Re: ATM Square mirrors?

[Anthony Stillman <atmer@flash.net>]

>...square blank of pyrex vs. a round one
>...I know that the image produced should be the same

No, the image is not the same.  Specifically the shape of the diffraction
pattern changes from circled and round to crossed and square.  The MTF
curve for the diagonal is inferior to a circular aperture of diagonal
diameter, being depressed for mid frequencies.  The MTF curve for a cross
section is a straight diagonal line and superior to the aformentioned
circular aperture.  Try apodizing a scope and taking a look.  Try various
orientions of the mask with the spider.  I'm assuming a four vain spider.
The loss of just few fractions of a square inch can really mess things up.


>...less wasteful

True enough, and one of the reasons given for the shape of the NIF
(National Ignition Facility) optics.  Not to save glass, but to increase
packing density in the beam lines.  In my estimation more thoughtless over
the wall engineering. Triple the fabracation time, triple the cost, reduce
the building size 10 percent.  Yea I know, the money comes out of different
pockets.


>Would the polishing be more difficult?

Yes, well figuring is more difficult.


Anthony