Re: ATM Interference Testing Technique and a D-K Secondary

["Bob May" <bobmay@nethere.com>]

You need to make a seperate piece of glass that has the right curvature on
it.  This piece can be tested in the traditional manner for spherical and
then applied to the secondary.
Part of the problem is that you need a fairly dark background (as well as
the only light source being the illumination of the interfering surfaces)
and any extraneous light tends to reduce the brightness of the fringes.
If you take two pieces of plate glass and superclean or polish them for a
moment, you can see fringes about as good as any other set that you will be
able to see.  It would be a good reference for what you actually can see for
fringes.  Two pieces of broken glass about 1" sq. are suitable for this
test.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net