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Meade 2120 SCT Secondary Recoating - Experiences

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#1 CltFlyboy

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Posted 18 November 2020 - 01:45 PM

I'm going to get the secondary on my Meade 2120 10" SCT recoated (it's one of the silver MCOG ones). It's got the freckles that are typical with this particular model. Before I start tearing it apart I wanted to reach out to get info from anyone here who has done this on this specific model (or the 2080 if it's identical). I really want to make sure I have all the gotchas covered before doing this. Appreciate any and all help.


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#2 CHASLX200

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Posted 18 November 2020 - 06:54 PM

My 1982 LX2 had the same prob.



#3 Rick-T137

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Posted 18 November 2020 - 08:24 PM

I had this done on my 1985 LX3 2080. The only real gotcha is to ensure that you mark the secondary such that you can return it to the scope in the same orientation as you removed it. This is critical to ensure the scope performs optimally once you get the recoated mirror back.

 

I had mine done by Optiques Fullum in Quebec Canada for about $120 and I am really happy with the results. The turn around was fairly quick (2-3 weeks I think?) and Normand was in constant contact with me (via email). Very nice.

 

BEFORE:

Meade 2080 Secondary
 
AFTER:

Meade 2080 Secondary FIXED

 

In my mind it was certainly worth it. If you decide to remove the corrector (which I did), ensure that it is marked as well so it can go back in the proper way. Also, if removing the corrector, be mindful of any cork shims that sit between the edge of the corrector and the OTA cell. They are easy to lose or damage.

 

I hope that helps!

 

Rick


Edited by Rick-T137, 18 November 2020 - 08:25 PM.

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#4 CltFlyboy

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Posted 18 November 2020 - 09:29 PM

Rick, did you have to remove the mirror from the mounting cell or did you just send it in as is? Thanks for the feedback, this helps immensely!


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#5 Rick-T137

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Posted 18 November 2020 - 10:20 PM

I think the mirror is glued or epoxied to the backing, so I sent in the whole thing  - they managed to recoat the mirror with no issues. I'm not sure that would apply to all places, but it worked for Optiques Fullum.


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#6 jgraham

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Posted 19 November 2020 - 11:29 AM

Agreed. I had the secondary on my 2080 recoated. Be sure to mark the orientation of the corrector. The secondary is spherical so its orientation shouldn’t matter, but marking it as well is a good idea. Check with your coaster if the back plate needs to be removed. Mine did, so I cut it off with dental floss and remounted it with clear silicone adhesive using toothpicks to space it off the plate. This was several years ago and it’s working great!


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#7 Bomber Bob

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Posted 19 November 2020 - 01:13 PM

I'm going to get the secondary on my Meade 2120 10" SCT recoated (it's one of the silver MCOG ones). It's got the freckles that are typical with this particular model. Before I start tearing it apart I wanted to reach out to get info from anyone here who has done this on this specific model (or the 2080 if it's identical). I really want to make sure I have all the gotchas covered before doing this. Appreciate any and all help.

Henry, Welcome to The Classics! from an Old Fly Boy (B-52s & C-130s in the USAF).  We need to pepper our posts with There I was... stories -- astro-related, of course.  Nothing like The Pleiades through good binoculars at 35000 feet...


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#8 RichA

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Posted 22 November 2020 - 12:54 AM

Agreed. I had the secondary on my 2080 recoated. Be sure to mark the orientation of the corrector. The secondary is spherical so its orientation shouldn’t matter, but marking it as well is a good idea. Check with your coaster if the back plate needs to be removed. Mine did, so I cut it off with dental floss and remounted it with clear silicone adhesive using toothpicks to space it off the plate. This was several years ago and it’s working great!

That might be a bit wishful.  Even spherical optics can be rotated against each other to achieve better performance, which is a sign that the surface can deviate a bit from being accurately spherical.



#9 CltFlyboy

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Posted 22 November 2020 - 04:41 PM

Thanks Rich. I always was under the impression that SCTs secondaries were uniformly spherical and thus had no discernable alignment nuances (outside of standard collimation). I'd love to see evidence one way or the other just for personal knowledge. I'm planning on marking the position of everything when I do the tear down, just the same as if I were performing a deep cleaning, but with the addition of placing witness marks on the secondary on the back of the mirror itself if I have to remove it from the mount.


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#10 DAVIDG

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Posted 22 November 2020 - 07:40 PM

 The secondaries are not spherical. They are aspheric since that is what is needed to reduce off axis coma. Take it from someone who  has been making optics for close to 40 years now including working on many SCT scope both Celestron and Meade.  Rotating them makes no difference. What is critical is getting them centered. 

  Meade had their secondaries figured to aspheric figures I believe in Japan. When the scope optics were assembled on the test bench in the factory in the USA in  the optician would swap secondaries in and  out to find one that met Meade's spec. There are always errors when making optical surfaces  which is true of the secondaries as well so they varied in their exact figure so by swapping them in out  they were trying to find one that offset the errors in the primary and corrector plate.

 

                    - Dave 


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#11 starman876

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Posted 23 November 2020 - 08:14 AM

The issue is, depending how far apart the scope has been, is the primary properly centered in the visual back and then making sure the corrector is centered properly on the visual back and then making sure that it all is centered on each other.  I use a laser (make sure the laser is centered properly) Also, make sure the laser beam is straight and true.  I am sure Dave has a sure fire way to do all this.   I have heard of reports in where the heavy back plate is not attached to the tube properly and causes a small tilt of the whole assembly. Remember the corrector is attached to the tube on the front and has no adjustment for tilt.  The back plate fits really tight to the tube and maybe that is to make sure that this alignment never changes.  


Edited by starman876, 23 November 2020 - 08:27 AM.

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#12 CltFlyboy

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Posted 23 November 2020 - 09:36 AM

Thanks Dave - I really appreciate that detailed answer! That gives me a lot more confidence in doing this.



#13 DAVIDG

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Posted 23 November 2020 - 10:31 AM

 Both Celestron and Meade have a main selling point that their scopes are design for both visual and astrophotography. So for the scope to take descent pictures it can not have a lot of off axis coma. A Schmidt system with spherical optics will have a good amount of off axis coma. It will have a similar performance to  a Dall Kirkham which uses a spherical secondary and pretty bad off axis coma . A classsic cassegrain is better corrected from coma with a hyperbolic secondary and parabolic primary. A Ritchey Chretien cassegrain which uses a hyberbolic primary and secondary is fully corrected for coma.  So what the optical designers did in a commercial Schmidt Cassegrain is use a spherical primary since that is easy to make and then for the corrector they made it have more power which over corrects the wave front.  So the combination of the two is acting more like the parabolic primary in a classic cass.  or the hyberbolic primary in Ritchey. So with this overcorrected wavefront you need an aspheric secondary. Celestron would hand figure their secondaries and test them until it met their spec, and  as I said Meade had them  made and would swap them in and out to find one that met their spec. 

    Optics are made to be figures of rotation. If not then they have astigmatism and that is major error in making them. So something is really wrong in their process if they make astigmatic optics.  Also you have three optical elements that vary greatly in their optical power. The corrector is just about zero, the primary being F2 is strong and the secondary with 5x magnification even stronger. So the odds that asymmetrical optical error in one which such varying amount of power can be reduced or cancel by another is very low. 

   So what is really is happening when people say they rotated the optics and the image got better is that they were correcting for them not being centered correctly. If the optics are not centered correctly you may be able to improve the image but you'll never get them perfect. Once correctly centered you can spin them around and it will make no difference what the orientation is. 

   You also have to remember that Meade is just going to get them close enough to meet their spec so not perfect. So many times you can improve upon what the factory did just by make sure the secondary is centered on it's mount, the corrector centered in the tube and the secondary centered in the tube.

 

                Happy Thanksgiving !

                       - Dave 


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#14 CHASLX200

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Posted 24 November 2020 - 06:42 AM

 Both Celestron and Meade have a main selling point that their scopes are design for both visual and astrophotography. So for the scope to take descent pictures it can not have a lot of off axis coma. A Schmidt system with spherical optics will have a good amount of off axis coma. It will have a similar performance to  a Dall Kirkham which uses a spherical secondary and pretty bad off axis coma . A classsic cassegrain is better corrected from coma with a hyperbolic secondary and parabolic primary. A Ritchey Chretien cassegrain which uses a hyberbolic primary and secondary is fully corrected for coma.  So what the optical designers did in a commercial Schmidt Cassegrain is use a spherical primary since that is easy to make and then for the corrector they made it have more power which over corrects the wave front.  So the combination of the two is acting more like the parabolic primary in a classic cass.  or the hyberbolic primary in Ritchey. So with this overcorrected wavefront you need an aspheric secondary. Celestron would hand figure their secondaries and test them until it met their spec, and  as I said Meade had them  made and would swap them in and out to find one that met their spec. 

    Optics are made to be figures of rotation. If not then they have astigmatism and that is major error in making them. So something is really wrong in their process if they make astigmatic optics.  Also you have three optical elements that vary greatly in their optical power. The corrector is just about zero, the primary being F2 is strong and the secondary with 5x magnification even stronger. So the odds that asymmetrical optical error in one which such varying amount of power can be reduced or cancel by another is very low. 

   So what is really is happening when people say they rotated the optics and the image got better is that they were correcting for them not being centered correctly. If the optics are not centered correctly you may be able to improve the image but you'll never get them perfect. Once correctly centered you can spin them around and it will make no difference what the orientation is. 

   You also have to remember that Meade is just going to get them close enough to meet their spec so not perfect. So many times you can improve upon what the factory did just by make sure the secondary is centered on it's mount, the corrector centered in the tube and the secondary centered in the tube.

 

                Happy Thanksgiving !

                       - Dave 

If i had to pick a winner it goes to Meade on better optics on avg.  My all time best SCT was a 1984 C8 , but five LX200's made in the later 90's had very good optics. I have had duds with both brands  My all time worst was a 2005 gray tube C8.


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