C9.25 mirror locking

5 years 6 months ago #101646 by albertw
C9.25 mirror locking was created by albertw
Hi,

I've got things pretty well set up in the observatory now and have started so do automated imaging but one of the limitations of the CPC 9.25 is now becoming a problem. There is no way to lock the mirror in place. This results in very noticeable out of focus points on ccd images as the scope moves around the sky.

One solution to this would be to get another mount and scope, but creche fees rule that possibility out!

So I'm left with wondering about modifying the scope to lock the mirror. I've a separate focuser screwed into the rear cell to do the actual focussing.

There don't seem to be any modifications out there on how to do this, which is a little surprising as I'd have thought it would be a common enough mod.

Not having opened up the scope it seems like there are two options.

1. Do like the larger scopes do and drill holes in the rear of the scope to screw into the support of the mirror cell.
2. construct some sort of O-ring retainer around the edge of the mirror and put screws in from the side of the ota that will lock the mirror in place.

Has anyone here tried anything like this before? :)

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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5 years 6 months ago - 5 years 6 months ago #101647 by dave_lillis
Replied by dave_lillis on topic C9.25 mirror locking
Hi Al,
Did it come with a locking shipping bolt, if so then you could use that.
Otherwise I'd go with drilling holes in the back of the ota and get some nuts and bolts in there.
You'd probably need someone to weld the threaded nuts to the back of the scope. I'd have them on the inside of the ota if there was room and if you're going to go to this kind of trouble, then I'd put in 3 of them in the shape of a triangle.
You'd want the bolts to touch the back of the mirror cell and not the glass for obvious reasons...
I'm sure I saw a page many many years ago of someone who did this to an older meade sct.

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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5 years 6 months ago #101652 by albertw
Replied by albertw on topic C9.25 mirror locking
Thanks Dave.

Celestron only start the shipping bolts with the C11. But that's exactly what I'm thinking of putting in. Guess I'll have to just take it apart and see what's back there to screw into - not the glass anyway :)

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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5 years 6 months ago #101653 by albertw
Replied by albertw on topic C9.25 mirror locking
Actually this is more what I was thinking of. Might be a bit safer, and doesn't assume there is a nice plate to screw into behind the mirror and I'm less likely to mess up collimation.

page 5-8 www.marsastro.org/presentations/MARS_Mirror-Lock.pdf

If there is enough space in the 9.25 to do it...

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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5 years 6 months ago #101661 by dave_lillis
Replied by dave_lillis on topic C9.25 mirror locking
It's a clever idea, but this could knock out the collimation as easily as using the 3screws from behind, I think either system would be ok so long as you don't over tighten the bolts.
Would you have less focusability/travel using this system compared to the 3bolts from behind the cell ?

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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5 years 6 months ago - 5 years 6 months ago #101662 by dave_lillis
Replied by dave_lillis on topic C9.25 mirror locking
Just had a thought, I might go for the side bolts as bolts on the back might get in the way of a camera or be more difficult to get to cos of the camera

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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5 years 6 months ago #101670 by albertw
Replied by albertw on topic C9.25 mirror locking
Goo points Dave. I think I'll have to pop the OTA open and see whats behind the mirror and measure how much the mirror does travel before making any decisions.

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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5 years 6 months ago - 5 years 6 months ago #101776 by albertw
Replied by albertw on topic C9.25 mirror locking
I was chatting to a couple of people yesterday about this again and after describing my problems it seemed that a moving mirror would explain it, but as Carl said, a C9.25 shouldn't suffer from that problem to the degree that I was seeing.

So today I decided to strip things down and have a look at the mirror and see if anything looked wrong.

First thing to do was to take the the autoguider rail and couterweight rail off. Once that was done I noticed that there was a little bit of play between the OTA and the fork. I'd noticed this before but put it down to the mount design and that pushing against the corrector plate probably wasn't a fair thing to do! However today I noticed that the tube (and hence the corrector plate) were able to move independent of the rear cell by several mm. Here's a video to show what I mean:




Time to take things apart.

Getting the OTA off a CPC fork isn't too difficult. Only 4 screws hold the OTA in place, two on each fork. However you need to take the panel off one fork and then loosen the bolts that secure the fork arm to the base (these are really tight - 'I think I'm going to break my Allen key trying to turn this with a pliers' kind of tight) to get enough slack to get the OTA off once you have the four supporting bolts off.

I thought I could then just unscrew the screws holding the rear cell onto the tube. Oh no. For a start, the four screws are held in with nuts so there would be no way to put it back together. Secondly, there were all loose. All of them.

The scope is second hand and I bought it from someone here. The previous owner hadn't done any modifications or cleaning so I can only assume the scope was like this when he bought it. Though perhaps it has loosened a little over time. The support it got from the metal bars for the guidescope and counterweight masked how obvious the problem really was.

So how to get at these screws. The corrector 'cell' is held in by three screws and nuts so you can't take those off either. What you need to do is remove the corrector plate, by taking the six screws on the front out. Then you can reach down with a pliers and tighten the screws. Here's a pic of three of them.



I decided not to mess with the mirror after that. I'll see how much better things are now before taking a drill to the rear cell :)

Here's a video of the lack of movement now:



Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/
The following user(s) said Thank You: dave_lillis, michael_murphy

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