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]

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

[The_bluester]

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/8cjqxagyivafj79/Photo%2017-2-19%2C%2018%2050%2030.jpg?dl=0

https://www.dropbox.com/s/pa9clob3n7vig2r/Photo%2017-2-19%2C%2018%2051%2049.jpg?dl=0

https://www.dropbox.com/s/17befidxwuyuwli/Video%2017-2-19%2C%2018%2050%2038.mov?dl=0

https://www.dropbox.com/s/4rs4r5ex1i1k1rc/Video%2017-2-19%2C%2018%2055%2029.mov?dl=0

[The_bluester]

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]

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]

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]

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

[Saturn488]

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

[Ukastronomer]

If new return for replacement

[Saturn488]

It has passed the 2 year Celestron warranty unfortunately.

[The_bluester]

Yes. So we can see how everything looks together.

[Saturn488]

Uploaded photos here

https://www.dropbox.com/sh/2stg3461s35j6za/AAByTeK43DHbI3Unv-ABpX7ia?dl=0

[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/258556-nexstar-8se-focus-problem-please-help/

[Saturn488]

Thanks I’ll have a read.

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

[Merlin66]

https://www.cloudynights.com/topic/609612-focus-stuck-broke-on-8se-otai-think/

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

[The_bluester]

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]

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

[mental4astro]

Hi Chrys,

The mirror moves up and down the baffle tube when you turn the focuser. So it isn't a focuser problem.

The washer being chewed up is odd, but it isn't affecting the action of the focuser, so leave this to one side for now.

Have you looked into the back of the OTA, up through the hole in the primary to look into the baffle? You should be able to see straight up, clear and without any obstruction like plastic, or fogging of the lenses in the baffle tube.

To not notice ANY change in focus tells me there may be an obstruction somewhere, especially if the primary mirror is moving when the focuser knob is spun. It's happened to me many times that I've gone an left a dust cap on an eyepiece... Could also be that something was accidentally shoved up the baffle tube, like a lens cap.

Just trying to eliminate possible reasons.

Alex.

[The_bluester]

I did not see it when I fitted my feathertouch some years back but found it on the focus shaft yesterday when I removed the focuser to look at something else.

[Saturn488]

Thanks for the reply.

Yep I looked through from the back and all looks alright.

The best way I can describe what I see is when we had it pointed to the Moon.

- It was blurry and out of focus so I turned the knob clockwise.
- After many, many turns the Moon started to move off the FOV and it was still not focused.
- I then turned the knob anti-clockwise and the Moon moved off the FOV of view in the opposite direction and still not focused.

The same results using both a 2" and 1.25" diagonal with a 25mm eyepiece.

[mental4astro]

Ok, that's different.

It may be that the focus mechanism has somehow gone askew and that the primary mirror is not correctly positioned in its cells. The image of the Moon should not be moving to one side and then the other when the knob is spun one way and then the other!!

May also explain the tightness of the focus knob too that the primary is out of kilter - it's putting a strain on the focus mechanism.

[Merlin66]

Alex,
No lenses in the baffle, this appears to be a standard version.

[mental4astro]

Thanks Ken.

One thing first. When you look up the baffle, does everything look co-centric? All the mirrors, reflections, tube outlines, mirror outlines, everything should be co-centric. I want to make sure it is the primary mirror that is out of kilter first before opening up the OTA.

If it is the primary mirror that is out of position, it will mean needing to open the OTA via the rear, removing the back section an inspecting the mirror/focus mechanism. Sounds horrific, but it's more daunting than difficult. There's plenty of info on doing this, including videos. But one thing at a time, and lets look for the tell-tale signs that the primary mirror is out of position on the focuser mechanism. Another way, though more tricky, is to see if the outline of the primary mirror looks co-centric with the rest of the scope when you look down through the corrector plate. May be difficult to gauge this as the primary may just be a few mm out of position. If it is the focus mechanism that out of whack, then we'll look at fixing this.

The primary mirror being out of position may also explain the rubber washer being chewed up as there is a strain on the focuser mechanism that's pushed the driving threads out of whack, and hence drove the rubber washer out when the focuser knob was turned so far.

Alex.

[Saturn488]

Thanks Alex.

I will check as you suggested when I get home tonight and let you know.

[Saturn488]

Had to move all photos to one folder, here is the new link.

https://www.dropbox.com/sh/2stg3461s35j6za/AAByTeK43DHbI3Unv-ABpX7ia?dl=0

[Ukastronomer]

It is the standard version

[Saturn488]

Apologies, I didnt get a chance last night to check.

I will check it out tonight.

If there is anything else I can have a look at please let me know.