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Focusing 'wobble' in CPC11 fixes?

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#1 KerryR

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Posted 10 June 2013 - 12:26 PM

When I focus my CPC11, the object gently oscillates back and forth in the ep, just a little, in time with the rate the knob is turned. Very slight binding of the knob is also felt that is coordinated with this wobble.

Note that the wobble I'm talking about is distinctly different from oscillations related to dampening times or oscillations induced by touching the scope. They are also distinctly different from the mirror shift often observed when reversing focus direction.

The best fix I've found so far: back the mirror all the way to the rear of the scope, the 'parked' position used while shipping, loosen the orange focuser retaining ring, ensure the focus knob is centered in the whole, and re-tighten the orange retaining ring while the mirror is tightly in the parked position.

The location of the rubber ring in the bearing stack appears to have an impact as well. It appears to have the most positive impact when it's located between the retainer and the outermost bearing, as opposed to between the rear casting and the inner bearing. The outer position is probably 'stock', but I played with it's location after complete disassembly of the entire ota when I added active venting.

I removed removed most of the issue at this point, but I'd like to know what others have done or observed...

#2 rmollise

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Posted 10 June 2013 - 02:40 PM

A little focus shift is normal for any SCT. I've never seen a scope's focus "wobble" and you describe, however. Sounds like you've got a problem focuser and need to call Celestron.

#3 KerryR

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Posted 10 June 2013 - 03:43 PM

I'm not worried enough about it to hassle with Celestron tech support. I'd expect, perhaps deservingly, to be handled with disdain for having had the OTA completely apart.

Curious, though, if other DIYers have observed the phenomenon and/or have found solutions.

Seems like I remember other folks having mentioned this here on CN in the past. I believe that's where I got the 'fix' mentioned in my OP, but I can't remember for certain.

#4 astrovienna

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Posted 10 June 2013 - 11:25 PM

I may have been one of the folks who mentioned this in the past. I had a C11 with a weird focus issue where the target would sort of corkscrew across the field while focusing, and suddenly snap 10-20 arseconds across the field. I figured the focusing rod was slightly bent. My solution won't help you though - I returned it to my dealer for different C11.

Kevin

#5 KerryR

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Posted 11 June 2013 - 09:25 AM

A very very slightly tilted or bent focus rod, such that the axis of the rod doesn't perfectly match the rotational axis of the threaded focus knob is one of my hypothesies. Luckily, it was nowhere near as bad as Kevin's example, and was dramatically reduced after the 'fix' in the OP.

#6 KerryR

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Posted 11 June 2013 - 09:44 AM

I did some snooping around via google, and found a couple of instances of the issue in other forums. The best example is:
Link

The 'fix' discussed there is pretty much what I did.

Good to know I'm not the only one who's seen this...






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