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Some good info on SCT collimation

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#1 RichInLesta


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Posted 24 August 2019 - 10:31 AM

Recently, I had the misfortune (of my own doing!) of stripping one of the screw holes in the secondary mirror tilt plate of my Celestron C8.  While removing the secondary mirror to examine the problem, the entire assembly within the corrector plate began to turn, and it rotated quite a bit before I realized it was happening.  I had previously read that the optical path of the SCT is factory set and there was relevance to the orientation of the secondary mirror to the corrector plate and--by default--the primary mirror, so I knew I had to try to get it back to where it originally was.  The problem, however, is that I couldn't find any index marks (other than the notch in the holder for the secondary mirror that receives a set screw on the secondary mirror tilt plate) anywhere on either the secondary mirror assembly, the corrector plate, or the edge of the tube itself.  Some posts claimed that the secondary should be oriented to the 12 o'clock position, some said the 3 o'clock position, and some said it didn't matter as much WHERE it was oriented on some of the newer SCT's.  So I had to figure out for myself exactly what I had to do to get it back where it belonged.

Fortunately, I had just purchased the Hotech Advanced Laser Collimator.  The instructions that came with the collimator were "OK" but left out what turned out to be some important steps and explanations.  As I was looking for more detail, I happened across this instruction set, which I think is a fantastic reference to orienting the optical path of the SCT.  Although it's specific to how to calibrate everything with the Hotech collimator, it can also serve to shed some light on how the items in the optics path are interrelated.  I thought it was good enough to share, so here is a link to those instructions.  Worth a read-through if/when you find yourself having optical issues.




If you're curious, I was able to re-tap the hole in the tilt plate and replaced all three 3mm screws with 6-32 screws.  I find that I like the slightly larger screws because it feels a bit beefier.  Only downside I can see is that it's a coarser thread so I have to be more careful with tweaking, but knowing that, I think I'll be find.


And as for the Hotech, it took time to get through the steps to aligning the optics and I have one more step to finish up.  I need to add a tilt plate to the visual back so I can tweak the orientation at that point, but by Monday (I ordered one from Teleskop) I should be all set.


And as for the orientation of the secondary mirror to the corrector plate, I'm leaning towards agreeing with the "it doesn't matter as much" because by using the Hotech I was able to get the corrector plate perfectly centered and the secondary mirror perfectly aligned and I don't seem to be oriented to either the 12 o'clock or 3 o'clock positions.  But we'll see as soon as I can get some clear sky to test it with.

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#2 petert913



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Posted 24 August 2019 - 11:01 AM

Yeah, but $500 for a collimator I may use twice a year...... not gonna happen :)

#3 Michael2


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Posted 25 August 2019 - 04:50 PM

Rich, thanks for sharing this. I have only skimmed the pdf, but lots of good info. bow.gif

Please keep us informed on your progress.



#4 luxo II

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Posted 27 August 2019 - 05:20 AM

Laser collimators should get you close but frankly nothing beats a star test ... and it’s free. Learn how, it is not difficult.

Edited by luxo II, 27 August 2019 - 05:22 AM.

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