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Old 16-04-2021, 07:41 PM
Ramon (Ramon)
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SCT Focuser upgrade questions/help

I recently acquired an older 8" Celestron SCT. I have noticed what I
understand to be some mirror flop and changing focus.

If I add a secondary Crayford focuser, my understanding is that it would help with the focusing but it wouldn't with the mirror flop, would it?

Also, when using a secondary Crayford focuser, where should I have the standard focuser position? would winding it all the way in or out, prevent the flop and I'd then use the Crayford to make focus??
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Old 16-04-2021, 09:26 PM
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Merlin66 (Ken)
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The flop/slop is caused but the clearance between the baffle tube and the main mirror sled.
Nothing really to do with the focuser.
You need to run the focuser on the SCT 30 turns either way a couple of times to re-spread the grease between the baffle and the sled to reduce the slop.
Having said that the focuser design on the Celestron is pretty good fpr locating the main mirror.
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Old 16-04-2021, 10:45 PM
Ramon (Ramon)
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Quote:
Originally Posted by Merlin66 View Post
You need to run the focuser on the SCT 30 turns either way a couple of times to re-spread the grease between the baffle and the sled to reduce the slop.
Cheers, I've made the turns and will test on the next clear night. Fingers crossed.
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Old 17-04-2021, 09:06 AM
alval (Alan)
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For image shift during focusing that is the image moves side to side as you focus in and out, as well as the redistributing the grease when focusing you should always finish focus in one direction, the one that is pushing the mirror forward so it cant move back slightly under its own weight, if you pass focus go back past the focus point and creep up on it again, that should stop the focus changing. Centre object in this position and then as you change eyepieces and tweek focus in the same direction as before it should recentre the object. Mirror flop I take as being when you pass zenith the mirror centre of balance shifts to the opposite side and the mirror flops, just refocus as above. This is more of an issue with imaging.
Al
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Old 17-04-2021, 11:07 AM
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iborg (Philip)
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Hi Ramon

I strongly second Al's advice.

Even if you do get a Crayford, I suggest setting the Crayford about half way along its travel, then fairly closely focus with the mirror ('pushing' the mirror last) and finely fine focus with the Crayford.

Philip
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Old 20-04-2021, 07:00 PM
Ramon (Ramon)
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That makes really good sense. Thanks alval and iborg.
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