Why is there a black spot in my telescope’s field of view?

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

Celestron CPC Deluxe 1100HD

When I look at Jupiter with my Celestron CPC I see a huge black spot in the centre. What could be causing this?

Steve says: “The Celestron CPC range of telescopes are of the Schmidt-Cassegrain design, with a large concave primary mirror at the base, a corrector plate at the front and a convex secondary mirror attached to the corrector plate on the inside of the tube.

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The light they collect follows a complex path: it travels through the corrector plate to the primary mirror; back up the telescope’s tube in a converging beam to the secondary mirror; back down the tube again and through a hole in the middle of the primary mirror; and finally through the focuser and star diagonal to reach the eyepiece.

All in the name of an effectively long focal length.

That black spot you are seeing is the shadow of the secondary mirror, indicating that you have not achieved correct focus.”

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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.