optical reflecting telescope

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Why do telescopes have holes in the middle?

“You can build a refracting telescope, where the lens focuses the light, as large as you want, with no problem. But lenses are heavy, expensive, and limited in size by practical constraints. Reflecting allows you to go bigger, since large (or segmented) mirrors are easier to build than lenses, but the light must be focused in front of the primary mirror. That means you need a secondary mirror/apparatus, which otherwise interferes with the incoming light and produces unwanted image artifacts.”

When you look at the largest, most powerful optical telescopes in the world, they all have something in common: they all have holes in their central, primary mirrors. This is for a few reasons, including that they’re all reflectors, they all need to focus light somewhere in front of the mirror, and they all need to send that light somewhere to be recorded and analyzed. You can, in principle, focus the light somewhere off-axis, and many amateur telescopes do, but for the professionals, you lose more light that way than you would by simply having a hole in the center. In order to conserve the most light and maximize the image quality with the fewest artifacts, leaving a hole in the mirror is by far the best way to go.