My telescope’s mirror is dull. Should I have it recoated?

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

Thanks to Kari Brown for her question, and for sending us in a picture of her mirror. Credit: Kari Brown

I am the owner of a second-hand 10-inch Newtonian. Despite cleaning the mirror with distilled water, mild detergent and alcohol it is still quite dull. Should I have it recoated?

Steve says: “In general, more modern mirrors will very rarely require re-coating but so much depends on how they have been looked after, and there is certainly no standard length of time you should adhere to. It is not at all uncommon for coatings to last 30 years or more.

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The main causes of mirror damage are constant exposure to moisture, contaminants through poor storage and, ironically, cleaning them too often.

A Newtonian mirror will very rarely require a clean unless it’s been woefully neglected and even then it should only be cleaned with a very mild detergent and washed off with copious amounts of distilled water.

Taking a look at the photograph of your mirror surface (with thanks to reader Kari Brown for this question and for sending us the image above), the finish looks pretty acceptable and there are no black spots, which often indicate that the surface is failing.

The surface does have some fine scratches but unless these are producing obvious artefacts around bright stars, even these shouldn’t be an issue.

They are, however, a reminder of the danger of over-zealous cleaning.

A well-illuminated, close-up photograph of a mirror such as the one you sent to us will tend to exaggerate any imperfections in the surface anyway, and so cannot be relied on for assessing the true state of the reflective coating.

However, if you shine a light from behind the mirror and can see lots of pinpricks on the surface this would give an indication that the surface is nearing the end of its life and you should consider having it recoated.”

Some useful guides that might help:

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Email Steve your astronomy queries to contactus@skyatnightmagazine.com and they could be answered in a future issue of BBC Sky at Night Magazine.