To: atm{at}shore.net
From: MLThiebaux 
Reply-To: MLThiebaux 


Anthony Stillman wrote:

...
>Well, yes.  And, a flat needn't be so flat.
>
...Anthony
>

True, but can we be quantitative?  Despite Texereau's admonishent for 1/8
wave diagonals (p101, footnote reference to A. Couder) it doesn't have to
be.

The diagonal could be hyperbolic (hyperboloidal has a nice ring), convex or
concave, and would work perfectly well.  A little less perfect is toroidal
with curvature along the minor axis twice the curvature along the major
axis, for the usual 45 degree diagonal.  Least perfect is spherical but
with computable limits.  Spherical with a sagitta (convex or concave) of a
wavelength or so depending on other things, might work as well.

There is a formula relating the sagitta of a slightly spherical diagonal to
an equivalent level of astigmatism in the primary mirror, but since ATM's
are commendably skeptical of formulas thrown at them, I'll give a numerical
example
-- tomorrow.

Martial Thiebaux
Rawdon Hills, Nova Scotia

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