On Jul 15, 3:58*pm, Quadibloc wrote:

Also, if, say, the six mirrors at the corner of the primary were
coated with copper instead of gold - assuming copper also has low
infrared emissivity like gold - that would involve a gain because
while copper falls from near 100% to 40% reflectance somewhat earlier
than gold, it stays around 40% right down to 220 nm, while gold
becomes less reflective at 260 nm.

Unfortunately, apparently this won't work, as copper has about the
same emissivity as aluminum, instead of the lower emissivity of gold.

John Savard