dawn and twilight sky flats

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Tom points out the Mark IV takes so long to read out that
one can't get very many flats while the sky is bright, but not
too bright.  D'oh!  I had completely forgotten about that.
It would help if one set the exposure time to, say, 2 or 3 seconds
instead of 60 seconds, but that fixed 40-second readout period
is still a problem.  That makes twilight flats less attractive,
for sure.

  Tom asks if one could combine flatfield images from dawn and
from dusk.  Yup.  If you point south (as the Mark IV typically
does), then the sky gradient will go one way at dusk and the
other way at dawn, so combining them will tend to remove the gradient.
Sort of ...

> Note that a lot of stars can be seen in a dawn flat ...

  If one changes the operating procedure, so that the exposure
time is only 2 or 3 seconds instead of the usual 60, there will
be many fewer stars visible in these exposures.  That's a lot
easier at dusk (when the operator is awake) than at dawn (when
he's asleep), of course :-/

>  The levels will thus vary wildly, and one must do something to 
>  normalize the data before combining.

  Absolutely true.  I normalize all the exposures so that the mean
level is the same in each before I combine them. 


                                               Michael