[AZ-Observing] Re: Flats and Bias
- From: Brian Skiff <bas@xxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Sun, 15 Jul 2007 14:12:45 -0700 (MST)
Jeff and Mike have mentioned most of the various trade-offs
with regard to flat-field calibration images. The usual procedure
at most observatories is to take twilight flats with the telescope
either not tracking or commanded to dither some small amount between
each exposure, the former being lots easier. As Jeff noted, often
the problem is getting enough flats for multiple filters in the
time available. Sometimes one ends up doing a few filters each
session, and getting "enough" over the course of a few days.
As an example, with our 72-inch + regular CCD set-up, one starts at
14 minutes after sunset with the B filter, and there is just enough
time to get 10 exposures in B, V, and R, working with something like
16-second exposures for the final batch of R images, which have lots
of stars. If one gets distracted and doesn't start the first flats
until say _16_ minutes after sunset, it's too late, and you won't be
able to get them all. There is no such thing as a star-free patch to
use for flats, despite efforts some years ago to identify such regions
in dark clouds etc, so you just deal with the stars in another way.
Dave Schleicher sometimes does comet imaging using eight rather
narrow passband filters to isolate various emission species, and this
includes filters down toward the atmospheric cutoff at 3200A or so.
So his first flats are 10 minutes _before_ sunset, and he gets only
three or so images for each filter.
The problem with twilight flats is that the spectrum of the
twilight sky does not resemble the (dark) night sky; the former is
a very blue continuum like that of the Sun (unsurprisingly), whereas
the night sky is rather red and has emission lines, something like
the spectrum of a planetary nebula (the 'nebula' being the glowing
ions in Earth's atmosphere). The characteristics of the flats
change as a function of color, and the emission lines can produce
interference fringes in thinned chips (which you folks probably
aren't using). All of this is minimized by the filters, but not
completely eliminated.
It is worth noting that there is a gradient in the twilight sky,
such that it is best to point the telescope at a particular place to
minimize this. The location, often referred to as the 'Chromey spot',
is 15-20 deg off the zenith in the azimuth direction _away_ from
the location of the Sun. Thus this time of year the 'sunset' spot is
southeast of the zenith. This is something that's amenable to being
programmed given a Sun ephemeris and local date/time, so could be a
telescope-control command in software (and is in the Lowell telescope-
control systems).
Jeff mentioned a specific well-depth level for the flats, but the
main thing here is simply getting enough photons in each pixel in the
combined median-averaged flat rather than the well-depth per se.
With ordinary telescopes, getting enough images with enough signal
(per pixel) on blank night sky is difficult. This usually comes about
for two reasons: the relatively long f/ratios of telescopes means the
sky gives a relatively weak signal on ordinary exposures, and it is
common to take a lot of pictures of relatively few fields, so the star
pattern remains fixed on most of the exposures. By contrast, with the
LONEOS Schmidt we're running at f/2 unfiltered, and we commonly cover
two dozen separate fields in a block of exposures. So even on our
standard 45-second exposures the sky is very bright, and the flat is
generated directly from the median of 20 or so exposures of different
fields. The "thousands" of exposures that Jeff mentioned are not needed;
more than a couple dozen in our case is total overkill. This is lucky
for us: the f/2 optics are so fast that we cannot take twilight
flats with exposures long enough to avoid shutter-illumination problems,
nor short enough to avoid getting zillons of stars, and the night-sky
emission produces fringing on some parts of the two CCD chips in the
camera, which can be taken out only with night-sky flats. We also need
to make different flats when there is significant Moonlight, since the
sky goes back to being like daytime (sky-blue with no emission lines).
This business about the change in the character of the night-sky
spectrum is "interesting" when considered for an urban location. The
sky is relatively stable thanks to the saturation of street lights,
business lighting, cars, etc. On the other hand you still have the
emission line spectrum from still-prevalent mercury, sodium, and
metal-halide lights. Then again, even the Full Moon hardly changes
the sky brightness...just one more street light.
There are some excellent articles on the art of flat-fields in
various issues of the late, lamented "CCD Astronomy" magazine,
including an article by Fred Chromey about the brightness gradients in
the twilight, and about building various light boxes, dome flats, etc.
\Brian
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