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Where best to lock SCT mirror

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#1 Lapisowl

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Posted 04 December 2021 - 10:13 PM

Long time CN member with a question for experienced ATM'ers or optics people.

 

I have been using a Crayford focuser on my C8 for years and now and I am considering adding a mirror lock to secure the primary "permanently".

 

I know the design is optimized for a specific corrector/primary separation, so how do I find the proper position to fix the primary?  Will it really make a difference?

 

Thanks for reading



#2 MitchAlsup

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Posted 05 December 2021 - 07:42 PM

a) if you are within a millimeter (or so) it really does not matter (visually)

 

b) any lock you put on the primary will be "setup" while you are using the scope. So you use the scope normally and get it focused, Then you move the locking bolts until they touch the backside of the mirror, while looking through the focuser, so you and see when the bolt touches the mirror*. Do this 3 times, and then the mirror is locked in place, and you can <gently> tighten down the holds(=nuts) on the bolts.

 

c) astrophotographically, you do as in (b) but watch that test images have equal sharpness in all 4 corners.

 

d) after locking the mirror "down" you can go through a collimation procedure to dial in the final degree of sharpness (not needed very often if already collimated)

 

(*) when the bolt touches the backside of the mirror the mirror will be tweaked a few thousandths of an inch which is easily seen as image shift.



#3 Lapisowl

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Posted 05 December 2021 - 08:01 PM

Thanks for the reply MitchAlsup.  I'm looking to permanently fix the primary in place. Possibly with 3 screws and some silicone. Deal with the alignment and collimation then leave it there, so I only have to use the Crayford.

 

I was thinking about calculating the back-focus I would need for an imaging train and setting it that way when I thought about the "optimal" spacing. Probably over-thinking it, but that's what bad weather is for; obsessing about astro stuff, right :)




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