Re: ATM Polishing/Figuring Done!

[epabcc@epa.ericsson.se (Bratislav Curcic)]

Rob "Butterfingers" O'Toole wrote :

> At first glance I would reckon that picking the criterion for minimizing 
> transverse aberration would be most appropriate. When I'm focusing my 
> scope I try to settle on a focal position that gives me the tightest 
> stellar image. Would it not be human nature to end up focused where 
> transverse aberration is least detectable?

No. There is another variable that we didn't mention - the area (or
contained energy). You may have a certain amount of rays that focus WAY
outside Airy disc, but small in percentage compared to the rest of
light that is concentrated inside the disc. This is the place where
wavefront error might be minimal: the perfect example is marginally
acceptable spherical mirror - like spherical 6" f/8 - you will NOT
focus where transverse aberration is minimal, as in this particular
focal position this will represent loss of resolution several times
compared to theoretical (as easily confirmed by calculating spherical
aberration of such mirror - in NO case it will be smaller than Airy
disc).  Instead, you will focus where MAJORITY of rays end up within
the Airy disc; thanks to light's dual nature, the rays ending outside
that will interfere and simply increase energy in diffraction rings.
But the resolution (i.e. size of Airy disc, or OTF at high frequencies)
will be the same as in perfect mirror.  It will resolve as close equal
doubles as any 6"; what suffers is resolution on unequal doubles and of
course planetary images.

Bratislav