Advice on loose SCT secondary please

[si_]

Apologies if this has been discussed elsewhere, I did look but although there are some posts on this, they are for different models of 'scope.

I recently (January) bought a new Celestron C9.25 Fastar.

I have had the secondary out previously but now it does not come out, instead the whole secondary assembly just rotates when i try to loosen the retaining ring. I don't want to go to the trouble of boxing this up and sending it back if it is an easy fix to secure this again and get it re-aligned (I'm used to the fact that astronomy equipment often needs some hands-on treatment!) but having said that wouldn't want to undertake something that might cause more problems or invalidate any warranty.

I'd appreciate the thoughts of members with more experience of this model of 'scope than I have.

cheers

[Waldemar]

If the whole corrector plate including the fastar assembly rotated, your scope is now probably out of alignment.
The easy way would be to turn it back into it's original position and tighten the screws om the mounting ring at the edge of the correctorplate a little bit. Not too much, though!
Once you can get the secondary out again to put in the HyperStar, you can use a very small amount of PTFE (teflon) grease on the thread of the retaining ring, so in future it will loosen a bit easier.
The process of anodizing the aluminum parts really dries out the material and makes it more prone to locking. A little ptfe solves that for ever.

Maybe you need to put some more work in the alignment, if you don't know the opriginal position.  Celestron used to put alignment marks on the side of the main mirror and the corrector plate, usualy at 9 o'clock, but over the years they changed that strategy a bit.

In order to determine weather that is the case with your scope, you will need to remove the corrector plate and the shims used to centre it. Mark the position of the shims as well as the position of the correctorplate.
I do somewhere have a file with instructions how to exactly do this, so if you need that, pm me.

Regards,

Waldemar

[Stu]

First things first, it is well worth checking the warranty position. I would think that removing the corrector plate would probably invalidate it.

It sound to me to be a similar thing that I am faced with on an old C9.25. @Waldemar, I don't think the whole corrector has rotated, just the secondary holder. I managed to remove the secondary holder and tighten it up but it is an old scope, not in warranty so I would err on the side of caution!

[si_]

On 5/20/2016 at 09:09, Waldemar said:

If the whole corrector plate including the fastar assembly rotated, your scope is now probably out of alignment.
The easy way would be to turn it back into it's original position and tighten the screws om the mounting ring at the edge of the correctorplate a little bit. Not too much, though!
Once you can get the secondary out again to put in the HyperStar, you can use a very small amount of PTFE (teflon) grease on the thread of the retaining ring, so in future it will loosen a bit easier.
The process of anodizing the aluminum parts really dries out the material and makes it more prone to locking. A little ptfe solves that for ever.

Maybe you need to put some more work in the alignment, if you don't know the opriginal position.  Celestron used to put alignment marks on the side of the main mirror and the corrector plate, usualy at 9 o'clock, but over the years they changed that strategy a bit.

In order to determine weather that is the case with your scope, you will need to remove the corrector plate and the shims used to centre it. Mark the position of the shims as well as the position of the correctorplate.
I do somewhere have a file with instructions how to exactly do this, so if you need that, pm me.

Regards,

Waldemar

Thanks for the reply Waldemar. The corrector plate isn't rotating, just the secondary within it. I have no idea how the secondary unit fixes to the corrector but it appears that this is the problem. Your advice about teflon grease on the release ring is a good idea.

[Stu]

12 minutes ago, si_ said:

Thanks for the reply Waldemar. The corrector plate isn't rotating, just the secondary within it. I have no idea how the secondary unit fixes to the corrector but it appears that this is the problem. Your advice about teflon grease on the release ring is a good idea.

Si, if you can remove the front ring, then you can carefully take out the secondary mirror (the fastar bit I guess). The holder is in two pieces which screw together from either side of the corrector. You do really need to remove the corrector to be able to grip the rear section whilst unscrewing the front.

With the one I'm dealing with, the assembly was rotating but it was not possible to either tighten or under the holder. However, with a bit of careful heat from a hairdryer applies to the inside of the front section, the two parts released. I removed them completely, cleaned them up to check for muck on the threads, then reassembled them and have been able to get a tight fix now.

[si_]

So, it's sounds like it's not difficult a thing to remedy but does require removal of the corrector. Did you remove just the plate or the entire front assembly?

 

[Stu]

14 minutes ago, si_ said:

So, it's sounds like it's not difficult a thing to remedy but does require removal of the corrector. Did you remove just the plate or the entire front assembly?

 

Just the plate. If you remove the six screws which hold the plastic ring on then the corrector will come out. Make sure you mark the orientation so it goes back in exactly as you remove it.

[si_]

Thanks Stu.

[si_]

OK so good news is that the fastar retaining ring came loose and I have been able to remove the secondary. The secondary cell has a reasonable grip on the corrector so I think i can get away without needing to tighten it up if I remain careful in how much I tighten the Fastar retaining ring.

The bad news is that I can't see any alignment marks on the corrector plate or secondary cell to help re-align the secondary cell. There are two letter "A"s hand written in green ink on opposite sides on the edge of the secondary mirror itself, one is a lot fainter than the other, but I can't see what they are for.

Any suggestions?

[Mr Spock]

On mine the word 'Fastar' has the same alignment as the word 'Celestron'. Though it doesn't really matter. If you've had the secondary assembly out or loose it will need collimating.

[si_]

Thanks Michael - are you saying the rotational position of the secondary within the corrector plate isn't important then and all I need to do is re-collimate?

 

[Mr Spock]

Yes. The primary and corrector are aligned but the secondary isn't.

[si_]

That's great to hear - I've been fretting a lot about it - thanks for taking the time to respond

[Stu]

I agree, I've not paid any attention to the rotational positional of the secondary so I'm sure you will be ok.