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Classic Telescope Testing

beginner classic collimation optics refractor
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#1 AllanDystrup

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Posted 27 December 2017 - 08:17 AM

.

Vixen FL55S/440mm    #1

 

 

     A row of holidays and overcast weather, -- so I've taken the time to start a formal test of some of my small classic refractors.

 

     Here's the first result,  for a small Vixen 55mm f/8 telescope:

 

1a FL55S -01.jpg
*click*

 

1a FL55S-02.jpg
*click*

 

     My preliminary (and inexperienced) attempt to quantify the result is ~1/30λ overcorrection.
     Any help in interpreting the Ronchigrams is of course most welcome.

 

Allan


Edited by AllanDystrup, 27 December 2017 - 08:21 AM.

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#2 DAVIDG

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Posted 27 December 2017 - 11:12 AM

 Your lens has spherochromatism, so your only see the errors at one wavelength by testing in green.  So those errors are increased across all the other wavelengths. The results is that the total error in the wave front is worse then what  you see only in green. 

   I don't know the design of the lens but for example  a typical 60mm f/15 achromat made from BK7 and F4 glass when made perfect will have a total wavefront error of around 1/8 wave because  a chromatic,  and spherical  aberration.  That lens will be designed to have no  spherical aberration in the green but will shows spherical at other wavelengths. So if it shows problems in the green then that error then adds to the  spherical aberration it already has at other wavelengths. 

  So you have to be careful in estimating the error in only one wavelength and assuming that the is total error in the wavefront.

 

              Happy New Year !

                 - Dave 


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#3 starman876

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Posted 27 December 2017 - 11:13 AM

Nice lens.  Very nicewaytogo.gif


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#4 AllanDystrup

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Posted 27 December 2017 - 12:21 PM

Dave,

    

Here's some information on the lens design for the scope I tested:

    

Vixen FL-Doublet.png

*click*

    

Thanks, and happy new year to all!
Allan


Edited by AllanDystrup, 27 December 2017 - 12:23 PM.

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#5 davidc135

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Posted 27 December 2017 - 01:33 PM

Great thread. The more light being shone on measuring, the better.

 

But there seems in the Foucault photo to be a noticeable difference between the centre, which appears to be in focus and the outer part which looks to be well inside focus- if it is overcorrected. How does this square with 1/16 wave (given dpac) optics? Maybe the contrast is illusory. Would measuring this delta null be a useful way to judge an objective?

 

David


Edited by davidc135, 27 December 2017 - 01:44 PM.

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#6 DAVIDG

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Posted 27 December 2017 - 02:40 PM

 To check the spheromatism you have to check the lens in three known wavelengths that are fairly narrow in band width. So using filters that have bandwidth of around 10 nm. You also have to check were those wavelengths come to focus in relationship to green and how that compares the theoretical data.    As your lateral color plot shows green looks to  be a  perfectly vertical line so it shows if your lens was perfectly figured it should show a clean null. The other wavelengths are not straight lines so you would see spherical aberration.  What causes the colors to come to focus are the radii on the surfaces, which are calculated for the glass types used. So when you have  the correct radii  and of the correct surfaces figures ( mostly spherical) everything works as theory shows the design to be but if a surface or surface(s) are off, everything changes ie the color correction and spherical aberration. When you see  a problem were you should have no spherical then everything is changed.

   DPAC testing  is  a great test but it doesn't tell the whole story with  a lens especially if the lens shows errors, because  most of the time your only testing in one wavelength.  If the lens tests well in green thou that is strong indication that it was made correctly and everything else will fall in line. 

 

                     Happy New Year,

                        - Dave 


Edited by DAVIDG, 28 December 2017 - 01:20 PM.

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#7 AllanDystrup

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Posted 28 December 2017 - 04:01 AM

.

     So to sum up, a Ronhci test in one color (usually green, for which wavelength the optics is typically optimized) can tell you if there are any gross errors such as a flipped lens assembly or elements, incorrect rotation or spacing of elements, incorrect collimation, tilt or figuring (TDE, coma, astig).

     

     It can also give you a rough estimate of the wave front error for the test color (again assuming green, which is supposedly the primary figuring and configuration target of the optical design), and a low wave front error in green is one major requirement for an excellent lens.

     

     BUT the error for green cannot in itself be used as an overall quantifier of how excellent the lens is (I never said that, btw grin.gif), because every lens design shows both longitudinal and spherical chromatism, and these have to be estimated to get an overall figure of how the total wave front is corrected as a function of focal length. The problem here is, that it’s hard to measure and quantify the combined effects of longitudinal and spherical chromatism in a Ronchi test.

     

     We do know however that longitudinal chromatic aberration (the change in focal length with wavelength) is typically reduced in a doublet to a fraction of the FL as follows: BK7|F4 achromat 0.05%, ED APO 0.012%, Fluorite APO 0.006%. Spherochromatism (change in spherical aberration with wavelength) is typically minimized in green and the figure corrected in blue and red to counteract the residual longitudinal chromatism. We then end up with the chromatic error plots, which most of the high end suppliers provide for their telescope optics. Example from Vixen (20 micron / division):

    

Vixen-Objectives.jpg

*click*

 

    

     Such data has been provided by Zeiss Jena from when Abbe designed the first astro-objectives, and also by Vixen since at least the 1980’ies. It was not available from Nihon Seiko when I bought my first refractor (a 3” Unitron 142C) back in 1965, so I had to rely on their guarantee of the optics being “diffraction limited” (which it was).

 

     As I do not have a fully equipped optical bench, I will not attempt to reconstruct this kind of data for my telescopes. The visual experience primarily (along with an occasional star test and a simple Ronchi test) is quite adequate for my use. I did have one of my Vixen objectives professionally tested though, out of curiosity. I’ll return to that later in this thread.

    

Allan


Edited by AllanDystrup, 28 December 2017 - 01:25 PM.

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#8 DAVIDG

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Posted 28 December 2017 - 10:34 AM

 My correction to the above is that a Ronchi test used with double pass can shows very small errors, on the order of 1/20 wave or less. If your testing a totally reflective system which is perfectly  achromat, then your seeing the total error in the system. With any system that has refractive elements you have to understand the total design and understand what the test is telling you since your most likely testing at only one wavelength.  As I said before if your lens tests well in green then the odds are very good that everything else in fine with it since all these correction interact with each other.

   Even when one does interferometry to determine the wave front error it is done in only one wavelength at a time. So it needs to  be done in multiple wavelengths for  a system that has chromatic aberration  and all those wavefront analyzed to determine the true polychromatic wavefront of the system.  Most of time a  report from interferometry for a commerical lens will show an excellent wavefront error but again  it  is done in only one wavefront so it can be misleading to the real quality of the lens.  Another issue can be that  most interferometry is done using  a HeNe laser at 632nm while most lenses are designed to be best corrected at around 550nm in the green. Like I said before I can test a singlet at one wavelength and get an excellent wave front error as well, but we all know that it would have ton of chromatic aberration. 

    Like I have said many times, test your optics so you know what you have. If you know the quality of your optics and your happy with the image, that is great.   There have been many example presented here that errors have been found like a flipped element that have gone uncorrected for years and the results has been taking a poor performing scope and turning it into a real winner. 

 

                           - Dave 


Edited by DAVIDG, 28 December 2017 - 01:22 PM.

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#9 DAVIDG

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Posted 28 December 2017 - 10:45 AM

 One other thing I'll add is that  to be sure that you let your optics become  temperature stabilized when you test them. Optical glass has a much higher coeff. of thermal expansion then Pyrex or other  materials used to make mirrors. So they change shape much more as they are becoming stable. 

 Allan's lens  a has CaF2 rear element which is temperature sensitive and one of  the reason why it placed as the rear element.  So it is possible that the over correction he saw was from the lens not being stabilized. 

 

                     - Dave 


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#10 AllanDystrup

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Posted 29 December 2017 - 01:59 PM

.

Vixen FL-55S/440    #2

    

    

     Here's the test of my other FL55S (this one is normally mounted as a finder on my FL102S). The test result is close to that described above for the same type of scope. -- All pictures are of course straight from the camera (no processing).

 

Vixen-FL55S-440-#2.jpg

 

 

Vixen-FL55S-440-#2-result.jpg

 

 

Allan


Edited by AllanDystrup, 29 December 2017 - 02:05 PM.

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#11 DAVIDG

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Posted 29 December 2017 - 03:14 PM

 Allan,

   I don't think your software is calculating the PV error in a wavefront  that is produced by parallel light and  an object at infinity but the PV error of the defocused image that is certain distance inside or outside of the focus for a mirror tested at the radius of curvature.   Having used double pass for 35 years and compared it's results to many other methods, the amount of bowing in your images  looks to me to be closer to 1/4 wave. 

   One example I show my students is 6" f/13 perfectly spherical mirror while being tested by double pass. One can just detect the bowing of the lines and it's true wavefront error is about 1/15 of the wave. Your images easily shows much more bowing then that,  so I don't see how the error can be 1/32 of  a wave. 

  I modeled a 55mm f/8 mirror with a conic of -0.6 which is what your images are showing,  and  that you are comparing  the bowing of the bands too.  OSLO shows that  it has  a PV wavefront of 1/7 wave. So in double pass you would see 2x this amount off errors and the lines would show a fair amount of bowing like I'm seeing in your images. Here is the screen shot of the data. 

 

                   - Dave 

55mf8mirror.jpg


Edited by DAVIDG, 30 December 2017 - 10:30 AM.

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#12 DAVIDG

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Posted 30 December 2017 - 10:40 AM

 Double pass is also excellent at detecting astigmatism and I might be seeing it in these images. Place the grating inside of focus so there are  three lines showing. Rotate the grating so the pattern  is  perfectly vertical. Now carefully move the grating to the outside of focus so again you have three lines showing. If the pattern  rotate even by the smallest amount what your testing has some level astigmatism. 

 

                        - Dave 


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#13 AllanDystrup

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Posted 30 December 2017 - 01:26 PM

Dave,

 

     Thank you for the analysis of the Ronchigrams, - this is the kind of feedback I had hoped to get!

 

     I'm still struggling with trying to model the result of my DPAC tests to get some quantitative approximation of the wavefront error for refractive systems.

 

    You're quite right that the model I tried for the Vixen 55S is PV error for defocused images, and an error of 1/32 wave is clearly way off. It would be very helpful with a set of inside/focus/outside Ronchigrams illustrating at least approximately how 3 lines defocus translates to typical wavefront errors of say; 1/2, 1/4, 1/8, 1/16 lambda. Your OSLO model of the 55S is no doubt reliable with respect to basic optic figure (we've discussed the inadequacy of single color test to quantify chromatic error, scatter and other defects), but OSLO is quite an expensive piece of software as I understand it.

 

   Here's the test I made today of a Zeiss Teleminor and a Telementor refractor; It would be interesting to compare these with tests of say comparable Tak, AP and Nihon Seiko refractors. Anyone?

 

 

CZJ-01.png

 

Allan


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#14 DAVIDG

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Posted 30 December 2017 - 02:44 PM

 Allan,

   Both Zeiss lenses are very good. The 63mm has  slight aspheric correction vs a perfect spherical correction, but it is slight and only 1/2 has bad as shown since this a double pass test.  I can see a slight outward bows of the lines one the inside of focus and inward bow on the outside of focus image which indicates a slight amount of the over correction.

   It is difficult to use a double pass to qualify an exact wavefront. A 1/4 wave  or worse easily shows with the bowing of the lines. You can't miss it.  At the null positive the zones are very easy to see. When you get to around 1/8 wave, there is  a slight bowing but you still can see it without to much trouble. At the null you can see the zones. At a true 1/10 wave you have to start to look for the bowing and usually have the rock the the grating for one side to the other to see the lines slightly bowing,  in and out.  At the null the position you can detect any faint zones. At 1/15 wave or better your fighting to see any departure from dead straight and  at the null position it looks like a uniform null. You have to fight to detect any zones. 

   I have been using double pass for many years and I figure my optics until the ronchi bands are dead straight and I can't detect any zones at the null position.  What the exact wavefront is I don't know but they show a perfect star test. Double pass is why I have 10 optical awards at Stellafane and is independent judge that the test works extremely well yet is easy to do.  That is why I have been using for years in the classes I help teach. 

  If you want to put an exact number on  the wave front then you need to measure it with a well calibrated interferometer.   What double pass gives you, is "poor", "good" and "great" range. 

   OSLO.EDU  which is the educational version is  free and as you said very powerful. It limits one to 11 surfaces but for 90% of the telescope design out there, that is more then enough. Some of the more exotic functions are turned off put for 90% of telescope designs one would want to check, you don't need them.  I have been using it for years to design and analysis designs.  Here is  a link to the OSLO7.edu version https://www.lambdare...m/support/5900/

 

              - Dave 


Edited by DAVIDG, 31 December 2017 - 10:23 AM.

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#15 AllanDystrup

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Posted 01 January 2018 - 09:28 AM

Dave,

     Thank you very much for your rules of thumb for a qualitative assessment of optical quality, based on green color 3-bar Ronchigrams!  I’ll go with that for now (while I have a closer look at the OSLO EDU tool).

 

     Here's my test result for another Carl Zeiss Jena C63/840mm "Telementor" refractor, along with the Zeiss spec for the color correction of the lens. I can see a slight inward bowing for the inside focus, and straight lines for the outside focus (or at least I'm struggling to see any deformation here). The null looks clean as can be. Now that should place it between 1/8 and 1/10 wave overcorrection, right? 

 

 

CZJ63-840 #1.jpg
*click*

 

CZJ63-840 #2.jpg

*click*

 

---

 

PS: Here's a link to Ronchi tests of some high end refractors : http://www.rohr.aiax.de/refractors.pdf. -- Unfortunately they use 5 bars (instead of 3) and they are seemingly in white light to estimate chromatic aberration (?), but I assume they are double pass, so they are still interesting for comparison.

 

Allan


Edited by AllanDystrup, 01 January 2018 - 09:54 AM.

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#16 Bomber Bob

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Posted 01 January 2018 - 11:42 AM

Despite the stinky pictures, here's a summary of the Classics that I've tested with my DPAC rig:

 

Swift 838 [Takahashi] --> 3 perfect bars.

 

ATM 5" F5 Triplet --> bowing, Outside Focus only.

 

Dakin 4" F10 --> bowing, Inside Focus only.

 

Goto HS452 --> 3 perfect bars.

 

Lafayette Galactic [Royal] - 40mm Guidescope --> 3 perfect bars.  /  76mm --> 3 perfect bars.

 

Mayflower 814 [Yamamoto] --> 3 perfect bars.

 

Mogey 3" F14 --> bowing, Inside Focus only.

 

Sears 6336 [Royal] --> 3 perfect bars.

 

Space Scope 151 [pre-Royal] --> 3 perfect bars.

 

The results correlate with what I've seen at the eyepiece with these scopes.  My 2 Royal 76mm refractors don't go soft at 100x per inch, the Swift is practically an APO, the Goto is the very best 60mm refractor that I've ever used, and that 5" triplet is freaky sharp -- but only at the very center of field.

 

I haven't tested my 2 APOs yet, or any of my reflectors or CATs.  The Questar has that rear camera port, and I think I could get it positioned on the rig to use that feature.


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#17 AllanDystrup

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Posted 01 January 2018 - 12:40 PM

That's impressive BB, but -- as the saying goes: without pictures, it didn't happen grin.gif

 

I haven't testet a telescope that was perfect yet. The Zeiss APQs and Tak TOA in the PDF I linked to above comes close, but I see (mostly small) issues in most of the rest in the PDF. So 'perfect' (as in true 1/10 wave or less)..., -- that I think is a rare bird.

 

Allan


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#18 DAVIDG

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Posted 01 January 2018 - 12:42 PM

 Allan,

    This second Telementor looks to me to  be at least a true 1/10 wave in green. If your seeing straight lines on one side and slight bowing on the other, that might be a slight misalignment to the flat./objective.   The null looks very good without any evidence of zones. So a really excellent lens.  My own Telementor tests this way. 

  Are you sure that the data is for lens design used in the Telementor ? The reason is that the plot for focused image show that the e line is not straight.  This would mean that one would see spherical aberration when testing in green

   I have seen the Japanese site before. Testing with 5 lines showing reduces the sensitivity so I always recommend to go down to three and also look at the null position as well. Testing in white light gives ones an feel for how well the lens is color corrected. The better the color correction the less color fringing you''ll see on the edge of the Ronchi bands. Since the chromatic aberration is doubled in double pass autocollimation  the color correction or lack of it stands out.  Testing in white light softens the contrast of the edge of the bands so it becomes harder to determine how straight the bands are and also soften the zones at the null position because ones has all the different over lapping images from the different colors coming to focus at slightly different positions.  By the way don't use a white LED but a incandescent bulb so you have  a continuous spectra.   

 

                   Happy New Year

                     - Dave 


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#19 Bomber Bob

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Posted 01 January 2018 - 01:28 PM

I got pix, but it's not worth the flack posting them.  By perfect lines I mean measured width & spacing match with no deviations within / between.  With the images on the computer screen, I use a clear acrylic metric ruler -- takes the guess work out of it.  The rig is easy to adjust, and I get the middle bar on centerline.  Each of the scopes got checked at least twice -- the 838 was my first DPAC and was on the rig at least a half-dozen times.


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#20 AllanDystrup

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Posted 01 January 2018 - 02:50 PM

Dave,

 

     Yes, I think you're probably right that there's a slight misalignment to the flat. The flat I used was a small 3" mirror, and the alignment was just a fast "eyeballing".

 

     I got this Telementor OTA with another buy and planned to sell it, -- but the star test and visual performance were so good, that I've had a hard time letting it go... Now i understand why grin.gif.

 

     The figures showing the color correction for the Telementor objective are from Österreichische Astronomische Monatschrift, Der Sternbote, "Ein empfehlenswertes Fernrohr für Amateure: Telementor/Telemator", ISSN 0039-1271 / 26. Jahrgang, 326 / 1983-10.
      Fig.1 shows the color correction of the Telementor Fraunhofer C63/840 lens on the optical axis, i.e. where the colors come to focus: the secondary spectrum from C-F is ~0.55mm; Fig 2 shows the spherical aberration from the center optical axis to the rim of the objective: it is optimized for the e line (which is not quite straight, but close), and the max aberration is 0.05mm for F. I haven't checked these data with other sources.

 

Thanks,

Allan


Edited by AllanDystrup, 01 January 2018 - 02:58 PM.

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#21 PeriodicTrends

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Posted 01 January 2018 - 02:56 PM

Are you sure that the data is for lens design used in the Telementor ? The reason is that the plot for focused image show that the e line is not straight. This would mean that one would see spherical aberration when testing in green

Happy New Year
- Dave


This is a question I have as well Dave. Maybe you can answer but in mucking around with OSLO I've noticed that the aberration plot can vary the x-axis unless you specify it. Is there a standard for what the limits of the axis should be for a well corrected lens? For instance I notice in Alan's included plot the limit is only 0.03mm...is this a lot or a little? Would the line look less curved if the limit was 1mm?

#22 DAVIDG

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Posted 01 January 2018 - 04:04 PM

 The color correction can be no better then the dispersion of the two or more of glasses used.  So that sets the limit. Any good book on optics like "Telescope, Eyepieces and Astrographs" explains how to design a lens and has the math to calculate to difference in the focus position of the different colors.

 

                        - Dave 


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#23 CharlieB

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Posted 01 January 2018 - 04:45 PM

That's impressive BB, but -- as the saying goes: without pictures, it didn't happen grin.gif

 

I haven't testet a telescope that was perfect yet. The Zeiss APQs and Tak TOA in the PDF I linked to above comes close, but I see (mostly small) issues in most of the rest in the PDF. So 'perfect' (as in true 1/10 wave or less)..., -- that I think is a rare bird.

 

Allan

If you haven't seen these, here are DPAC photos of many of my refractors.  Some scopes are great, some are quite poor.  The quality of the photos themselves is variable, as I was on a learning curve.  Many tips of the hat to DAVIDG for his expert help.

 

https://www.cloudyni...820841=&page=10



#24 PeriodicTrends

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Posted 01 January 2018 - 05:13 PM

I got a new book coming in the mail...woohoo! Thank you for the recommendation Dave, and again many many thanks for all of your help with this stuff over and over again.

Also in case I haven't mentioned it, Charlie and Allan your DPAC pictures are so clean and crisp! I really need to move on from my pillows and toothpicks setup ;-)

#25 AllanDystrup

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Posted 02 January 2018 - 02:09 PM

Charlie, Dave --

    

     Wow, nice thread and some quite excellent DPAC results too! Thanks for sharing! -- I'll read it with great interest.

    

     And for the record, Dave is also the one who inspired me (in comments and descriptions in the ATM forum) to try DPAC evaluation of my scopes. Thanks Dave! -- I've build my test setup according to your instructions, and it works great waytogo.gif .

    

 

    

     Here's my latest test of two Nihon Seiko 60mm/900mm (F/15) Fraunhofer air spaced doublet refractors. The first is mine, the second that of a friend). Mine is good (~1/8 λ?) with a slight turned up edge, my friend's shows a more pronounced TUE and I think some astigmatism, but could be ~1/6 λ ? with a proper aperture diaphragm.(And btw, my lens is an earlier version than that of my friend.)

    

NS-60-900 #1 #2.png
*click*

    

Allan
 


Edited by AllanDystrup, 02 January 2018 - 02:19 PM.

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