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Walking on the Moon

Why are telescopes round?


kidlands
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A square mirror is not difficult to make, The same grinding and polishing procedures apply just the same, but for reflecting telescopes, it makes sense to make the objective round. It's easier to fit and adjust in a round tube. The secondary could be square too, but the corners would obstruct the incoming light, and cause unnecessary light loss. The corners would add nothing but problems to the unit.

Square mirrors are produced, but they are probably used in applications far removed from astronomy. Suffices to sayu, round mirrors make life easier all round (No Pun.), for both user, and manufacturer.

Ron.

PS. The square scope you saw, would only be the tube. The mirror would have been round.. Believe me I know. I've made a few.

Edited by barkis
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I guess it could be any shape as long as the surface is correctly shaped, parabolic, spherical etc but a circular mirror is going to give you the most efficient surface area ie equal in all directions . As for production- I suppose it's easier too :eek: only guessing !

I took up the thread 'cos I thought it was the 1st line to a joke

" I don't know, why are telescopes round ?" ;)

Oh, when they were first grinding optics using canon balls, anything other than a round piece of glass spinning on a wheel would have chipped or shattered as the CB caught the protruding edges ????

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Oh so it was the tube not the morror that was square.

Yeh I had an thought it was due to the limitatons of the day back yonder and we just didnt change it.

Thanks guys

Edited by kidlands
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Round tubes are easier to mass produce than square ones and argueably more aesthetically pleasing. For one off amateur construction, square tubes are easier to make and have slab sides which lend themselves to the addition of fittings, the square section also mitigates tube currents. Square mirrors would be a difficult option to manufacture and non circular mirrors (other than elliptical secondaries) are really only relevant for segmented mirrors in large professional telescopes.

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Round mirrors (or lenses) have a nice symmetric diffraction pattern ... a square aperture would introduce marked diffraction smearing which would reduce contrast. Some people get all het up about the minor contrast loss a central obstruction causes, square corners would be a damn sight worse.

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Every edge to anything in the light path (secondary supports, mirror edges) causes diffraction with scatters light and decreases contrast. A circle is the most efficient shape with the smallest edge length to surface area ratio meaning that it will create the minimum amount of diffraction and loss of contrast for it's surface area.

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Round mirrors (or lenses) have a nice symmetric diffraction pattern ...

Wot he said.

And a circular mirror is easier to make, as you are trying to generate a rotational symmetric shape (sphere/parabola) in the mirror. As soon as you go to a non-circular mirror, you break the symmetry and have an extra factor to consideration in the manufacture. (for small mirrors with oversized tools, this might not make any difference; but for professional manufacture with sub-diameter tools, it is a significant difference).

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Not all telescopes have round mirrors. Large telescopes currently being built have mirrors composed of hexagonal segments - the James Webb space telescope, the European Extremely Large Telescope and so on.

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Radio telescopes aren't! They often use elliptical dishes which have something to do with side lobes but it is too late for me to look up my notes from Jodrell Bank's course and I have just realized they are in England am I'm not - but I daresay someone less hopeless than I will clarify all...

Olly

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In the early 1970s I had a reflector in a square wooden tube which was mounted on a Charles Frank eq mount. I found an old photo - not too good but it shows the scope.

Mark

That looks like a great location to observe. :mad:

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That looks like a great location to observe. :mad:

This is still my observing location and the bank shown in the photo is a disused railway line. My east/west horizon (the direction that the scope is facing) looks towards Wales and rural Herefordshire - at the moment no light pollution. The square newt was too heavy and was sold 10 years ago - now painted white.

Mark

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If the moon and planets were square or triangle instead of round

That would imply very weak or no gravitational force - in which case there would be neither moon nor planets, nor stars, to observe, and no elements heavier than helium, which would make constructing a telescope at all somewhat awkward.

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