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[ATM] Re: DP=-H2/R or DP=-H2/R+H4/2R3??
Both terms are for a perfect parabola: third order term and fifth order
therm, respectively. The fifth order term is negligible for all but very
large/fast mirrors
(when it becomes less than negligible, it is advisable to think about
some test other than Foucault's, at least as a double check). I believe
the general focal shift formula at the r.o.c. for any conic K that
includes both terms is DP=-(Kh^2/2R)-K(3+K)h^4/8R^3 for moving source,
and twice as much for fixed source ( "h" is the zonal hight, and R the
paraxial r.o.c. which has negative sign).
Vlad
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