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Hubble doesn’t use lenses. Sorta.
Even today, 19 years after Hubble’s launch, it’s not all that uncommon to hear a newscaster refer to “Hubble’s lens”. I once heard it used by an announcer on a science show produced by the Space Telescope Science Institute, the agency that runs Hubble!
The thing is, Hubble doesn’t use a lens. It has a mirror.
Galileo used a telescope with a lens, as did everyone up until Isaac Newton. He was the genius who figured out that a properly shaped mirror could focus light as well, and has advantages over a lens: mirrors need only be ground on one side (lenses have two), and mirrors can be made larger than lenses because they can be supported all across their back side, while lenses must be supported around their circumference, where the glass is thinnest and most vulnerable.
Over a certain size, lenses are simply impractical, so mirrors are used. Hubble’s primary mirror is 2.4 meters across, about 8 feet. Although it’s the biggest mirror for astronomy ever lofted into space, it’s considered small by ground-based standards; many telescopes today have mirrors 4 or more meters across. The mammoth twin Keck ‘scopes in Hawaii have mirrors made of segments that total 10 meters across each!
It turns out the cameras on board Hubble use mirrors too. Why? Glass absorbs light. Not much, maybe 2% of the incident light, but that adds up. A lens has two surfaces, each of which reflect a little bit of light, so you lose more through a lens than with a mirror. Also, mirrors can be made to reflect light of different colors about the same, but lenses bend light at different colors differently. So all in all, doing it with mirrors makes a lot more sense.
However, there are lenses on board: they are used in the Fine Guidance Sensors, small telescopes that track stars with incredible accuracy and help keep Hubble locked onto to its targets.
I could not find any other lenses used on the entire observatory. I read the instrument guides and asked several people who work with Hubble, and no one knew of any either. It’s possible they exist and I missed them, but the point is lenses are a very rare commodity on the ‘scope.
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