Manok101 Posted January 28, 2011 Share Posted January 28, 2011 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? Link to comment Share on other sites More sharing options...
nightfisher Posted January 28, 2011 Share Posted January 28, 2011 i would guess that for a home telescope it would not give any real advantage, and collimation would be a real nightmare Link to comment Share on other sites More sharing options...
Manok101 Posted January 28, 2011 Author Share Posted January 28, 2011 Oh I understand that, but I was just wondering, do the extra mirrors give an extra feild of view? Link to comment Share on other sites More sharing options...
Moonshane Posted January 28, 2011 Share Posted January 28, 2011 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. Link to comment Share on other sites More sharing options...
FraserClarke Posted January 28, 2011 Share Posted January 28, 2011 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! Link to comment Share on other sites More sharing options...
LukeSkywatcher Posted January 28, 2011 Share Posted January 28, 2011 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. Link to comment Share on other sites More sharing options...
yesyes Posted January 28, 2011 Share Posted January 28, 2011 a segmented mirror does not necessarily imply adaptive optics. it could just be segmented because of its size. Link to comment Share on other sites More sharing options...
LukeSkywatcher Posted January 28, 2011 Share Posted January 28, 2011 a segmented mirror does not necessarily imply adaptive optics. it could just be segmented because of its size.Thats true i suppose. But to be of any use wouldnt those segments need to work in unison? Thats essentially adaptive optics. Link to comment Share on other sites More sharing options...
yesyes Posted January 28, 2011 Share Posted January 28, 2011 they could be in a fixed arrangement and not move with respect to each other; just forming a larger mirror made up of several segments. Link to comment Share on other sites More sharing options...
LukeSkywatcher Posted January 28, 2011 Share Posted January 28, 2011 they could be in a fixed arrangement and not move with respect to each other; just forming a larger mirror made up of several segments.Ah ok. i see what you mean now. Link to comment Share on other sites More sharing options...
umadog Posted January 28, 2011 Share Posted January 28, 2011 I'm not sure if this is what you mean, but the large binocular telescope has adaptive optics built into the mirrors and these aren't made up of separate segments: LBT - 2007http://en.wikipedia.org/wiki/File:LBT_3.JPGYou can deform a solid mirror. That's why a proper mirror cell is so vital for, particularly, larger amateur mirrors. Link to comment Share on other sites More sharing options...
FraserClarke Posted January 28, 2011 Share Posted January 28, 2011 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 Link to comment Share on other sites More sharing options...
LukeSkywatcher Posted January 28, 2011 Share Posted January 28, 2011 Ah so the secondary mirror (of a scope that uses them) has the built in "adaptive optics", while the primary has "active optics". I always assumed that the primary mirror had the adaptive optics. Link to comment Share on other sites More sharing options...
umadog Posted January 28, 2011 Share Posted January 28, 2011 Me too, thanks for the clarification. What you say makes sense. Link to comment Share on other sites More sharing options...
Peter Drew Posted January 28, 2011 Share Posted January 28, 2011 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. Link to comment Share on other sites More sharing options...
LukeSkywatcher Posted January 28, 2011 Share Posted January 28, 2011 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.. Link to comment Share on other sites More sharing options...
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