I have two flats, a 6" Zerodur one and a 12" fused silica one. Both are uncoated and have both sides polished flat. The problem I have is that when trying to use them for DPAC, I get reflections from both the front and rear surface. For the small 6" flat, the reflections are very close and using a red laser diode I get two airy disks, almost touching. For the larger flat, I see them separated by about 30-degrees at high power (200x). From this I can tell that the smaller flat has almost parallel sides while the large one has a convenient wedge. But even for the larger flat, when I use a Ronchi grating, the second reflection interferes (not in the wave sense) with the Ronchi pattern from the first one. Hence, I'm looking for a method to suppress the reflection from the back surface.
I tried spraying water onto the rear surface to optically scramble the reflection and it did dim the second Airy disk a bit, but because it was not not wetting it uniformly there was still a large area left dry between the droplets. Smearing oil seems too messy, requiring a later clean up. I also tried water-wetted lens tissues stuck to the rear and that did dim it, but not completely eliminate it.
Any suggestions here? Advice would be appreciated.