RE: Focus indication

[<aah@nofs.navy.mil>]

Chris,
  We use a similar technique with the PMM scanner since
it is taking an image of a photographic plate.  Since photographic
plates contain exposed 'grains', the optimal focus is the
one with the highest spatial frequency content.
  However, there are a few reasons why this technique may
not work or at least work optimally for astronomical imaging.
First, even if the technique works, you have no indication of
which way to move focus, which is the advantage of the focus
plate technique.  Second, multiple frames must be taken, each
with its 40 second readout.  Finally, most of the pixels just
contain noise, so your technique won't necessarily optimize focus
on the (relatively) few real stars.  This is different
from the daylight photo situation.  Focus with the Mark IV is done
once in a blue moon since Tom has shown there is little
temperature dependence and once the systems are working you
are unlikely to knock something.
Arne