Re: ATM Astigmatism

["Dave McCarter" <dmccarter@sympatico.ca>]

> What you have measured is one component of astigmatism, the "twist".
This
> is the twist of the top half of the mirror with respect to the
bottom half.
> There is another component that I call "potatoe chip" that this test
cannot
> detect.   That is where the top and bottom edges of the mirror are
bent
> forward, and the left and right edges are bent back

Testing the mirror when it is polished out to a sphere and just
looking for a null with Foucault is sensitive to errant shapes in the
glass. Using a really tiny pinhole, such as is found on the chip of an
inkjet ink cartridge, and observing the reflected image with a short
focal length eyepiece is also useful to detect miss-shaped optics.

Dave