[Aavso-photometry] Flat Stuff

[Arne Henden]

Chuck Pullen wrote:
 > Hi Steve - I'd heard that as well, but was corrected by Arne Henden.
 > Steve Howell cites 13 degrees east of the zenith for evening twilight
 > flats from Chromey and Hasselbacher 1996 (PASP 108  944).  I'm not sure
 > how much difference it makes unless you are trying to do extreme color
 > stars or very high precision work in the mmag range.
Chromey and Hasselbacher is a little ambiguous, but the basic idea
is that the null-gradient point is somewhere near the zenith, possibly
being offset in the antisolar direction by a few degrees depending on
the distance the sun is below the horizon.  For almost all CCD fields of
view, using the zenith for twilight flats is a good compromise with
little gradient.  It would be interesting for someone with a wide-field
but carefully flatfielded system to confirm C&H's results.

Robert Koff wrote:
 > Thanks for the comments.  I have recently stopped using the diffuser for
 > I-band flats, and in such cases I set the drive to slew at guide speed
 > (2X sidereal with the LX200).  This smears out the star trails pretty
 > well, and they seem to median out.  BTW, I generally point near the
 > zenith for the flats.
It is usually easier to just turn the drive completely off if you
are doing "trailed" flats.  The streaks are long enough in most cases
and you can leave the telescope pointed to the same zenith angle.
I usually don't use trailed flats, but instead tracked flats, with
a position offset between each flat so that stars can be median filtered
out.  With trailed flats, you can often get a bright star that still
leaves some residual when combining the flats.  However, both techniques
work.

Richard Miles wrote:
 > I also use 2 or 3 different exposures.  So taking a dawn set, the first
 > set are with say 2-sec exposures, and I image in the range 20-50%
 > full-scale. Then I move on to say 0.5 sec, finish that set before moving
 > onto the last at 0.2 sec.  The shutter on the camera is electronic and
 > is in effect <<0.001 sec so shutter speed is not an issue.
If I were you, I would test this.  I have yet to see an electronic
shutter that was perfect.  Our uniblitz shutters, for example, start
to leave an objectionable vignetting pattern for any exposure shorter
than 3 sec.  Flats are the most important calibration you will do,
and any error in the flats impact all science frames.  Test them in
many ways.
Arne