prism versus diagonal mirror

One of the problems I have been having making my 14" f/15 scope is that at f/15 the Foucault test is hideously accurate and shows up the tiniest flaw in the surface... and the cone of air between the mirror and the tester, making it difficult to interpret. On top of that, I can also clearly detect the aberations in the lens of the video camera I use to view the Foucault test image. After a while, you start to pull out your hair trying to figure out what errors belong to what source! In truth, the mirror under test appears to be pretty darn good, but I want to be sure to be sure

Soooo... I have just picked up the bits and pieces to assemble a Bath interferometer to test the optics by interference. When I get that assembled, I will be able to test any mirror to an alarming degree of accuracy while eliminating other sources of "noise" in the test set-up! I'm looking forward to testing my 20" f/4 (which I know is zoney and only barely meets the 1/4 wave limit) and most of all, my first ever mirror, a 6" f/7 that I reckoned was about 1/12th wave on the surface when I finished figuring it. I imagine it could be a little embarrassing! Interferometers have a nasty reputation for plunging your warm cosy fantasies into a bath of very cold reality!

Phil.

I've gone ahead and ordered one anyway from BC&F. Telescope service sell the same diagonal but with their own name on it. Regardless of its specification< the proof is in the eating>
I am also tempted by the 2" wide angle eyepieces offered by TS. They have a deal for 2 eyepieces (40,32 or 50mm) for just over €100. I think these are Revelation eyepieces. Anyone ever used one?