How to use an optical flat

An optical flat can be used to measure the flatness of another
surface. This is generally done with an extended mercury source and
placing one edge of the optical flat against the test piece. With
several wedge fringes the flatness of the piece under test can be
estimated.

This is the usual description of how flats are used. However, this is
a great simplification. As soon as the flat and test piece are
separated (as with the wedge or flatness deviations) errors creep in
due to the line from the observer's eye to the test piece not being
colinear with the source and the test piece.

My question is: Does anyone know of a good on line reference
describing and esimating the expected errors? I would find this
useful as a check to my own calculations.

www.richardfisher.com
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