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Recoating a C11HD primary mirror, Q on optical alignment

catadioptric Celestron DIY optics SCT ATM cassegrain
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#1 BWAZ

BWAZ

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Posted 30 May 2017 - 05:34 PM

So basically I came to the point of needing to rebuild the C11HD I was stupid enough to grab here on CN which the seller refused taking it back. The primary mirror is much worse than what I'd expected, and I need to have it recoated. There are dozens of missing/damaged spots plus numerous micro-scratches with the latter concerning me the most. 

 

I've disassembled the tube, cleaned every components, re-lubed the main baffle, and collimated the system as best as I could. A few glimpses on Jupiter were actually very very good. I could have put my mind in peace then, but an A/B comparison with my C9.25 revealed the real problem: scattering. I attributed the problem to these micro-scratches, a result of a careless cleaning for sure. 

 

I'm quite handy but after researching this topic a bit more, I came the conclusion this is not a task for an everyday DIYer like me. The most difficult part in the process is to reinstall the mirror carriage while maintaining the mechanical/optical alignment. Celestron chooses to use the wet shim to attach the mirror to the carriage, and thus resulting in a very challenging scenario for anyone who attempts to reinstall the primary mirror for whatever reason. 

 

I followed some work done by a few CNer's and found the key step is to use a laser beam reflected from the primary to check if the optical centerline of the mirror is aligned with the mechanical ceternline of the carriage, by spinning the mirror until the reflected beam hits the same spot. 

 

If you've done this before, I'd greatly appreciate your inputs here. My own visualization is lacking here, in particular on how to set up the fixture for the mirror during the whole alignment procedure.

 

Thanks!


Edited by BWAZ, 30 May 2017 - 05:36 PM.



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