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Old 17-02-2019, 08:38 AM
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Exclamation Celestron 8” SCT Focus Issue

My uncle has a Celestron 8Ē SCT OTA on a really nice new bit of kit! The other night we were pointing it at the Moon but we couldnít get the damn thing into focus! When focusing one way or the other the Moon was moving across the field of view but not focusing. The focus knob started to get tight if I turned it either way as I was turning it so much.

At certain points you can hear something coming from the back (possibly the mirror shift) but I got a feeling soemthing might be disconnected or lose.

I was hoping if someone can help me try to fix it? Today I am going to double check that it isnít a backfocus issue. If thatís all ok then it might need to be looked at and Iím not confident enough to take it apart especially if it needs to be collimated.

Was wondering if thereís anyone here in Melbourne who has the ability to inspect/repair the issue?
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Old 17-02-2019, 08:50 AM
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Merlin66 (Ken)
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Some detail on the model of the C8 and the way it is mounted may assist investigations....
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Old 17-02-2019, 09:09 AM
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If the focus knob goes tight at both ends of the travel then probably nothing has come adrift and it is just a matter of getting the backfocus distance right.


Have a look down the front of the tube while you run the focus from stop to stop, you should be able to see the primary mirror moving up and down the baffle tube maybe 40-50mm.
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Old 17-02-2019, 12:35 PM
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Sorry everyone!

Itís a Celestron C8-A XLT OTA.

Mounted on an EQ6-AZ and was using a 2Ē GSO diagonal with a 25mm eyepiece.

Yep I noticed the movement inside.

I will be doing the backfocus test this arvo and let you know.

Thanks!
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Old 17-02-2019, 05:54 PM
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After checking the focuser looks like some rubber washer has broken.

Check video and photos on the Dropbox links below.

https://www.dropbox.com/s/8cjqxagyiv...%2030.jpg?dl=0

https://www.dropbox.com/s/pa9clob3n7...%2049.jpg?dl=0

https://www.dropbox.com/s/17befidxwu...%2038.mov?dl=0

https://www.dropbox.com/s/4rs4r5ex1i...%2029.mov?dl=0
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Old 17-02-2019, 06:20 PM
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Funny enough I had the micro focuser off mine this morning and that dropped out of mine too. I think it is supposed to sit in the recess on the back the mirror cell and cushion the stop slightly.
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Old 17-02-2019, 06:30 PM
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Is it normal for the focuser to move in and out when the rubber knob and metal ring is off?
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Old 17-02-2019, 06:53 PM
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Yes, without the bolted down part of the focuser fitted the rest will move out of the tube as the mirror moves toward the rear cell.
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Old 17-02-2019, 07:03 PM
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Quote:
Originally Posted by The_bluester View Post
Yes, without the bolted down part of the focuser fitted the rest will move out of the tube as the mirror moves toward the rear cell.
OK.

Very strange though. I focused it all the way in and out and still no change in the eyepiece. I am focusing on a distant tree as it's much easier to do during the day.

Anything else I can try or check?
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Old 17-02-2019, 07:19 PM
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Can you post up a pic of the optical train? with everything attached as you had it?
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Old 17-02-2019, 07:25 PM
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Quote:
Originally Posted by The_bluester View Post
Can you post up a pic of the optical train? with everything attached as you had it?
Put the knob back on and put the diagonal and eyepiece back in?
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Old 17-02-2019, 07:30 PM
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If new return for replacement
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Old 17-02-2019, 07:35 PM
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Quote:
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If new return for replacement
It has passed the 2 year Celestron warranty unfortunately.
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Old 17-02-2019, 08:00 PM
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Quote:
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Put the knob back on and put the diagonal and eyepiece back in?

Yes. So we can see how everything looks together.
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Old 17-02-2019, 08:09 PM
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Quote:
Originally Posted by The_bluester View Post
Yes. So we can see how everything looks together.
Uploaded photos here

https://www.dropbox.com/sh/2stg3461s...v-ABpX7ia?dl=0
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Old 18-02-2019, 12:27 AM
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That all looks ok to me, I have the 8" edge, have you any way to check using a 2" diagonal

Another poster here, may help, delete if not allowed

https://www.cloudynights.com/topic/2...m-please-help/
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Old 18-02-2019, 05:39 AM
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Quote:
Originally Posted by Ukastronomer View Post
That all looks ok to me, I have the 8" edge, have you any way to check using a 2" diagonal

Another poster here, may help, delete if not allowed

https://www.cloudynights.com/topic/2...m-please-help/
Thanks Iíll have a read.

Tried with a 1.25Ē and 2Ē diagonal and both do the same thing.
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Old 18-02-2019, 07:31 AM
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https://www.cloudynights.com/topic/6...se-otai-think/

This may give some insight to the possible issue......
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Old 18-02-2019, 07:44 AM
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It does beg the question that has not been asked. How many turns of the focus knob are you getting between the stops? It might be in the videos but I can not open dropbox files at work and we are currently "Enjoying" an NBN connection with a download rate anywhere down to 1mbps, which apparently has nothing wrong with it, so opening the videos at home is not workable either.

You should see something in the order of 30 turns of the focus knob from end to end.
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Old 18-02-2019, 07:55 AM
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I'm also intrigued by that rubber washer...
I can't remember coming across that when I fitted the Feathertouch micro.....
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