Why do telescopes have holes in the middle?

It’s actually better to not catch that extra light. Here’s why.

“A mouse does not rely on just one hole.” –Plautus

If you want to see farther, deeper and more detailed into the Universe, you gather more light.

*Image credit: the CANDELS UDS Epoch 1 observations; image produced by Anton Koekemoer (STScI).*

You accomplish by either observing for longer, collecting more photons, or by building a bigger telescope.

*Image credit: Richard Wainscoat/Gemini Observatory/AURA/NSF.*

The larger the diameter of your primary mirror (or lens), the more light you can collect at once.

*Image credit: Wikimedia Commons user Cmglee.*

Doubling your diameter actually quadruples the amount of light you can pick up.

*Image credit: Refracting telescope of the Strasbourg observatory, France, from Wikimedia Commons user Pethrus.*

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.

*Image credit: 24 inch convertible Newtonian/Cassegrain reflecting telescope, formerly on display in the Franklin Institute rooftop observatory. Under a c.c.a.-s.a.-3.0.*

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.

*Image credit: flickr user z2amiller, via* *https://www.flickr.com/photos/z2amiller/, of the Keck secondary mirror.*

That means you need a secondary mirror/apparatus, which otherwise interferes with the incoming light and produces unwanted image artifacts.

Leaving a gap in the primary mirror’s center alleviates this problem completely.

*Image credit: Wikimedia Commons user Eudjinnius, of a diagram of Herschel-Lomonosov telescope system.*

Alternatively, you could have the light focused off-axis, but you wind up obtaining less light overall that way.

*Image credit: Giant Magellan Telescope — GMTO Corporation.*

Adding a hole actually improves your image overall, maximizing the useful light you gather, without extra errors.

*Image credit: Lori Stiles and John Florence (University of Arizona).*

Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images and video in no more than 200 words.

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