/23/13 02:39 PM

Good point actually Chris.

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RodgerHouTex
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Re: Triplet APO or Doublet APO? [Re: RodgerHouTex]
      #5880416 - 05/23/13 02:42 PM

It is not the critic who counts: not the man who points out how the strong man stumbles or where the doer of deeds could have done better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood, who strives valiantly, who errs and comes up short again and again, because there is no effort without error or shortcoming, but who knows the great enthusiasms, the great devotions, who spends himself for a worthy cause; who, at the best, knows, in the end, the triumph of high achievement, and who, at the worst, if he fails, at least he fails while daring greatly, so that his place shall never be with those cold and timid souls who knew neither victory nor defeat.

– “Citizenship in a Republic,” Speech at the Sorbonne, Paris, April 23, 1910 Teddy Roosevelt

I think that about sums it up.


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wh48gs
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Re: Triplet APO or Doublet APO? [Re: Fomalhaut]
      #5880625 - 05/23/13 04:09 PM

Quote:

Citation Takahashi Europe, and Takahashi China as well:
"The objective of TOA-150 (D=150mm, F=1100mm, F/D 7,3) is an air-spaced triplet of which two of the lenses consist of ED glass with very weak chromatic dispersion."

=> The TOA-150 is obviously said (by people who should know) to be a pnp-objective and not an npn such as the TSA. (p = positive and n = negative lens)




Astronomics says it's FPL53 between two crowns; Astromart says the same; Optcorp says the same; ScopeCity says the same (also that it is like TOA130 "absolutely color-free"); and others. So many people think they know everything!

The fact is, it doesn't matter what triplet configuration is TOA150, but whether it is different than 130. And Takahashi says the two are "similar in design". That kind of excludes one of them being a reverse design.

Vla


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wh48gs
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Re: Triplet APO or Doublet APO? [Re: roadi]
      #5880999 - 05/23/13 08:03 PM

Thanks, Roadi. Worry not - just another day on CN

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