Fungus is forming on my telescope. What should I do?

BBC Sky at Night Magazine's Scope Doctor Steve Richards solves your practical astronomy ailments.

You can clean fungus from your telescope, provided you know what you are doing. Credit: David Hannah / Getty Images
Published: March 15, 2020 at 6:24 pm
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My Celestron NexStar 6SE has fungus. Is there anything I can do to save my telescope?

Steve says: "Fungal growth is sometimes found on the inside of the corrector plate and the safest advice is to have the telescope cleaned and serviced by the importer.


However, non-Edge HD optics like yours can be cleaned by you if you are very careful and are confident you know what you are doing.

Unscrew the retaining ring inside the front housing and accurately mark the position of the corrector plate relative to the housing, making a note of any offset of the corrector from the centre of the housing.

Remove the secondary mirror housing, place it in a dust-free plastic box and place the optical tube somewhere safe with cling film over the front.

Soak the corrector plate in tepid water with a few drops of mild detergent for half an hour, then clean the corrector’s surface very gently with a small piece of surgical grade cotton wool and a 1:1 mixture of isopropyl alcohol and distilled water to carefully remove the fungus.

Rinse with large amounts of distilled water, dry naturally then re-assemble and collimate. For more on this, read our guide on how to collimate your telescope."


Email your astronomy queries to Steve at


Astronomer Steve Richards
Steve RichardsAstronomer and author

Steve Richards is a DIY astronomy expert and author of Making Every Photon Count: A Beginner’s Guide to Deep Sky Astrophotography.


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