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Classic Telescope Testing

beginner classic collimation optics refractor
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#26 DAVIDG

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Posted 02 January 2018 - 03:16 PM

 Hi Allan,

   You might try using a real knife edge to view and image the null  position. It will give you a cleaner image. Your getting a shadow at both the left and right side, that might be a real zone or more likely it is  part of one Ronchi band and part of another.

   Your friend's lens has some really problems. A turned edge really hurts the wavefront and with the astigmatism your most like worse then 1/4 in green and that would make the polychromatic wave front even worse. Before you call that lens bad thou, I would try loosening the retainer ring to be sure it is not being stressed in it's cell.  Also be sure that the spacers are at 120° centers.

 

                        - Dave 



#27 AllanDystrup

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Posted 04 January 2018 - 09:31 AM

Dave,

 

     Thanks for your analysis and advice. I've tried loosening the retainer ring, but that didn't change the result. The spacers are at 120° -- could respacing with thicker/thinner spacers solve the issue?

 

----------------------
 

Here are 3 lenses I'd like your comments on -- they all seem pretty good to me, maybe with some very minor issues. I find it difficult to rate these 3 lenses.

 

#1-H.jpg

 

#2-C.jpg

 

#3-S.jpg

 

Thanks,
Allan


Edited by AllanDystrup, 04 January 2018 - 01:50 PM.

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#28 DAVIDG

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Posted 04 January 2018 - 10:54 AM

 Allan,

   All three are good lenses. Number 2 is the best  a with clean null, #1 is second with tiny hill in the middle and #3 has a bit more roughness at the null. I would say #2 is true 1/10 wave in green maybe better. The other two are at least 1/8 wave in green and maybe better.  As I have said before, you have to analysis the total polychromatic wave front to get the true quality of the lens.  You can have a f/4 achromat made from BK7 and F2 that tests perfect in green yet the total polychromatic wavefront will be much worse then 1/2 wave because of the residue chromatic aberration that is just inherent in the design. Typical binoculars have a large amount of chromatic aberration from the lenses being in the F/3 to F/4 range but they use fixed low magnification in the 6x to 20x range so the chromatic aberration is not objectionable. 

  As for your friend's lens that tested with problems, respacing it won't fix those problems only a  pitch lap and polishing will.

 

                     - Dave 


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#29 AllanDystrup

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Posted 04 January 2018 - 01:41 PM

     Thank you again Dave!, -- your evaluation is in perfect alignment with my own assesment here!

 

     Judging the lens quality from Ronchi-grams is a learning curve, but the more tests I make and the more feedback you offer, the more I feel confident in my own assesments.

 

     Like you I find #3 to be lagging a tiny bit behind; #1 and #2 were more difficult for me to rank, but I can follow your description, and I do agree with your evaluation.

 

Best regards,
Allan

 

 

PS: The objectives here are apochromatic with excellent color correction, so I'm not too concerned about the polychromatic wave front.


Edited by AllanDystrup, 04 January 2018 - 01:51 PM.

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#30 PeriodicTrends

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Posted 04 January 2018 - 02:01 PM

It is interesting to read the last two posts for me.  I saw Allan's pictures and there were no comments from Dave yet, so I hustled and I got out a sheet of paper and wrote down the things I saw.  I tried really hard to do some interpretation after all the reading I've been doing the past couple months.  I even tried to use the right vernacular and had to look some of the old posts up to make sure.

 

Wah wah (trombone sound)

 

I totally missed a couple of things, including the roughness.  But that is part of my lack of experience.  I think it is phenomenal to see such good pictures that offer so much detail.  I can sorta see these details with my eye when I am doing a test but rarely can I capture them so well on camera as Allan and others have done.

 

But I'm making progress on both fronts.  This is super cool and hopefully I can contribute something worthwhile myself someday.  Way to go everyone and so fun!


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#31 AllanDystrup

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Posted 24 February 2018 - 01:14 PM

.

     A neverending row of overcast days up here in Scandinavia, so today I decided to do a fast Ronchi test of my most used Vixen FL-80S/640mm refractor, and compare the result with a wave optics diffraction simulation program in order to get a rough quantification of the quality of the objective.

 

     I already know (from comprehensive observations) that this doublet APO is an excellent performer, but just how well would the DPAC images stack up against those generated from optics theory for different levels of wave-front errors ( "deformation coefficient", aka Schwarzchild constant)?

 

     Here's the result; You can judge for yourself.

 

Vixen FL-80S-640 DPAC with Analysis.png

*click*

 

 

Allan


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#32 DAVIDG

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Posted 24 February 2018 - 01:33 PM

 Allan,

    It nulls  beautifully, so in green it is at least 1/8  wave if not  1/10 wave  or better.  When you get to this level of correction it becomes difficult to judge the straightness of  Ronchi bands in a  still image  so posting the null image is very helpful. Also when your testing if you  move the grating from one side of focus to other and watch for any bowing of the lines you can  detecting small deviation from straightness since the  eye is very could at detecting motion and deviation from straightness.  It may also help to hang a string in front the lens as straight edge to compare too.

 

                      - Dave 


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#33 deSitter

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Posted 24 February 2018 - 02:01 PM

Looks like a tiny bit of astigmatism? Lines are rotated relative to each other in/out. I think I see this in the null too - brighter at noon/six vs three/nine.

 

-drl


Edited by deSitter, 24 February 2018 - 02:05 PM.


#34 rolo

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Posted 24 February 2018 - 02:45 PM

Looks good Allan! I tested my FC50 a while back and was happy with the results. Wracking the focuser in and out both sides were identical.

Attached Thumbnails

  • fc50-1.JPG

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#35 starman876

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Posted 24 February 2018 - 09:28 PM

.

     A neverending row of overcast days up here in Scandinavia, so today I decided to do a fast Ronchi test of my most used Vixen FL-80S/640mm refractor, and compare the result with a wave optics diffraction simulation program in order to get a rough quantification of the quality of the objective.

 

     I already know (from comprehensive observations) that this doublet APO is an excellent performer, but just how well would the DPAC images stack up against those generated from optics theory for different levels of wave-front errors ( "deformation coefficient", aka Schwarzchild constant)?

 

     Here's the result; You can judge for yourself.

 

attachicon.gif Vixen FL-80S-640 DPAC with Analysis.png

*click*

 

 

Allan

I think many of us would be very happy with a lens that tests like thatwaytogo.gifwaytogo.gif


Edited by starman876, 24 February 2018 - 09:28 PM.

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#36 AllanDystrup

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Posted 25 February 2018 - 03:14 AM

drl:
     Yes, I can see what you're pointing out. I'm sure this is due to different alignment of the camera for the Inside/Outside pictures. I need a better camera mount or a least more care in handling the camera.

 

     Here's a retake on the DPAC for this Vixen FL-80Sfrom today. I took more care in keeping the camera position exactly the same, and I carefully observed transition Inside/Outside without being able to notice any rotation or deviation from straight.

 

     For comparison I've juxtaposed ½ Inside + ½ Outside. see below.

 

Vixen FL-80S-640 DPAC with Analysis 02.png

*click*

 

Dave:

     I agree with you, that this objective is a true 1/10 wave in green, and very probably better, but I think it will take more precise measuring and more careful modelling to get any closer to an exact wavefront rating. Anyway my purpose with this test has been accomplished, as I think DPAC to rating categories of 1/2, 1/4, 1/8, and 1/10 wave is plenty for at least my practical purposes. I think that's what you're advocating too.

 

rolo, Johan:

   Yes! I'm certainly happy to have this optically excellent objective for my astronomical explorations, and on top of that, in a lightweight package that's perfect for my grab/go setup. I have another FL-80S in our cottage in the woods, and I think this will test just as good.

 

     My smaller FL-55S scopes do show some line bowing, I estimate ~1/8, but I'd like to model it for a better approximation. My FL-102S is not 100% perfect with respect to color crossing, but the small deviation is due to the spacers having been compressed through the years, and I'm not going to tamper with that.

 

Allan

 

 

 


Edited by AllanDystrup, 25 February 2018 - 04:45 AM.

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#37 DAVIDG

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Posted 25 February 2018 - 11:12 AM

Allan,

 " Anyway my purpose with this test has been accomplished, as I think DPAC to rating categories of 1/2, 1/4, 1/8, and 1/10 wave is plenty for at least my practical purposes. I think that's what you're advocating too."

   Yes this what I have been saying. Also DPAC is  a cross check with other test methods, like the star test that I see so many doing wrong. DPAC also allows one to quickly tell what is going one since I have seen my times with vintage and antique scopes that the optics sometime in the past have been disassembled  for cleaning and then reassembled wrong and this error has been carried forward for years.  It only takes a minute to turn a lens around on the stand and if it tests better or a few minutes to flip an element around that one suspects might have been backwards and see if that results in a better image. 

 

              - Dave 


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#38 AllanDystrup

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Posted 26 February 2018 - 05:06 AM

.

"There is another one..." -- and here it is: My Vixen FL80S/640 out in our "Walden" in the woods:

 

Vixen FL-80S-640 #2 DPAC with Analysis.png

*click*

 

My estimate is at least 1/10 wave, like the one I have at home.

 

Allan

 

 

 

 

 

 

 


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#39 AllanDystrup

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Posted 26 February 2018 - 05:11 AM

.

     And I also had a closer look at my first Vixen FL55S/440mm :

 

Vixen FL-55S-440 DPAC with Analysis.png

*click*

 

     I'd say it's a solid 1/8 wave?

 

Allan


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#40 bremms

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Posted 26 February 2018 - 08:51 AM

I lived in upstate NY my first 24 years of life. Wasn't uncommon to have a month of clouds. I feel your pain. Lots of reading and fabrication ( more of an ATM here) Never wanted to grind a mirror though. Helped a friend with an 8" and it was TOO tedious and messy for me. After he put scratches in the mirror during polishing ( SiC particles lodged in the lap) that was a heartbreak. He was super careful too. Mirror turned out well in the end. Too fiddly for me. Might be fun once you get the hang of mirror making, but the learning curve is very steep and long.


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#41 AllanDystrup

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Posted 26 February 2018 - 09:42 AM

.

     I feel if I was younger, I would really enjoy trying to make a good mirror; -- it must be a great satisfaction to be able to figure your own optics to a high degree of precision, but I'm sure it will take quite a lot of experience to be able to do that. And that would mean a substantial investment in good  materials, plus a lot of time spent practicing to reach perfection.

 

     As it is now, even though I'm retired, I feel I may have only ~5-10 good years exploring the universe full steam (the spirit is ready, but the body is not 18 anymore), and so I'd rather go right ahead and buy a few good telescopes, and then be free to spend my time researching and observing what's to see and understand out there.

 

     I have however also much enjoyed "rescuing" more than a dozen classic telescopes in the past 5 years (mostly NS Polarex/Unitrons) and I have found it interesting and challenging to repair/restore and use these. Down to only one NS 128 mount and a FL80S OTA right now, that needs to go, and then I think I'll retire as a classics restorer (ahhh well, maybe until the next new Lazarus shows up at the roadside, -- you know how it goes wink.gif ).

 

Allan


Edited by AllanDystrup, 26 February 2018 - 01:42 PM.

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#42 starman876

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Posted 26 February 2018 - 11:16 AM

so who, why  and when was it decided that the lines needed to be straight.


Edited by starman876, 26 February 2018 - 11:19 AM.


#43 deSitter

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Posted 26 February 2018 - 11:32 AM

so who, why  and when was it decided that the lines needed to be straight.

If differing zones have differing focal lengths (SA), then the lines passing through those zones will have differing spacing for a given grating placement. So straight lines mean all the zones share the same focal length (no SA).

 

-drl


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#44 starman876

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Posted 26 February 2018 - 12:33 PM

 

so who, why  and when was it decided that the lines needed to be straight.

If differing zones have differing focal lengths (SA), then the lines passing through those zones will have differing spacing for a given grating placement. So straight lines mean all the zones share the same focal length (no SA).

 

-drl

 

No no no smirk.gif  I was looking for the history of DPAC testing, When it first got started, how it got started, who started it.  



#45 AllanDystrup

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Posted 26 February 2018 - 12:53 PM

who: G.W.Ritchey: Test for Paraboloid at focus, Astrophysical Journal 14, p 218-220
when: 1901
why: https://stellafane.o...ollimation.html

 

Allan


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#46 DAVIDG

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Posted 26 February 2018 - 01:21 PM

 The shape of the Ronchi lines are analogous to contour lines on an elevation map. Instead of showing changes in elevation the Ronchi bands are showing  were the light is coming to focus. If the lines are dead straight it means that light from every  part of the optical system is coming to focus at the same position. 

   What is also critical is were the light source is located. A telescope is designed to take parallel light and bring it to perfect focus.  So to have it show straight Ronchi bands with perfect optics the light source needs to produce a wavefront that is the same as one that object at infinity would produce. If not you will see spherical aberration and to determine if your optics have errors you need to calculate the amount of spherical aberration and then measure it to see how they compare.

   In the classic Foucault test the light source is not at infinity but placed at the radius of curvature of the mirror ie 2x the focal length. A spherical mirror with an object at this distance will perfectly focus light and show straight Ronchi line and a clean null  ie gray all over with a knife edge test. The problem is that a spherical mirror when used with a object at infinity will not bring all the light to a  common focus , we need a parabola or addition optical surface.   If we test a parabolic mirror with the light source at the radius of curvature it will not focus all the light to perfect focus but it will show a some level of over corrected spherical aberration. We calculate that amount and then measure it to see how it compares to what theory shows it should be. We  continue figuring until in theory they match. The problem with any test method that requires that you make measurements is that there are errors associated with those measurements so you need to know what those errors are. If not what you believe you have and what you really have mayt be very different.

    A better test is a null test which requires no measurement. You can just look and see if there is problem ie if the Ronchi bands aren't straight. To have a null test that requires a wavefront that has the characteristics of an object that is located at infinity. In Double Pass autocollimation we use the telescope to do that,   first working backwards to make  that  wavefront since in theory a perfect set of optics will produce a perfectly parallel light just like an object at infinity. If the optics aren't perfect the wavefront coming from the telescope  with have errors. The angle of the light won't be perfect parallel. Now this light is reflected off an optical flat mirror  and back into the telescope were the optics again add errors to the light, hence double the errors. So now we a have simple to setup test, that requires no measurements and twice the sensitivity of most other optical test.  The result is that one can easily test their optics and quickly determine they have problems  with making any measurements.

 

                   - Dave 



#47 DAVIDG

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Posted 26 February 2018 - 01:25 PM

 

 

so who, why  and when was it decided that the lines needed to be straight.

If differing zones have differing focal lengths (SA), then the lines passing through those zones will have differing spacing for a given grating placement. So straight lines mean all the zones share the same focal length (no SA).

 

-drl

 

No no no smirk.gif  I was looking for the history of DPAC testing, When it first got started, how it got started, who started it.  

 

 The Clarks seem to have invented  it and it was one of the main reasons why their lens perform better then then  rest. for their time. They could see the errors in their lens and then figure them so there were none. So it wasn't the design or the glass or some long lost "magic". It was their test system.

 

                      - Dave  


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#48 starman876

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Posted 26 February 2018 - 03:43 PM

 

 

 

so who, why  and when was it decided that the lines needed to be straight.

If differing zones have differing focal lengths (SA), then the lines passing through those zones will have differing spacing for a given grating placement. So straight lines mean all the zones share the same focal length (no SA).

 

-drl

 

No no no smirk.gif  I was looking for the history of DPAC testing, When it first got started, how it got started, who started it.  

 

 The Clarks seem to have invented  it and it was one of the main reasons why their lens perform better then then  rest. for their time. They could see the errors in their lens and then figure them so there were none. So it wasn't the design or the glass or some long lost "magic". It was their test system.

 

                      - Dave  

 

How do you think the Clarks developed DPAC? Through some sort of calculations or just an idea? Would they also have needed to make a really good flat or did they use the pan of oil?


Edited by starman876, 26 February 2018 - 03:44 PM.


#49 DAVIDG

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Posted 26 February 2018 - 03:56 PM

 Here is  a link to a story about their 48" optical flat that was  used to make some of their famous lenses. The flat doesn't need to be super optically flat just optically smooth. So it can be a very long radius spherical mirror.

https://archive.org/...ge/n97/mode/2up

 

                     - Dave 


Edited by DAVIDG, 26 February 2018 - 04:04 PM.

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#50 starman876

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Posted 26 February 2018 - 10:49 PM

It looks like they were not using  a RONCHI screen.  The other thing I wondering about is the eyepiece.  Can you explain what they were doing Dave?




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