[ATM] Testing a flat

[bobmay at nethere.com (Bob May)]

The testing with a spherical mirror is one of the ways to do the test and is
probably the easiest with an aluminized flat.
Test the spherical mirror for accuracy and then setup the flat near the
tester so that you do a 45deg. change of direction of the light path and
adjust until the spherical mirror fully fills the flat.  At this point, you
test again and any changes in the surface shape are from the flat, not the
spherical mirror.
It is wise to test in several directions of the flat so that you can see if
there is any astigmatism resulting from the operation.
This test is especially suited to interfrometer testers as they will
automatically show any curvature in one direction of the flat with just one
test.  The Foucault test, unfortunately is going to test only in one
direction unless you are using a pinhole for a light source as the second
(normally the vertical dimension) is strongly blurred from a slit source
plus it will naturally show an error as the light source and destination
(unless a beamsplitter cube is used to get the two beams coaxial) will show
an error due to their offset from each other.
Finally, long ROC spheres are better used for this than short ones as they
will show less error in the vertical direction - the offset of the source
and destination is less percentagewise.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net