Celestron GPS11 with fungus

Reflectors Backyard Astrophotography
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Paul Campbell avatar
Paul Campbell avatar

Found a used Celestron GPS11 for 750, including a working mount, in my town but it has a few bits of fungus on the corrector plate and primary mirror. For the foreseeable future, it would be used for planetary imaging. Maybe one day I could get an eq mount for it.

Worth it or hard pass? How hard is it to clean the corrector plate and mirror? I seem to be getting mixed reports on how complicated of a procedure this is. The seller is pretty well known guy in the community around here and I bought my c8 from him.

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Tony Gondola avatar
Tony Gondola avatar

For that price I would say yes, it’s a good deal sort off. If the fungus has not actually etched the glass the you are in good shape, it can be removed from the corrector plate. I dilute solution of Hydrogen Peroxide will do it. The primary is a bigger problem because whatever you do to remove the fungus will likely harm the aluminum coating of the mirror. Not that big a deal because the mirror can be recoated but something to take into account. It won’t be free to do.

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Lynn K avatar
Lynn K avatar

The correcter is not that hard to clean. Make sure you place the corrector back in with the mark on the correcter edge at 3 o'clock looking from front of scope. Mark the shimes placement and replace in the same spot. There is no need to remove the secondary.

Use 100% cotton balls on the mirror, wiping from center out to edge. There are many liquid cleaners altenitives. Never use a dry cloth/ cotton ball and blow off all dust first.

Lynn Krizan

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Paul Campbell avatar
Paul Campbell avatar

Tony Gondola · Jun 19, 2026, 08:55 PM

For that price I would say yes, it’s a good deal sort off. If the fungus has not actually etched the glass the you are in good shape, it can be removed from the corrector plate. I dilute solution of Hydrogen Peroxide will do it. The primary is a bigger problem because whatever you do to remove the fungus will likely harm the aluminum coating of the mirror. Not that big a deal because the mirror can be recoated but something to take into account. It won’t be free to do.

Thanks, Tony. Good to know.

Paul Campbell avatar
Paul Campbell avatar

Lynn K · Jun 19, 2026, 09:04 PM

The correcter is not that hard to clean. Make sure you place the corrector back in with the mark on the correcter edge at 3 o'clock looking from front of scope. Mark the shimes placement and replace in the same spot. There is no need to remove the secondary.

Use 100% cotton balls on the mirror, wiping from center out to edge. There are many liquid cleaners altenitives. Never use a dry cloth/ cotton ball and blow off all dust first.

Lynn Krizan

Thanks, Lynn. The idea of shims make me nervous but maybe i’m over thinking it. I’ll look at some videos

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