Why glass for primary mirrors?

[Norlesh (Shane)]

While looking up thermal expansion coefficients on wikipedia Invar (a table of Invar's properties here) caught my eye - it's a Nickel/Iron alloy with a linear coefficient of 1.2 compared to Pyrex's 3.2 (lower is better).
While a mirror the same dimensions as pyrex would be almost 4 times heavier (Specific gravity - Invar: 8.05 vs Pyrex 2.23) I would assume the mass could be reduced to match or exceed that of the equivalent diameter Pyrex mirror by judicious drilling and machining of the back.

Anyone ever worked with it?

FYI: apparently if you have an old CRT sitting around, there is a fair chance the shadow mask inside behind the face is made from Invar

[OzEclipse (Joe Cali)]

It is common to use invar rods to keep the spacing constant between primary / corrector / and film holder in Schmidt cameras and other fast systems to maintain focus in these f2 optical systems.

Joe

[Norlesh (Shane)]

On further reading it would appear Invar can be topped, according to this report on the fabrication of large scale mirrors from 1999 the best of the best for mirror substrates (comparing density/elasticity vs expansion/heatflow in Figure 3) is SiC - silicon carbide.