Re: Good Board Stack

[Arne Henden <aah@nofs.navy.mil>]

Thomas Droege wrote:
> In the past, I have prepared new darks and flats about once a month. 
> Looking at them, they repeat well within the noise unless I have done
> something.  The plan for future running is to continue doing this.  My
> thinking is to do a maintenance during the time of the full moon.  The idea
> is to dust/clean the lenses, and do darks and sky flats (before moon rise).
> I know that it is standard ritual to do flats every night.  These take a
> lot of time, and don't contribute anything.  Likewise the cameras are
> temperature stabilized and darks are constant.  What does affect both are
> the outside light conditions.  At my location these change constantly. 
> There is no way to correct for when my neighbors decide to brighten up the
> night sky.  These are much more a factor than "good" darks and flats.  So
> why even try to do them every night?  The only reason that I can think is
> that "every one does them" so I will be criticized if I don't.  But they
> are useless.  Comments, anyone?
> 

It is more of a cautionary measure than a necessary one.  If you have
a spot in the system where dust will be close to focus, and that spot
is open at some time during the night, then you have the potential to
get new dust.  If you take flats once/month and the dust falls on the
system the day after you take the flats, and then moves before you
get a chance to take additional flats, you can't fully correct.
In the Mark IV, the problem area is not the front lens surface, but the
shutter mechanism.  You can get dust from paint flecks, fuzz from the
cable, etc. that can make its way into the camera head.  If you haven't
seen this, then you can gamble and do monthly flats.  Just be aware
that you may lose a month of data.
   For our Observatory, and whenever you visit another observatory, you
only have one chance for some observations.  The comet moves, the
CV fades, the near-earth asteroid becomes more distant and fainter,
the nova finishes its outburst.  You don't want to gamble on losing
the data, and so take flats more often, usually nightly since flats
are most commonly taken when the sky is too bright for observing
(either twilight flats or closed-dome flats) and so no important time
is lost.
Arne