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Comment by knodi123 | Hacker News Reader

I'm not asking this skeptically, but out of curiosity and ignorance- how the heck does a 3 mirror setup make sense? Can't a single mirror always accomplish the same goal? (for that matter, why mirrors, with their blind spot where the sensor is, instead of lenses?)

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The image from a single curved mirror or lens will have distortions that can be reduced by adding additional curved mirrors/lenses. It's also harder to make and use large diameter lenses than mirrors. Each lens has two surfaces that need to be aligned without the possibility of post-fabrication calibration. Weight and sagging are a bigger problem for lenses. Mirrors can be thinner, partially hollow, and can have mechanical support behind them without blocking light. There are further considerations that might favor mirrors, like material cost and reaction to temperature changes. If nothing else, bending the light path back and forth with mirrors means the telescope can be shorter, easier to point, and will fit in a smaller building.

The largest exclusively lens based ("refractor") telescopes got up to about 1 meter diameter before the trade offs caused a shift to mirrors for larger apertures. Even so, it's common to have lenses near the focal plane of a mirror based ("reflector") telescope to improve the image. Vera Rubin is like that, including a 1.5 meter lens (among others) near the sensor.

The sensor doesn't actually form a blind spot in the image, because it is severely out of focus. Obstructions do affect the pattern of light a star forms on the sensor, but it's all relative, and no mirror or lens can produce perfect images.

Thanks for explaining!

> how the heck does a 3 mirror setup make sense? Can't a single mirror always accomplish the same goal?

Easier to make three smaller mirrors than one big one.

> why mirrors, with their blind spot where the sensor is, instead of lenses?

"Mirrors don't cause chromatic aberration and they are easier and cheaper to build large. They are also easier to mount because the back of the mirror can be used to attach to the mount" [1]. (And there are off-axis designs that avoid obstructing the incoming light.)

[1] https://lco.global/spacebook/telescopes/reflecting-telescope...

That's not intuitive to me, but I recognize that the problem is on my end. Thanks for explaining!

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