8" celestron refurb

[tonyindarwin]

I am hoping someone can give me some advice about re-conditioning an 8" Celestron a friend gave me recently. The optics are ok, but it needs to be disassembled, cleaned, painted and reassembled.
Are the any books that would allow a good do-it-yourselfer like me to attempt this task, because unfortunartely I dont have the money to pay someone to do it professionally and my star watching will be over before it began.
Any hints as to a direction to take would be appreciated.
Cheers

Tony

[edwardsdj]

Hi Tony,

The C8 is a great telescope. I've had mine for 10 years now and have been thinking of servicing it myself as well.

I've yet to try, but this is the best description I've found so far:

http://www.astromart.com/articles/article.asp?article_id=594

This also seems helpful:

http://www.ngc1514.com/Celestron/disassem.htm

And this is the most practical article of cleaning of optics (including corrector plates) I've seen:

http://www.arksky.org/asoclean.htm

Hope someone who's actually done this replies to this thread.

If not, hope these links help.

Have fun,
Doug

[Blue Skies]

That first link is a good one!

I've pulled apart an old Meade before (have meant to post the pics for reference, I'll get to it one day!) and it's not so hard to do.

IMPORTANT THING TO REMEMBER!
The corrector plate must go back on the same way it came off. The tape marking as demonstrated in the first link is the way to go. And dont break it! you can't get a replacement for it. The corrector plates are individually matched to each primary mirror so if you break one you've wrecked the scope.

The corrector plate can be cleaned just like a mirror once it's out.

Let us know how you go and send some pics! :thumbsup:

[koputai]

I don't think this is quite true. I have a Nextstar8 with a busted corrector, and Celestron sell the correctors as a spare part, I've just baulked at the $750 price tag. And you don't have to send the scope back, you can order the part through your dealer. I can't see that each corrector could be custom crafted to suit each mirror, it just wouldn't be commercially viable to do that much work on a mass produced scope.

Cheers,
Jason.

[Zuts]

I thought they were matched. At the very least they are aligned very carefully at the factory. I have heard that if you take one apart make very sure you put a mark so you can put the corrector back in exactly the same orientation or your performance will realy suffer.

So with a new corrector I am not sure how you would find the best orientation.

Paul

[Blue Skies]

Yes, that's it. They choose three or four that they then try with each scope, rotating them to find the best fit. Even if you could get a replacement I'm skeptical that you would get optimal performance from it, koputai.

[edwardsdj]

I thought they put the corrections on the secondary. My understanding is that it is the orientation of the secondary that is important. All the stuff about getting the corrector oriented correctly is to ensure the secondary ends up in the correct orientation. Or so I assumed.

This would imply you could install a new corrector as long as you ensured the secondary ends up in the same orientation with respect to the primary.

I could be completely wrong about this, but that's what I assumed.

[Blue Skies]

eeermm, I don't think so (the secondary being adjusted, that is). However, I had an interesting conversation last night with an ex-telescope technician who used to fix a lot of Meade scopes in the UK. He indicated that he would be happy to use an off-the-shelf corrector plate as the quality these days is much better than it used to be. The scope would work, but you might not achieve the "200%" quality that the original might have had (ie, it might not be able to be pushed to optimal performance, but would still be a usable scope.)

Further to what we've been discussing previously, the orientation of optical elements is important, no matter what. Rotating optics even just a smidgin to get a good match can have a big impact on the quality at the eyepiece end, so it's still a good idea to keep the same orientation of the corrector plate if possible.

[edwardsdj]

I couldn't agree more. All I was trying to get at is that I can see how a new corrector could be installed as long as the orientation of the secondary is maintained.

If removing the corrector I'd ensure it was orientated as close as possible to its original position when reinstalling it. As long as you don't remove the secondary from the corrector, this will ensure the correct orientation of the secondary.

As I understand it, corrections are only applied to the secondary after the primary and corrector have been chosen.

What I'd really like to know is how much of this correction is due to errors in the primary and how much to errors in the corrector.

My gut tells me:

Primary errors: 90%
Corrector erros: 10%

as the primary is a very steep f/2 curve while the figure of the corrector is very slight.

If this is true, I can see that correctors can be replaced with minimal optical degredation as long as the secondary orientation is maintained.

Anyway, this is just my gut feeling. It would be great to hear some more about this.

Have fun,
Doug