In a reflecting telescope, why doesn't the secondary mirror cause an image with a "hole" in it?

Question

In a reflecting telescope, why doesn't the secondary mirror cause an image with a "hole" in it?

Answer 1

It does---- if it's out of focus.

But in focus (by definition) focuses all light collected at the primary to a single point. The fact that it (or the light path) has a hole in it makes no difference.

Answer 2

Every single "piece" of the mirror contributes to the whole image.

If you were to cover most of the mirror (with a mask -- we do that for bright planets, sometimes) so that just a tiny portion is used, you'd still get a complete image.  That image would have less detail (and less brilliance) but it would still be complete image.

If two mirrors have the same focal length, what the larger mirror does (larger aperture) is to provide more detail and more light to the image.  Both images would be the same size.

Answer 3

Say you're aiming your telescope at the full moon.  If you were to trace the billions of light rays coming into your telescope, you'd see that _every_ spot on the (exposed part of) the main mirror, is receiving (and reflecting) rays from _every_ part of the moon's surface.  So, although the central part of the mirror does not get struck by any light, each of the parts that _do_ get struck receives a "whole" image of the moon.  When those little pieces are recombined, you get a bright image in which no part of the moon is missing.