Those sorts of questions are interesting to me, and make me wish that I had taken at least one Astronomy class in college…
I did come across this reference-- which, for our purpose may as well be one of many.
http://www8.nationalacademies.org/astro2010/DetailFileDisplay.aspx?id=540
(downloads a pdf)
Glass is the legacy material for mirror substrates, and there are several reasons why it performs so well: the material is thermally stable; it can be engineered into a stiff structure with minimal residual stress; and the face sheet can be polished to a high-quality optical surface.
…
While glass has been the traditional substrate of choice, other materials have been used. One example is the Spitzer Space Telescope (SST), which has an 0.85 meter beryllium primary mirror. Launched in 2003, the SST is a general-purpose observatory that is diffraction limited at 5.5 microns and operates at 4K. Beryllium offers several advantages when used as a mirror substrate. One of the most significant advantages is that Be has a large specific stiffness (or stiffness-to-mass ratio): this ratio is five times greater than ULE and 6 times greater than aluminum [Yoder 1993]. Beryllium’s superior rigidity means that it can be used in mirror substrates that take up less volume than a ULE substrate designed for the same mission. In addition to a high specific stiffness, Be has a near-zero coefficient of thermal expansion (CTE) when used below 100 K which makes it an ideal material for cryogenic mirrors.
the “why polish the back” is a interesting question. still can’t nail down an authoritative answer. May have something to do with making the mirror reflective enough for interferometry, may have something to do with removing material without damaging the structural integrity of the blank, may have something to do with proving that the glass is structurally sound before the much more delicate and expensive process of carving the front into an optically perfect surface.
