This means that the mirrors in an X-ray telescope have to be oriented so that their surfaces are nearly parallel to the incoming light. However, if we simply turn a regular optical telescope mirror on its side, we would miss a lot of the incoming X-rays – the part of the mirror that the X-ray could hit (or its “cross-section”) would be very small.
To solve this problem, X-ray telescopes use cylindrical mirrors and nest them, one inside the other. This is the general design used by both the telescopes on Suzaku and will be used by those on Astro-H.
