I'd have to disagree!
I would certainly bow to your superior knowledge and experience. I can imagine that finding somewhere to test an f/30 mirror would be next to impossible anyway.
Quick question. If you had a reference flat of very high quality could you use this in the testing to reduce the length of the testing tunnel?
Hi Dave - Sorry for the long delay replying - I'm just back from a couple of weeks flying in France.
You could indeed use a flat mirror to fold the optical path, though it might introduce a little astigmatism into the test image. That shouldn't be a problem with long slow mirrors - just short fast ones, because the viewing angle would be greater. Two flat mirrors at 45 degrees would remove this problem altogether.
In a reversal of this, I will be using the spherical primary to allow me to test the surface of the flat. Any errors on the flat will appear doubled in magnitude as the light will reflect off the flat twice before returning to the tester. I haven't got the primary aluminised yet so I haven't messed with this.
Phil.
  