From: Mark Holm 
To: pk6811s{at}acad.drake.edu, atm{at}shore.net
Reply-To: Mark Holm 


Paul,

I'm not sure I understand your diagrams.  I imagine that what you are
describing has the folding mirror after the knife edge/mirror edge.  In
this case, the optical quality of the folding mirror should be relatively
unimportant.  This doesn't mean you can use a really junky mirror, but a
reasonably flat first surface mirror should work.  The only problem I see
might be that it will be hard to get your eyeball as close to the
"knife edge" as you need.  Even on rather moderate f-ratio
mirrors, my eye is often so close to the knife edge that my eyelashes hit
it when I blink.  Maybe this is a case where the old trick of putting a
small telescope (similar to a conventional finder scope) behind the
"knife edge" will help.  You need the auxillary telescope to have
enough focus out travel to focus on the test mirror.  Otherwise the
auxillary telescope doesn't have to be terribly good quality.  Maybe even
one of the awful things from a "department store" telescope will
suffice.

Mark Holm
mdholm{at}telerma.com

--- BBBS/NT v4.01 Flag-5