Celestron 8” SCT Focus Issue

[Saturn488]

My uncle has a Celestron 8” SCT OTA on a really nice new bit of kit! The other night we were pointing it at the Moon but we couldn’t 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 isn’t a backfocus issue. If that’s all ok then it might need to be looked at and I’m not confident enough to take it apart especially if it needs to be collimated.

Was wondering if there’s anyone here in Melbourne who has the ability to inspect/repair the issue?

[Merlin66 (Ken)]

Some detail on the model of the C8 and the way it is mounted may assist investigations....

[The_bluester (Paul)]

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.

[Saturn488]

Sorry everyone!

It’s a Celestron C8-A XLT OTA.

Mounted on an EQ6-AZ and was using a 2” 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!

[Saturn488]

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

[The_bluester (Paul)]

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.

[Saturn488]

Is it normal for the focuser to move in and out when the rubber knob and metal ring is off?

[The_bluester (Paul)]

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.

[Saturn488]

Originally Posted by The_bluester

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?

[The_bluester (Paul)]

Can you post up a pic of the optical train? with everything attached as you had it?

[Saturn488]

Originally Posted by The_bluester

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?

[Ukastronomer (Jeremy)]

If new return for replacement

[Saturn488]

Originally Posted by Ukastronomer

If new return for replacement

It has passed the 2 year Celestron warranty unfortunately.

[The_bluester (Paul)]

Originally Posted by Saturn488

Put the knob back on and put the diagonal and eyepiece back in?

Yes. So we can see how everything looks together.

[Saturn488]

Originally Posted by The_bluester

Yes. So we can see how everything looks together.

Uploaded photos here

https://www.dropbox.com/sh/2stg3461s...v-ABpX7ia?dl=0

[Ukastronomer (Jeremy)]

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/

[Saturn488]

Originally Posted by Ukastronomer

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 I’ll have a read.

Tried with a 1.25” and 2” diagonal and both do the same thing.

[Merlin66 (Ken)]

https://www.cloudynights.com/topic/6...se-otai-think/

This may give some insight to the possible issue......

[The_bluester (Paul)]

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.

[Merlin66 (Ken)]

I'm also intrigued by that rubber washer...
I can't remember coming across that when I fitted the Feathertouch micro.....