[ATM] Testing a flat

[adam at strachan-stephens.com (Adam Strachan-Stephens)]

There is a commercial company here in the UK which advertises testing
flats either by fringe testing or (if aluminised) by autocollimation
with a spherical mirror.  To save me money, the owner has suggested the
following DIY test method using a parabolic primary:- 

"If you set the flat at 45 degrees in front of your mirror, in contact
with it, then go back to radius of curvature point. With careful
adjustment you will see yourself reflected larger and larger (eye). If
from this position you can use some very small torch bulb (no reflector)
to illuminate the flat, which will in turn reflect the light back, into
an eyepiece you have placed next to the bulb, examine the image. If it's
a good image, your flat is good enough, in fact excellent. If the image
is totally haywire, bad luck."

It should be said that the flat in question is an ex-military relatively
thin mirror, glued in someway to a metal backplate and what I want to do
is to use it in a quick and dirty star test rig to see if I can stand
the coma of a f/3.5 mirror that I have picked up.  So, the flat does not
have to be absolutely perfect - merely reasonable.

Is this guy's suggestion reasonable?

Thanks and regards

Adam