The Inrad website quotes "sub-20 Å surface roughness". Our requirement is 3 Å RMS surface roughness on spatial scale below 1000 nm. If their process can achieve that, we might consider metal, but would need to evaluate a number of other factors such as the deposition of a multilayer coating (to be supplied by another vendor) on the front surface.

Question: Why is the reflective coating on the rear plano surface, rather than the front concave surface?

Answer: For alignment purposes - the front surface is to be coated by another vendor with a multilayer to reflect EUV wavelengths around 10 nm.

Question: What is your preferred material (fused silica, ULE or Zerodur)?

Answer: Whatever material/process combination can achieve 0.3 nm RMS surface roughness on spatial scales below 1000 nm (as well as the other specifications).

Question: What is the application?

Answer: A telescope mirror for an EUV imaging spectrograph (EUNIS) to fly on a
sounding rocket. It will be in full sunlight for about 7 minutes. 

 (see http://discovermagazine.com/2016/janfeb/14-hot-answer-to-a-solar-mystery)