some observational observations

[<aah@nofs.navy.mil>]

After using the Mark IV for a few weeks, I have
a couple of comments regarding techniques.
  The sky background on a dark night is about
2.7 ADU/pixel/second.  The dark current is about
1.5 ADU/pixel/second according to TN62, so the
two are about comparable.  I'd limit exposures
to less than 15 minutes on the basis of these
numbers, but the drive will limit you to shorter
exposures anyway.  Readnoise will be the dominant
noise source for exposures shorter than 60 seconds
or so.  Don't worry, since the shutters are pretty
crude and you should probably not use shorter exposures
without a lot of care if you need timing accuracy.
  I don't have the mount properly aligned to the
pole, and so am getting trailing in both RA and
Dec that limits me to ~3minute exposures.  This
will be fixed as soon as we get a proper mounting
plate installed; probably next week.  It would
be nice to have a pole sight on the system for
initial alignment.  Tom's inclinometer has been
really helpful, since it points me to within
a degree or so and that is sufficient with these
big fields.  The drive speed in declination is pretty
fast; I'd say you can move from one field to another
within a minute or so.  The RA drive is much slower;
it moves about 1 minute of time for every two seconds
of motion.  That means it takes two minutes to move
an hour of RA.  It is best to run the system as a transit
telescope except for a few isolated special cases.
  Flatfielding is a problem.  Ice crystals do come
and go (though they improve with cycling with Tom's
dessicant system).  That means you have to take flats
every night.  If you have local lights, or a moon in
the sky, then master sky flats are difficult to do
properly.  I find I have to change the dessicant about
every third day; this may make a remote installation
more difficult without a different drying scheme.
  Data reduction is a *real* problem.  Generating
2kx2k median darks and sky flats takes a lot of
disk space and computer time.  You acquire 2-3GB per
night, and so need to keep ahead of things; forget to
reduce a few nights and your disk gets full.  Or worse,
you have archived the data and now have to pull it off
your archive media.  I find to do a proper job requires
a fair amount of hands-on manipulation.  For making flats,
you want to delete images with bright stars, moon gradients,
satellites/planes, etc. so you really want to inspect them
all.
  Be sure to install some sort of lens shade.  These lenses
have a definite ghosting/scattered light problem.  Of course,
lens shades add weight to the front end, and that may be
a problem in getting balance correct.
  The system is pretty exposed.  Don't expect it to last
through too many rain storms.  Figure out how to cover
it permanently.  The 90' cable that Tom brought is working
well and I don't see any data loss or noise problems.  So
this length is doable; don't expect to go much longer.
  I think the BASIC program is getting sophisticated enough
that you can do quite a number of useful projects almost
"out of the box."
  Having Tom come out with a system is a real plus.  I hope
he can do that with most of the Mark IV's, or at least can
videotape the next installation in gory detail.
Arne