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Post Script: Improving the wavefront quality of the C14 Edge

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#1 jhayes_tucson

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Posted 02 May 2017 - 05:40 PM

This is a postscript to my testing article found here:  https://www.cloudyni...elescopes-r3095

 

A while back, I posted an article about testing the two C14 Edge scopes that I have in my shop.   The first scope that I tested is the one that I most often use for imaging and it tested at a Strehl of 0.845 (0.065 waves rms.)  Anything over 0.8 meets the Maréchal criteria for a diffraction limited system, so that's not a bad system.  Still a number of folks were surprised that the system didn't measure better.

 

Since I did the original test, I've installed a carbon fiber tube on the telescope so I scheduled another session on the interferometer, which I just finished today.  Along the way, I've seen a number of articles showing that it is possible to improve the wavefront quality of Celestron systems by simply rotating the corrector plate to minimize wavefront errors.  That seemed like a good thing to try since astigmatism was the predominant wavefront error.  I also knew that the secondary was nearly a perfect sphere (tested in another session on a WYKO 6000.)   I was able to get the telescope aligned fairly quickly and positioned the field point to be within about +/-0.5 mm from center of the field.  I used all of the same components as in my original report so I didn't go through all of the calibration steps that I did the first time around.  

 

I then rotated the corrector plate assembly by~ 90 degrees and realigned the secondary.  The wavefront improved dramatically, and I could see that it might do better with a bit more rotation.  So I rotated the corrector assembly by another ~15 degrees.  Measurements showed that the astigmatism term was reduced by more than half to about 1/20 wave.  I was able to dial the coma term down to 0.010 waves and the total wavefront error decreased to 0.235 waves PVq(99%)  [meaning that 99% of the data fell within a range of 0.235 waves] with an rms of only 0.043 waves and a Strehl of 0.93!  The "measurement" was actually an average of 164 phase measurements to average out air turbulence.  I've attached the "shop screen" display from the 4D Foresight software.  This level of wavefront performance on a 14" telescope is exceptional--regardless of the manufacturer.

 

This result clearly implies that the clock alignment marks on the Celestron components are not very accurate.  I think that this may occur either because the sensitivity of the test used to place the marks is not very sensitive, or that there is a systematic error in the procedure.

 

I didn't try to realign the second scope but I suspect that it might benefit from a similar "re-alignment."

 

John

 

Attached Thumbnails

  • C14 Edge Num1 Shop Screen Final Test.jpg

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#2 mistateo

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Posted 02 May 2017 - 05:49 PM

Wow, this is incredible!  I wonder if even the synta manufacturer knows about this.



#3 jhayes_tucson

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Posted 02 May 2017 - 05:54 PM

Uh...they do now!


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#4 Cpk133

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Posted 02 May 2017 - 05:58 PM

That's quite an improvement for such a simple thing to do.  Wish I had the ability to do those kinds of tweaks.  Do you think you'll be able to see any difference in your imaging?  



#5 J A VOLK

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Posted 02 May 2017 - 06:13 PM

I rotated my corrector on my classic c14 in 20 degree increments and carefully star tested. I was able to improve the performance at an orientation that does not match the "factory" setting. The improvement 'appeared' as reduced roughness, I really didn't see astigmatism in any orientation, but might have manifested itself as I stated. Of course the spherical correction never changed (est. 1/6 wave undercorrected). Since I have a later model with nylon screws for centering the corrector was not too hard, just left it on the mount. First loosening the retaining ring a bit - loosened the centering screws exactly 2 turns, rotated the corrector with the secondary assembly, tightened them 2 turns. Would be a real pain with the older ones with shims.
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#6 mistateo

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Posted 02 May 2017 - 06:18 PM

Uh...they do now!

I can see it now.... Celestron Edge UltraHD... Featuring the same exact equipment, more thoughtfully/carefully assembled.


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#7 Cpk133

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Posted 02 May 2017 - 06:19 PM

I rotated my corrector on my classic c14 in 20 degree increments and carefully star tested. I was able to improve the performance at an orientation that does not match the "factory" setting. The improvement 'appeared' as reduced roughness, I really didn't see astigmatism in any orientation, but might have manifested itself as I stated. Of course the spherical correction never changed (est. 1/6 wave undercorrected). Since I have a later model with nylon screws for centering the corrector was not too hard, just left it on the mount. First loosening the retaining ring a bit - loosened the centering screws exactly 2 turns, rotated the corrector with the secondary assembly, tightened them 2 turns. Would be a real pain with the older ones with shims.

Artificial star in a controlled environment?



#8 J A VOLK

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Posted 02 May 2017 - 06:27 PM

I rotated my corrector on my classic c14 in 20 degree increments and carefully star tested. I was able to improve the performance at an orientation that does not match the "factory" setting. The improvement 'appeared' as reduced roughness, I really didn't see astigmatism in any orientation, but might have manifested itself as I stated. Of course the spherical correction never changed (est. 1/6 wave undercorrected). Since I have a later model with nylon screws for centering the corrector was not too hard, just left it on the mount. First loosening the retaining ring a bit - loosened the centering screws exactly 2 turns, rotated the corrector with the secondary assembly, tightened them 2 turns. Would be a real pain with the older ones with shims.

Artificial star in a controlled environment?

real stars - took a couple months as had to wait for good seeing for each test. Logged the results as best I could. I'm pretty experienced at star testing - learned to figure mirrors solely on star testing Dobson style.
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#9 jhayes_tucson

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Posted 02 May 2017 - 06:30 PM

That's quite an improvement for such a simple thing to do.  Wish I had the ability to do those kinds of tweaks.  Do you think you'll be able to see any difference in your imaging?  

Probably not by much if at all in the final result.  The biggest difference is that I could see the little bit of astigmatism when I focused and that should disappear.

 

John


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#10 Cpk133

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Posted 02 May 2017 - 06:33 PM

 

 

I rotated my corrector on my classic c14 in 20 degree increments and carefully star tested. I was able to improve the performance at an orientation that does not match the "factory" setting. The improvement 'appeared' as reduced roughness, I really didn't see astigmatism in any orientation, but might have manifested itself as I stated. Of course the spherical correction never changed (est. 1/6 wave undercorrected). Since I have a later model with nylon screws for centering the corrector was not too hard, just left it on the mount. First loosening the retaining ring a bit - loosened the centering screws exactly 2 turns, rotated the corrector with the secondary assembly, tightened them 2 turns. Would be a real pain with the older ones with shims.

Artificial star in a controlled environment?

 

real stars - took a couple months as had to wait for good seeing for each test. Logged the results as best I could. I'm pretty experienced at star testing - learned to figure mirrors solely on star testing Dobson style.

 

Wow, that would be tough.  If I wasn't looking at a live image in a controlled environment, forget about it.



#11 jhayes_tucson

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Posted 02 May 2017 - 06:36 PM

I rotated my corrector on my classic c14 in 20 degree increments and carefully star tested. I was able to improve the performance at an orientation that does not match the "factory" setting. The improvement 'appeared' as reduced roughness, I really didn't see astigmatism in any orientation, but might have manifested itself as I stated. Of course the spherical correction never changed (est. 1/6 wave undercorrected). Since I have a later model with nylon screws for centering the corrector was not too hard, just left it on the mount. First loosening the retaining ring a bit - loosened the centering screws exactly 2 turns, rotated the corrector with the secondary assembly, tightened them 2 turns. Would be a real pain with the older ones with shims.

I'm sure that it's possible but without a way to accurately record the results this seems like hit-and-miss proposition.  Star testing has its value but I'm not much of a believer in it as a way to produce accurate numbers (such as 1/6 wave under-corrected.)  Without the interferometer, I personally never could have done this alignment in a way that I would have believed.

 

John


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#12 jhayes_tucson

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Posted 02 May 2017 - 07:08 PM

BTW, I ran into some of the folks on the JWST optical test team while I was at 4D.  I've know these folks for many years since I was on the JWST IPT2 test review committee.  I asked what kind of wavefront spec they are aiming for on the final telescope after it is fully deployed.  It is mostly an encircled energy spec but the rms will be 0.0625 waves at the operating wavelength.  That's a Strehl of 0.822.  The JWST telescope is now assembled and being shipped to Houston for full cryo testing in a large vacuum chamber.

 

John


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#13 J A VOLK

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Posted 02 May 2017 - 10:42 PM

I rotated my corrector on my classic c14 in 20 degree increments and carefully star tested. I was able to improve the performance at an orientation that does not match the "factory" setting. The improvement 'appeared' as reduced roughness, I really didn't see astigmatism in any orientation, but might have manifested itself as I stated. Of course the spherical correction never changed (est. 1/6 wave undercorrected). Since I have a later model with nylon screws for centering the corrector was not too hard, just left it on the mount. First loosening the retaining ring a bit - loosened the centering screws exactly 2 turns, rotated the corrector with the secondary assembly, tightened them 2 turns. Would be a real pain with the older ones with shims.

I'm sure that it's possible but without a way to accurately record the results this seems like hit-and-miss proposition.  Star testing has its value but I'm not much of a believer in it as a way to produce accurate numbers (such as 1/6 wave under-corrected.)  Without the interferometer, I personally never could have done this alignment in a way that I would have believed.
 
John

Confident I found a "sweet spot" - I wouldn't generally recommend the procedure though, it is laborious and a bit subjective. Did it partially as a fun/academic exercise and partly because the scope was somewhat distressed when I got it, and the previous owner had stated he messed with the optics (and more than likely not reassembled it properly). Rotating the secondary had pretty much no effect BTW. Suiter does have diagrams to evaluate spherical correction for the approximate obstruction of an SCT - I think you can get pretty close if not too many other error types conflate the star test.

Edited by J A VOLK, 02 May 2017 - 10:45 PM.


#14 SandyHouTex

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Posted 03 May 2017 - 10:30 AM

Wow, this is incredible!  I wonder if even the synta manufacturer knows about this.

Last I heard, the C14s are assembled in Torrance:

 

http://www.chuckhawk..._connection.htm



#15 Wildetelescope

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Posted 03 May 2017 - 12:26 PM

This is a postscript to my testing article found here:  https://www.cloudyni...elescopes-r3095

 

A while back, I posted an article about testing the two C14 Edge scopes that I have in my shop.   The first scope that I tested is the one that I most often use for imaging and it tested at a Strehl of 0.845 (0.065 waves rms.)  Anything over 0.8 meets the Maréchal criteria for a diffraction limited system, so that's not a bad system.  Still a number of folks were surprised that the system didn't measure better.

 

Since I did the original test, I've installed a carbon fiber tube on the telescope so I scheduled another session on the interferometer, which I just finished today.  Along the way, I've seen a number of articles showing that it is possible to improve the wavefront quality of Celestron systems by simply rotating the corrector plate to minimize wavefront errors.  That seemed like a good thing to try since astigmatism was the predominant wavefront error.  I also knew that the secondary was nearly a perfect sphere (tested in another session on a WYKO 6000.)   I was able to get the telescope aligned fairly quickly and positioned the field point to be within about +/-0.5 mm from center of the field.  I used all of the same components as in my original report so I didn't go through all of the calibration steps that I did the first time around.  

 

I then rotated the corrector plate assembly by~ 90 degrees and realigned the secondary.  The wavefront improved dramatically, and I could see that it might do better with a bit more rotation.  So I rotated the corrector assembly by another ~15 degrees.  Measurements showed that the astigmatism term was reduced by more than half to about 1/20 wave.  I was able to dial the coma term down to 0.010 waves and the total wavefront error decreased to 0.235 waves PVq(99%)  [meaning that 99% of the data fell within a range of 0.235 waves] with an rms of only 0.043 waves and a Strehl of 0.93!  The "measurement" was actually an average of 164 phase measurements to average out air turbulence.  I've attached the "shop screen" display from the 4D Foresight software.  This level of wavefront performance on a 14" telescope is exceptional--regardless of the manufacturer.

 

This result clearly implies that the clock alignment marks on the Celestron components are not very accurate.  I think that this may occur either because the sensitivity of the test used to place the marks is not very sensitive, or that there is a systematic error in the procedure.

 

I didn't try to realign the second scope but I suspect that it might benefit from a similar "re-alignment."

 

John

Hi John!  

    Those results are very interesting.  So the main thing I am taking from your original article, and these latests results are that the optical components, in particular the primary mirror, are not necessarily the weak link in these higher end mass produced scopes, but rather the mounting hardware and care in assembly?  I understand your sample is too small to generalize across a single product range let alone the entire market, but at least in some cases this may be true?  

 

Maybe a better way to put it is that mounting hardware and care in assembly may be as likely to contribute to variability in scope performance as variability in the optical components themselves.   Is that a fair assessment?  

 

Thank you very much for sharing your experiences.

 

Cheers!

 

JMD



#16 gnowellsct

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Posted 03 May 2017 - 08:22 PM

It would be nice to know how to optimize this without having a home interferometer.  

 

But, I have often said, that Celestron's biggest enemy is not in its optics as such but in the mechanics of putting it altogether.  This is a new version of that argument.  Greg N


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#17 tolgagumus

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Posted 03 May 2017 - 09:05 PM

Have you guys seen this? 

 

https://youtu.be/2TXIr-x-p1o



#18 Alph

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Posted 03 May 2017 - 09:05 PM


I then rotated the corrector plate assembly by~ 90 degrees and realigned the secondary. 

John

To be clear. Did you rotate the corrector and the secondary together?



#19 jhayes_tucson

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Posted 03 May 2017 - 09:11 PM

 

Wow, this is incredible!  I wonder if even the synta manufacturer knows about this.

Last I heard, the C14s are assembled in Torrance:

 

http://www.chuckhawk..._connection.htm

 

Yes, that is true; but all of the components except the corrector plate come from China (AKA Synta.)  The primary is fully manufactured and qualified in China.  The secondaries are delivered as nominally spherical; but as many have reported here, the secondaries are reworked in Torrance to minimize the total wavefront error.   There is a lot of work done in Torrance but it's an effort split between China and the US to deliver a finished C14.  Many of the other scopes (of all other sizes) still go through Torrance as well.

 

John


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#20 jhayes_tucson

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Posted 03 May 2017 - 09:20 PM

Hi John!  

 

 

 

    Those results are very interesting.  So the main thing I am taking from your original article, and these latests results are that the optical components, in particular the primary mirror, are not necessarily the weak link in these higher end mass produced scopes, but rather the mounting hardware and care in assembly?  I understand your sample is too small to generalize across a single product range let alone the entire market, but at least in some cases this may be true?  

 

Maybe a better way to put it is that mounting hardware and care in assembly may be as likely to contribute to variability in scope performance as variability in the optical components themselves.   Is that a fair assessment?  

 

Thank you very much for sharing your experiences.

 

Cheers!

 

JMD

 

 

Some of the astigmatic error in the system could be due to mounting issues but my measurements were not designed to determine the origin.  At this point, I'm not sure where the astigmatic error originates, but it may not matter.  The key take away is that on this scope, the wavefront could be optimized by rotating the corrector plate assembly (=corrector+secondary) to minimize the total wavefront error.  Others have seen this effect as well so it may be safe to assume that the ability to make the wavefront better may not be isolated to just this one telescope.  The problem for anyone without an interferometer is that this type of alignment is likely to be a very difficult procedure to carry out with any precision.

 

John


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#21 jhayes_tucson

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Posted 03 May 2017 - 09:30 PM

 


I then rotated the corrector plate assembly by~ 90 degrees and realigned the secondary. 

John

To be clear. Did you rotate the corrector and the secondary together?

 

 

Yes.  As I said, previous interferometric measurements showed that the secondary in this particular telescope was a very high quality spherical surface.  I showed the results of that test in the test report so you can see for yourself.  That means that in this particular case, the only two components that have astigmatic error (AKA anamorphic surfaces) must be the primary mirror and the corrector plate.  On other systems with secondaries that may not be radially symmetric, rotating the whole assembly may not be the best thing to do.  I should point out that I measured two secondaries and that both were radially symmetric--even though one showed significant zonal corrections.  I haven't measured very many secondaries so I can't say if others contain non-axially symmetric corrections.  So, extrapolate these results at your own risk.  The only way to know for sure is to make a good measurement of the individual components (and that's not trivial.)

 

John


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#22 jhayes_tucson

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Posted 03 May 2017 - 09:48 PM

Have you guys seen this? 

 

https://youtu.be/2TXIr-x-p1o

Tolga,

I didn't have the patience to watch this entire thing (at 1x) but I'm going to provide some comments on it anyway.  If anyone wants to try this procedure here is what you should know:

 

1)  Holly smokes...you don't need to try the alignment at 26 different positions around a full rotation!  Since astigmatic error is slowly varying and is an even aberration, here's a strategy that will work much more efficiently:  First clock the corrector by +/-90 degrees and determine which is better.  Then clock the corrector from that position by +/- 45 degrees and determine which is better.  If you want to go crazy, then clock the corrector by +/- 22.5 degrees and determine which is better.  Then stop.  That's only 6 measurements!  You will NOT be able to tell if it gets better or worse with any smaller increment than 22.5 degrees!

 

2)  When you change the clock angle of the corrector, you will change the tilt of the secondary and induce on-axis coma.  So for each adjustment, it is absolutely necessary to very carefully realign the secondary in order to look for astigmatic error.  On the interferometer, that was very easy because I could simply bring the return beam right back to where I started after a rotation (to within a fraction of a wave.)  I could also read out the digital value of coma to make sure that it was less than ~0.05 waves and the value for astigmatism to see where it was minimized.  If you do this on a star, you should have a mount that tracks well and a wide field eyepiece with a reticle.  Place the star on the reticle, then rotate the corrector plate, and finally re-adjust the secondary tilt to bring the star back to the middle of the reticle.  A high power eyepiece could then be used to evaluate the Airy pattern.

 

 

John


Edited by jhayes_tucson, 03 May 2017 - 09:58 PM.

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#23 tolgagumus

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Posted 03 May 2017 - 10:29 PM

 

Have you guys seen this? 

 

https://youtu.be/2TXIr-x-p1o

Tolga,

I didn't have the patience to watch this entire thing (at 1x) but I'm going to provide some comments on it anyway.  If anyone wants to try this procedure here is what you should know:

 

1)  Holly smokes...you don't need to try the alignment at 26 different positions around a full rotation!  Since astigmatic error is slowly varying and is an even aberration, here's a strategy that will work much more efficiently:  First clock the corrector by +/-90 degrees and determine which is better.  Then clock the corrector from that position by +/- 45 degrees and determine which is better.  If you want to go crazy, then clock the corrector by +/- 22.5 degrees and determine which is better.  Then stop.  That's only 6 measurements!  You will NOT be able to tell if it gets better or worse with any smaller increment than 22.5 degrees!

 

2)  When you change the clock angle of the corrector, you will change the tilt of the secondary and induce on-axis coma.  So for each adjustment, it is absolutely necessary to very carefully realign the secondary in order to look for astigmatic error.  On the interferometer, that was very easy because I could simply bring the return beam right back to where I started after a rotation (to within a fraction of a wave.)  I could also read out the digital value of coma to make sure that it was less than ~0.05 waves and the value for astigmatism to see where it was minimized.  If you do this on a star, you should have a mount that tracks well and a wide field eyepiece with a reticle.  Place the star on the reticle, then rotate the corrector plate, and finally re-adjust the secondary tilt to bring the star back to the middle of the reticle.  A high power eyepiece could then be used to evaluate the Airy pattern.

 

 

John

 

Thanks John, I just meant to show someone else confirmed your findings on a different model. So it seems this is a common thing. 



#24 GlennLeDrew

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Posted 03 May 2017 - 11:13 PM

If the secondary is known to be a good figure of revolution, then allowing it to rotate with the corrector is in principle permissible. If purely spherical, definitely so. If possessing zonal corrections that's still OK, unless perhaps the secondary is rather notably offset from the center of the corrector. A significant offset for a 'touched up" secondary would have it oscillate when carried around with the rotating corrector, thereby translating the figure with respect to the axis defined by the primary baffle/primary mirror/eyepiece support. But it would require a pretty significant de-centering in order to sensibly impair the wavefront; I'd suppose by about 1/20 the secondary diameter, or an offset of 2.5mm for a 50mm diameter optic.

 

Nulling astigmatism via rotation of the corrector (discounting the co-rotation of the good secondary) implies both the corrector and primary impart at least somewhat similar, generally anamorphic (potato chip) wavefront deformation. Somewhat to be expected, given the thinness of the substrate (the outer part in the case of the primary.) Eliminating astigmatic figures would likely require frequent enough rotational adjustment of the optic on its support, so as to defeat the establishment of a saddle-shaped component in the figure that a systematic warping can too easily impart. If such frequent rotational adjustment is not conducted, it might be due to cost savings.


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#25 jhayes_tucson

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Posted 03 May 2017 - 11:32 PM

Glenn,

I didn't want to go into these particular details so thank you for doing it for me.  As you have so eloquently pointed out, things quickly become a lot more complicated as soon as you have a non-axially symmetric secondary that may be mounted off-center on the corrector plate.  I left out the details but I've carefully centered the corrector plate assembly relative to the baffle tube axis on this scope and since I had already measure the secondary figure quite accurately, I didn't have to worry about this particular detail.  However, it is a consideration for anyone who might want to try this without knowing the shape of the secondary or how the corrector assembly is aligned.  Rotating the corrector assembly willy-nilly may make things better and it may make things a lot worse.  The good news is that things can always be put back in place if it doesn't work.

 

John


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