For optical telescopes, we want light to reflect from the mirror more or less back along the direction of incidence. If we are detecting radiation that strikes the mirror at a grazing angle, then we are observing:

Question

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When observing radiation at a grazing angle, we are dealing with high-energy electromagnetic waves such as X-rays, which have a shorter wavelength and higher frequency than visible light, making them more likely to reflect at grazing angles. Radio waves have a longer wavelength and are therefore less likely to reflect off a mirror, while ultraviolet and gamma rays have even shorter wavelengths and higher frequencies, making them even less likely to reflect.

For Optical Telescopes, We Want Light to Reflect from the Mirror