My Celestron NexStar 8SE has silver spots on the inside of the lens. Should I remove the lens to clean them?

Steve says: "The Celestron NexStar 8SE is a very popular 8-inch Schmidt-Cassegrain telescope (SCT) mounted on an altaz Go-To mount.


Schmidt-Cassegrains are reflecting telescopes with a spherical primary mirror but they have a specially figured correcting plate on the front of the optical tube to remove the aberrations that such a mirror would normally produce, namely spherical aberration.

Unless you know that the silver spots are spoiling the views through the telescope, which is unlikely, or that the spots are in any way organic or corrosive, which means that they could cause problems in the future, the best plan is to leave well alone.

Cleaning optics can lead to more problems than you solve as there is always the risk of micro-scratching the surfaces or worse.

However, if you do decide that the corrector plate must be cleaned then proceed carefully and methodically.

There is much debate over whether or not the corrector plate and primary mirror in a given telescope are matched at the factory but it makes sense to ensure that you can return the corrector plate to its original orientation.

This can be done by removing the screws in the retaining ring and marking the edge of the corrector plate and the optical tube with an indelible marker before removal and subsequent cleaning.

However, a much safer approach is to return the telescope to the importer’s repair centre and let them do the work for you."


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


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.