Sky & Telescope - December 2020 - 61

neighbor, be more affected by noise, or respond to the chip's
temperature a bit differently. The key to making scientiﬁcally
accurate images is to map these small ﬂaws and correct them.
How is this done? First, we need to quantify all the ﬂaws
in the detector. This is accomplished by recording and applying images known as calibration frames, commonly referred
to as dark frames and ﬂat-ﬁeld images.
Dark frames are maps of the small electric current known
as dark current that accumulates even when no light is striking the photodiode. They also record the ﬁ xed-pattern noise
signal from the sensor. Dark frames are created by covering
the telescope or closing the camera's shutter and recording
an image with the same duration and at the same temperature as the light images you make of your target stars.
Because dark current varies randomly, we need to take
many dark frames and average them together. If I were planning to record 5-minute exposures of my target, I'd also take
a few dozen 5-minute dark frames.
The other calibration frame is a ﬂat-ﬁeld image. This
records each photodiode's sensitivity to light, as well as any
variations in the optical train of
your telescope, including vignetting or dust motes on any optical
element in the light path that can
slightly skew the measured result.
Typically, these are images of the
twilight sky or a uniform light
source, such as an electroluminescent panel. Flat-ﬁeld images
also require their own dark frame
calibration, which you take using
the same exposure you used for
the ﬂat frame. These are called
dark-ﬂat frames, or sometimes
ﬂat darks.
To beat down random variations in these images, an observer
typically records multiple dark,
ﬂat, and ﬂat-dark frames and
pu QUICK CHART
then averages each of the results
Above: The Variable Star
to produce low-noise "master"
Plotter on the AAVSO
website (aavso.org/
dark and ﬂat frames.
apps/vsp) produces
After a night of imaging, you
finder charts for known
typically open the images of your
variable stars to help
target stars in your preferred
you pinpoint your target.
astronomical imaging program.
Above right: Using the
advanced options, you
Depending on your camera, the
can also create a chart
images may look a bit noisy and
with DSS (Palomar Sky
mottled with "donut"-shaped
Survey) images, as well
dark areas. You then apply your
as produce a table of
master calibration frames - subsuitable comparison
stars in a specified field
tracting the master dark frame to
seen at right.
remove dark current, then dividing it by the master ﬂat-frame to

p SIMPLE SETUP Most any telescope and camera combination on an
accurate tracking mount will produce excellent photometric data.

sk yandtelescope.org * DECEMBER 2 02 0

61

http://www.aavso.org/apps/vsp http://www.aavso.org/apps/vsp http://www.skyandtelescope.org

Sky & Telescope - December 2020

Table of Contents for the Digital Edition of Sky & Telescope - December 2020

Contents
Sky & Telescope - December 2020 - Cover1
Sky & Telescope - December 2020 - Cover2
Sky & Telescope - December 2020 - 1
Sky & Telescope - December 2020 - Contents
Sky & Telescope - December 2020 - 3
Sky & Telescope - December 2020 - 4
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