Celestron 8” SCT Focus Issue

[mental4astro (Alexander)]

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 (Paul)]

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 (Alexander)]

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 (Ken)]

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

[mental4astro (Alexander)]

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/2stg3461s...v-ABpX7ia?dl=0

[Ukastronomer (Jeremy)]

Originally Posted by Merlin66

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

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.