Ask Ethan: Why don’t we build a telescope without mirrors or…

Ask Ethan: Why don’t we build a telescope without mirrors or…

Ask Ethan: Why don’t we build a telescope without mirrors or lenses?

“Why do we need a lens and a mirror to make a telescope now that we have CCD sensors? Instead of having a 10m mirror and lens that focus the light on a small sensor, why not have a 10m sensor instead?”

Every time you shine light through a lens or reflect it off of a mirror, no matter how good it is, a portion of your light gets lost. Today’s largest, most powerful telescopes don’t even simply have a primary mirror, but secondary, tertiary, even quaternary or higher mirrors, and each of those reflections means less light to derive your data from. As CCDs and other digital devices are far more efficient than anything else, why couldn’t we simply replace the primary mirror with a CCD array to collect and measure the light? It seems like a brilliant idea on the surface, and it would, in fact, gather significantly more light over the same collecting area. True, CCDs are more expensive, and there are technical challenges as far as applying filters and aligning the array properly. But there’s a fundamental problem if you don’t use a mirror or lens at all that may turn out to be a dealbreaker: CCDs without lenses or mirrors are incapable of measuring the direction light is coming from. A star or galaxy would appear equally on all portions of your CCD array at once, giving you just a bright, white-light image on every single CCD pixel.

It’s a remarkable idea, but there’s a good physical reason why it won’t pan out. For the foreseeable future, we still need optics to make a telescope! Find out why on this week’s Ask Ethan.