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View Full Version here: : SCT c/plate: Where can it get re-coated?


Star Hunter
01-12-2017, 02:16 PM
Where can I get my 12" SCT corrector plate re-coated with Mfg or some other coating in Oz? It's a few years old now and it needs a new coat.

Also, Ia mate of mine has a 1990's C14 and he too was wondering if the c/plate can be replaced or recoated?

Brian

Wavytone
01-12-2017, 03:40 PM
Order a replacement from Celestron - they are apparently interchangeable for stock scopes that weren’t hand-selected. At worst you may have to rotate the corrector during a star test in good seeing to find the optimum orientation.

I stumbled on another supplier of corrector plates by accident some weeks ago and will try to find it again.

While it is technically possible to remove MgF coatings by boiling in a mix of sulphuric acid and boric acid - the latter to deal with the hydrofluoric acid produced - it is a dangerous process and secondly the glass surface will be etched in the process, so it would have to be repolished and refigured, which can only be done by Celestron (and they won’t do it - they will simply put yours in the trash and send a new one).

Tasastro
01-12-2017, 05:42 PM
If it is not severely scratched or otherwise damaged I wouldn't worry about it. MgF2 is durable and doesn't degrade much due to atmospheric attack. Also the later Meades (I assume yours is a Meade), and Celestrons are multi-coated, which is even harder - the corrector should outlast the rest of the telescope.

toc
02-12-2017, 07:49 AM
I had always thought that Celestron matched optical sets, and if you damaged the corrector they replaced the entire set.

Wavytone
02-12-2017, 08:58 AM
Many thought so for a long time but apparently not the case - some on CN have discovered they are more-or-less interchangeable. There is some variation, of course, but its within being well worth trying.

It may also require some firm talking on your part to get just the corrector - it also appears Celestron does try to talk you into a set of primary, secondary and corrector plate.

Decades ago without modern computer-controlled grinding and polishing machines it probably was true that the optic sets were hand-matched at the factory, but not any more.

Secondly the machines are able to produce mirrors to the required shape within 1/8 wave without hand finishing. That is far better than the accuracy required for the figure on a lens - or a corrector plate. Hence correctors don't really need to matched to the mirrors anymore - they're randomly matched - which may also explain the "hand-picked" ones off the factory floor that are better than the rest which are within tolerance (however Celestron defines that), but not particularly perfect optically.