Celestron C8-CF Restoration

[Robert72]

Last night I purchased a fairly tatty C8-CF from RVO https://www.rothervalleyoptics.co.uk/second-hand-celestron-8-sct-carbon-optical-tube-assembly.html

I probably should have asked this question here first, but just how much can these things be disassembled?

Not too sure about the mirror retaining/focussing system, meniscus lens etc.

Looking forward to carrying out a bit of a restoration on this scope, seems like a lot of scope for not a lot of cash.

[johngm]

This may help you on your way.

John

[johngm]

I know its a Meade, but the principle will be the same

John

[Skyline]

Nice scope, for a cheap price

[Robert72]

Thanks for the replies.  After finding a bit of time to do a more detailed search, it seems (unsurprisingly) that there are some decent articles about disassembly etc. out there. 

[Robert72]

Hi, managed to get home and inspect the C8. I was half expecting a cracked/chipped/scratched meniscus. however the optics look pretty darned decent.  I have included a couple of pictures to demonstrate (not sure the best way to photo this)........

The focusser is also very smooth, although i don't have another to compare it with.

The tube has seen better days, the bolts are corroded/missing, and there is corrosion under the paint on the end caps. Large areas of bubbling are visible.

Not too worried about the end caps for now, but can any of you knowledgeable C8 enthusiasts help me identify what all the bolts and bolt holes are for? I don't know if some have been added by previous owner(s).

 

Top view, the three middle bolts are obviously for the upper dovetail bar and the two top left are for a finder i guess, the rear cell has 4 c/sunk bolts for cell attachment, and the front has 3 c/sunk bolts,

again for cell attachment.  I think this is correct?:

Side view, right hand side looking from aft. The three rear holes, two are tapped and the rearmost one is plain, the corresponding front hole is tapped. What are these for?:

Underside. More holes for a lower dovetail:

Left hand side, looking from aft. More mystery holes. That front hole is quite close to the front cell bolt:

I would be extremely grateful if anyone could confirm that my guesses on the bolt functions is correct, and identify the mystery holes.

The current plan is to eventually do some kind of star testing in the current state, and then perhaps strip down and do a mild refurb. However given our current situation and work commitments none of this will happen till 2021!

Regards, please stay safe and healthy.

[AstroFin]

Hi Robert!

To me it looks like your OTA has once been attached to a Nexstar GPS fork mount, which would explain the "mystery holes" on both sides, since they were intended for attaching the scope to the fork mount. Your guesses about the other holes seem correct to me. The scope must be an early Celestron Nexstar GPS 8 because it still has the older Celestron font on it. It's one of the last models made in the USA. I hope this helps.  

Clear Skies from Finland,

Tomi

Edited May 2, 2020 by AstroFin

[Robert72]

Tommi, yes i had a feeling there was a fork mount involved somewhere, but couldn't confirm it because of the front holes.  Going off what you said i dug up this:

And this explains the front holes, and also demonstrates the two dovetails in action.

Eventually I will probably plug the fork mount holes, if re-spraying.

Thanks for your reply Tommi, that helps explain what all the holes are for.

[Explorer42]

Hello Robert,

I just bought an identical C8 which needs a restoration. Can you tell me if it is easy to separate the carbon tube from the rest of the telescope?

Regards,

Explorer42

 

 

[billhinge]

Years ago there used to be a lot of holy war discussions on CN about removing the correctors from SCT's.

Some argued that Celestron orientate the corrector to be a specific optimal fit with the primary, you should find some marks on the edge when you unscrew the restraining ring

Others disbelieve this and say there is no best orientation - even if they did lining up some pen marks isn't high accuracy considering how the corrector is held in place

Or you could mark the edge of the corrector and just put it back how you found it - there were no original marks on mine

Take your pick 😉 

Taking it apart (to flock it with protostar or clean it) is easy as using a screw driver, putting it back is the challenge since you have to re-collimate it from scratch

[Explorer42]

On 10/09/2022 at 20:21, billhinge said:

Years ago there used to be a lot of holy war discussions on CN about removing the correctors from SCT's.

Some argued that Celestron orientate the corrector to be a specific optimal fit with the primary, you should find some marks on the edge when you unscrew the restraining ring

Others disbelieve this and say there is no best orientation - even if they did lining up some pen marks isn't high accuracy considering how the corrector is held in place

Or you could mark the edge of the corrector and just put it back how you found it - there were no original marks on mine

Take your pick 😉 

Taking it apart (to flock it with protostar or clean it) is easy as using a screw driver, putting it back is the challenge since you have to re-collimate it from scratch

Indeed it's very easy to take it apart. The mine is old anf has many marks on the edge of  the corrector and on the tube. I guess previous owners have tried several positions 🤔

[billhinge]

16 minutes ago, Explorer42 said:

Indeed it's very easy to take it apart. The mine is old anf has many marks on the edge of  the corrector and on the tube. I guess previous owners have tried several positions 🤔

Now you've seen how easy it is to disassemble and that its only cardboard shims and some screws actually holds the corrector plate in place to the high accuracy of a complex scientific instrument you can decide yourself if orientation matters 🤣

I was being sarcastic in case you didn't catch that. I wouldn't worry too much IMHO, I doubt the Chinese factory did  exhaustive field trials on each scope to get best orientation  🙂

[Robert72]

Hello Explorer42, sorry for taking so long getting back to you. I didn't continue with the restoration, I just decided to use it as is for now. I hope to get back to it at a future date.

Now It's time to concentrate on completing my CF spider for my home made tube and get that project completed.