A very long focal length eyepiece used on a Newtonian telescope may exhibit the "kidney bean effect" – a black spot that seems to float in the center of the field of view. This is the shadow of your secondary mirror. It’s caused when the exit pupil of the eyepiece becomes too large, and it’s related directly to the focal ratio of the telescope. Here’s the formula:

exit pupil = fl of eyepiece ¸ f ratio of scope

If the exit pupil gets up around 8mm, the secondary shadow shows up and you have blackout. For example, a 50mm eyepiece in an f/5 Newtonian gives an exit pupil of 10mm. Blackout city!

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This page last updated on March 31, 2001