why are all telescopes round?

Why do scopes always have a round mirror, wud it make a difference if it was square?
thanks

Why do scopes always have a round mirror, wud it make a difference if it was square?
thanks

they have to be round to bring all the light rays to a focus at one point you cannot do this with a square mirror. the concave mirror brings the light to one point so you can magnify it with the eyepiece.
stephen.

I think Fionn is talking about the plan of the mirror, not its cross-section. The cross-section must indeed be parabaloidal (or spherical if you're willing to put up with lots of coma).

I don't know if the plan needs to be circular. If you obstruct part of an SCT's aperture, you can see the obstruction in the out-of-focus star images. This means that each star is getting light (or in this case, not getting light) from each portion of the mirror. So I suspect that a square mirror would give uneven illumination (stars would get a bit more light at their corners), but I'm only conjecturing.

Interesting question.

-- Jeff.

Aren't some of the large telescopes (> 6m or so) made up of composite hexagonal mirrors? I guess they don't have to be round but I've never seen a small scope with anything but....

I cant think of any reason why a telescope with say a square primary would not work. Assuming its collimated and parabolic/sherical as the case may be.

However thats in theory. Practically you would have a problem creating such a beast. I assume its much easier to polish a circular disc than a square one of the same area. Also if the mirror needs colimation you will have a harder time working out what screws to turn. On top of this the stress points of a square mirror might cause tension in unexpected ways.

Also if the collimation wasnt right, you would end up with a square shaped Airy disc (I think) which would be interesting, out of focus stars would be interesting to see.

When you think about it many telescopes dont have circular mirrors. The clips to hold them in place take up some of the area and leave you with less than a circle. And as dave pointed out the large scopes, particularly with adaptive optics are made up from arrays of hexagons.

Cheers,
~Al

It would be an interesting experiment to make a reducing mask for a scope with a square instead of a circle, I think the star points would be squarish as the airy rings would be square, making focusing a bit more difficult.
It would be interesting to see how those sharp corners would affect the light.

Making a square mirror/lens would be probabily more difficult then a circular one.