Re: How to use an optical flat

On Dec 13, 3:00 pm, Louis Boyd <b...@xxxxxxxxxxxxxxxxxxxx> wrote:
Helpful person wrote:
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

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.

Since you brought up optical flats I have question. I bought a single
sided 6" fused silica flat made by Edmunds on ebay. Both sides are
polished and visually look identical. On it's lable is an ^ mark
pointing at one side. Does that mark point to the reference flat side
or does it mean "this side up" or what? I don't have another flat to
test it against. I bought it have the flat(est) side aluminized to use
it as a secondary mirrroy to fold a 20" f/4 parabolic telescope to put
the focal plane between the primary and secondary. The primary isn't
perforated.- Hide quoted text -

- Show quoted text -

One thing to remember. After coating, the flat will be less flat then
it was due to variations in coating thickness. However, the small
difference should not be a problem for your application.