"Another difficulty is that a liquid metal mirror can only be used in zenith telescopes" [Wiki]

Why is that? Why can't a series of flat mirrors be used to reflect light coming from any angle to be perpendicular to the plane of the mirror?

Here a ray diagram. Apologies for the size and the quality. enter image description here

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    $\begingroup$ Perhaps you could sketch a ray diagram to show us what you mean? The liquid mirror is the primary mirror. $\endgroup$ – ProfRob Jun 12 '15 at 9:54
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    $\begingroup$ I think your flat mirrors would need to be as large as your primary mirror! $\endgroup$ – ProfRob Jun 12 '15 at 11:09

The problem is that you'd need your flat mirrors to be the same sort of size, if not larger, than your liquid mirror. But the whole point of liquid mirrors is that you can make very large mirrors without the problems of the mirrors deforming under their own weight. So your suggestion just exchanges the problem of making a very large parabolic mirror for the problem of making a very large flat mirror, and gets you no further forward.

  • $\begingroup$ Simultaneous thinking! $\endgroup$ – ProfRob Jun 12 '15 at 11:09

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