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Multi mirrored reflectors?


Manok101

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I've seen once or twice in my research into astronomy the idea of a standard reflector (well with a few minor differences) with more than one mirror cell, see the VLT( very large telescope) I was wondering what the view would be like with more than one main mirror?

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the field of view is 'controlled' by the focal length not the mirror size. I think you need a circular collection of mirrors and if you put say seven in a circle (one in the middle) then you'd basically gather more light so see fainter objects or get more resolution. for the same focal length though you'd have a faster scope and therefore more coma to correct etc. you'd also need to figure the whole area as one too.

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It doesn't make much difference to what you see. If you get into the details of analysing the images, the diffraction patterns look very odd because you have a very odd shaped "pupil". The classic airy pattern people are used to is because the mirror ("pupil") is round -- if you have a different shape mirror, then the airy pattern looks different. Other than that, it's just a way of increasing the aperture of the telescope.

You build multiple mirror telescopes if you think the technical/cost tradeoff makes it cheaper/more feasible to build many smaller mirrors (and keep them collimated) rather than one larger mirror. For example, you can't practically build a single mirror bigger than 8 meters -- because you can't transport it up mountain roads!

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This type of optics is called adaptive optics (one mirror made up of many smaller mirrors). It is used in such telescopes as the VLT. It cuts down the atmospheric distortions, thus giving a better quality image. Also because the primary mirror consists of many smaller mirrors (cells)........this allows you to have a larger mirror.

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Adaptive Optics uses very fast (100's of Hz) correction of the mirror surface, and does not use the primary mirror (segmented or not; you can't flex them that fast!). You typically use a small mirror in an instrument to do this correction, but some telescopes (e.g. LBT as Umadog says) are starting to use deformable secondary mirrors to do the correction. This is has quite a lot of advantages (less light loss!) but is by no means easy -- the adaptive secondary mirror of LBT is 1meter across but just 2mm thick!!!

The primary mirrors of professional telescopes (both segmented and not) are usually flexed to control the aberations, but this happens on a much slower timescale (minutes to hours) and is called "active optics".

Annoyingly similar terminologies :)

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What you have to bear in mind is that each of the components of a segmented mirror have to be an exact part of what the mirror would be if it was a large single item. It might be easire to visualise if you imagine being able to smash a large single mirror into several perfectly identical pieces and then re-assemble like a crazy paving to reform the original but instead of gluing them together, each piece was on its own individual adjustable mount. You would in effect be collimating each component untill it acted like one large mirror.

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What you have to bear in mind is that each of the components of a segmented mirror have to be an exact part of what the mirror would be if it was a large single item.

Thats exactly why i thought it was the primary mirror on such scopes that had the adaptive optics rather then the secondary mirror.

Thinking about it thought, it makes MORE sense for the secondary mirror to have adaptive optics rather then the primary. The secondary IS where the light is concentrated..

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