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How do I clean an SCT optics

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I have had a a flood in my observatory and my Celestron 9.25" SCT, has been very damp for a long time and mould has started growing inside the OTA on both the primary mirror and the corrector plate, can anyone tell me, how easy is it to remove the corrector plate and clean the optics and can you give me any tips regarding the work.



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H Neil,

Sorry to hear of your problems...

SCT cleaning is not to be undertaken lightly... the aligmnet of the Front Correcto Plate with respect to the mirror is important. there are also shims that hold the FCP properly centered and these need to go back exactly as they came out if you want to maintian the scopes optical performance.

Saying I have had the FCP out of mine a few times luckily all my shims stay in place and having marked the FCP's position in the OTA its all gone back together and been brought back to collimation each time.

You would also have to stirp the OTA to get at the mirror...

I would seriously consider having the job done proferssionaly.


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My C9.25 managed to fill up with water when its cover blew off, so I feel your pain :(

It is very simple to remove the corrector.

1) stand the OTA on its end, and have it supported so that it cannot fall. I used a spare pair of hands for this.

2) Remove the small screws holding the retaining ring around the edge in place, taking care not to touch the corrector plate with the screw driver. Before removing the final screw, make a little mark on the ring and OTA so that the ring goes back on in the same place. I dont think this is critical, but it cant hurt :D

3) The ring may be stuck to the corrector. I found a little gentle sideways pressure with a flat bladed screwdriver was enough to pry it loose.

4) IMPORTANT: Mark the plates position. You will likely find there is a mark on the plate anyway, but you will want to ensure it goes back in exactly the same place. If you plan on removing the secondary mirror too, then mark its position as well, but do not try to remove it at this time.

5) You should see a few shims made of cork or similar around the edge of the plate. I found that with gentle sideways pressure away from them in turn, I could lift them out with tweezers. I marked the position of each shim with a pencil so that they too went back where they came from.

6) The plate itself is ready to be lifted free, but you may find it is stuck to the cork ring underneath it, again, very gentle sideways pressure should break the seal. I was able to lift the plate out carefully by holding the secondary housing, or you could use suckers etc.

7) Clean the stuff. I used Baader Optical fluid and cloth available from FLO, but I cant really offer advice on the cleaning side, so please consult another source.

8) Reverse the procedure to put the stuff back.

Hope that helps a bit. You need a steady hand and patience, and preferably somebody else to help hold/fetch things, but it can be done on your own too.



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The FCP is staright forward it was gettign at and cleaning the Primary that was my concern - I would have thought that a check and clean of the focusing mechanism may be required as well...

found this...


Celestron helpfuly tell you how to do it in a few lines then remind you it voids your warranty and suggest its better to return it to a service center...


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