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Questions on repairing a broken C8

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#1 bykhed

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Posted 08 September 2013 - 04:05 PM

Fedex broke the corrector on a Fastar-compatible C8 (Super C8 vintage OTA). The scope had exceptional optics and was the best-performing C8 I've ever looked through. After inspection for the insurance claim, the scope was returned to me. I removed the corrector and inspected the primary and secondary and both are undamaged, as is the rest of the scope (a Feather Touch microfocuser was also damaged, but that's another story).

I have since found a great deal on an orange-tube, vintage C8. My plan is to use the corrector from the older scope to repair the damaged C8. My thinking is that C8 correctors were mass-produced so it shouldn't matter if the corrector came from an older scope.

Questions:
- Will the replacement corrector work with the newer mirrors?
- Is the size of the corrector opening the same for a Fastar-compatible secondary holder as a standard holder?
- Am I better off parting out the remains of the broken C8 and just using the orange-tube instead?

Any advise is greatly appreciated - thanks in advance!

-Matt

#2 coopman

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Posted 08 September 2013 - 08:20 PM

Based on what I have seen stated here, the correctors are matched to the primaries at the factory by the opticians. I'm sure that others will chime in soon.

#3 JimK

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Posted 08 September 2013 - 08:26 PM

Based on what I have seen stated here, the correctors are matched to the primaries at the factory by the opticians ...

... by touch up of the secondary mirror, and marking the optical orientation each component that provides the best view in the system.

#4 bykhed

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Posted 09 September 2013 - 10:57 AM

Hmmm - I've read Piekiel's book about Celestron and, according to his sources, Celestron opticians sometime swap secondary mirrors until they find a good match to make up for optical variances in the corrector. But the corrector is a refractive lens and, assuming they have good QC in manufacturing them, any variances should be relatively negligible in most cases. In addition, such testing and swapping around adds a lot of cost to the process and it would be *much* cheaper to produce uniform correctors.

Anyone else have more info or has anyone swapped a corrector from another C8?

#5 Masvingo

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Posted 09 September 2013 - 01:50 PM

This thread may be of interest - see page 9 onwards (assuming default of 20 posts per page).

#6 bykhed

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Posted 09 September 2013 - 03:16 PM

Thanks so much for the link! That was an interesting read and very encouraging. At this point I've decided to check the optics on the orange tube first before taking any further steps.

#7 bykhed

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Posted 11 September 2013 - 01:14 PM

I PMed Bob Pielkiel and he said he and others have had mixed results with corrector swaps. Sometimes it works out great, sometimes there's all kinds of spherical abberation, etc.

#8 rmollise

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Posted 11 September 2013 - 02:58 PM

That's about what I would expect. If you can find a corrector, you will probably get average images. Probably not better, maybe worse.

#9 Starhawk

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Posted 11 September 2013 - 03:59 PM

For what it's worth, modern day Taiwanese SCT glass kits used to repair SCTs sent to Celestron appear to be extremely uniform and generally quite good.

-Rich

#10 bremms

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Posted 12 September 2013 - 09:21 AM

Just put the Fastar secondary holder and a dovetail on the other C8. DONE.

#11 bykhed

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Posted 12 September 2013 - 12:35 PM

Agreed, if the optics in the orange C8 are great optically.

#12 Geo.

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Posted 17 September 2013 - 08:51 AM

Isn't there a notch cut into the corrector to retain the Fastar cell?

BTW, I don't think any Super C8 was ever shipped with the Fastar secondary. Fastar first appeared on the second generation Ultima 8 in 1990. Long after Super C8 production ended. :question:

#13 rmollise

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Posted 17 September 2013 - 09:10 AM

No, there is no notch in the corrector.

#14 mclewis1

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Posted 17 September 2013 - 09:38 AM

There is however a notch on the secondary mirror holder which aligns it with the ring/cell that attaches to the corrector. When you remove and re install the secondary mirror that notch provides the correct orientation of the secondary mirror.

It's that outer ring that starts to rotate (and then renders the notched registration pointless) which annoys folks with the faster compatible secondaries.

#15 bykhed

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Posted 17 September 2013 - 01:20 PM

Follow up: I received the orange C8 and the optics are quite nice. So I've decided to sell the broken scope and keep the orange-tube C8 as a replacement.

#16 orion61

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Posted 18 September 2013 - 12:24 PM

I am the one who mix and matched the optics for Joe.
A lot of people said it couldn't be done but it worked out fine, using the Primary AND secondary with another corrector.
They were BOTH Black Starbright tubes tho.
We may have simply gotten lucky having both tubes made on the same corrector making machine??? or the mass produced
systems come out very close,
I tend to go with the Luck/same corrector grinding machine.
I would NEVER guarantee those excellent results again,
especially between an Orange and black tube.
If you are buying it anyway it would only be a matter of a waste of time if it didn't work.
remember to mark the corrector and cell.
Try placing the S/N on the corrector at the 3:00 position in the broken Black tube to start. I simply rotated the corrector 1/4" at a time until I achieved the best results.
I then fine tuned the components by first marking the secondary and turning it as well 1/4" at a time, then back to the corrector..you get it.
It was very time consuming, but it came out great!
The Doubles I split, and Planetary details were diffraction limited (at least) using known separation angles of the Stars.
I used up to a 4mm Ortho to visually test.
How did that system end up with a fastar Secondary on an old SC8+? I wasn't aware they were interchangeable?
Then again I never tried or asked if it could be retrofitted.
Good Luck.. and SHAME on the Shipper!






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