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I've been reading about reflecting telescopes and I understand how the secondary mirror doesn't cause any obstruction to the image itself, but I've read that it does dim the image by blocking the total amount of light reaching the primary mirror. However, nothing I can find seems to go into more detail than that, and I can't quite understand what factors (besides the size of the secondary mirror) are in play with regards to the blocking of light. What kind of relationship exists between the primary and secondary mirror (in other words, how does the size of a secondary mirror affect the amount of light that reaches the primary)?

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  • $\begingroup$ The obstruction is just the shadow of the mirror and the struts that are holding it on the primary. There are some minor points about stray light and diffraction on those structures, of course. The secondary gets larger with decreasing ratio of focal length to aperture (f-number). $\endgroup$ – CuriousOne Feb 23 '16 at 23:30
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Basically, take the area of the primary mirror subtract the area of the secondary mirror and that difference divided by the area of the primary mirror will represent the fraction of light that is transmitted through the telescope.

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