Sky & Telescope - November 2019 - 37

A LL IM AG ES BY THE AU THOR

Dark frames are captured in complete darkness (with the
ust ﬁve words is all it takes to highlight one of the main
telescope or camera covered), at the same detector temperathings that makes astrophotography so challenging: It
ture and exposure duration as your target images. All other
is dark at night. With very few exceptions, there are big
settings, such as ISO, gain, and offset (if your camera allows
differences in how to photograph dim targets under low-light
you to change these settings), should also be the same as
conditions as compared with daytime photography.
for the light frames. Make sure that no light can reach the
Beyond the solar system, targets visible in the night sky are
camera's sensor, and record a series of dark frames using your
very dim and often barely brighter than the background sky.
preferred acquisition software in the same way as you would
The cameras we use to image deep-sky objects are inherently
capture light frames. You can then combine them into what's
noisy. And to further compound the problem, images from
called a "master" dark frame using either an average or pixeltelescopes, astrographs, and camera lenses suffer from uneven
rejection method, which helps to remove random signal. A
ﬁeld illumination due to a host of reasons, including varying
camera's noise characteristics will change
sensitivity across your astronomical camover time, so shoot new dark frames every
era's CCD or CMOS detector, vignetting,
Calibration should be the
few months even if all other shooting
and even dust on optical surfaces. These
ﬁrst step in any imageparameters remain the same.
deﬁciencies are seen in every image that
processing workﬂow.
Note that off-the-shelf DSLRs and their
downloads from your camera's detector.
mirrorless
cousins do not include temperaFortunately, the standardized process
This is true for both CCD
ture regulation. When imaging with these
known as image calibration can clean up
and CMOS cameras,
cameras, try to match the temperature of
the majority of these issues in our photos
regardless of whether
the calibration frames to the light frames
prior to combining and processing them
as closely as possible. You can record dark
further, which then allows you to get the
they are monochrome
frames periodically throughout your imagmost out of your astrophotography.
with ﬁlters, cooled oneing run so that their average temperature is
Raw, unprocessed deep-sky images can
shot color (OSC), DSLR,
similar to that of the light images. Another
look quite unappealing directly out of the
strategy for cold-weather imaging is to put
camera. They generally appear dark with
or mirrorless cameras.
the camera in the refrigerator or freezer
just a smattering of stars. If you stretch
and record dark and bias calibration frames
that image, you'll begin to see your target,
to mimic expected nighttime temperatures.
but such image manipulation reveals countless white and
This brings us to the third type of calibration used in
black specks scattered across the image. This is a combination
deep-sky imaging, called ﬂat-ﬁeld calibration. Flat-ﬁeld
of unwanted signal and noise that has to be removed from
calibration frames are images of an evenly illuminated
each of your target exposures before they can be combined
blank target, such as a projection screen. Their acquisition
and then further processed.
is more complex than either bias or dark frames. A properly
Most experienced deep-sky astrophotographers conexposed ﬂat-ﬁeld image records uneven ﬁeld illumination
sider calibration an essential step in the process of making
(vignetting), regional differences in sensor response, and the
top-notch images of galaxies, nebulae, and star clusters.
shadows of dust spots, which appear as round spots when
Although calibration isn't difﬁcult in practice, it does require
shooting through a refractor or doughnuts when imaging
a modest investment of time to learn, and it has associated
through reﬂecting telescopes.
jargon that can make it seem hard. Don't let this discourage
Unlike bias and dark frames, you can acquire ﬂat-ﬁeld
you from learning how to calibrate your images. Here's what
images with different camera settings than used for your light
you need to know.
images, though if you shoot your ﬂat-ﬁeld frames at a different
temperature, you'll need to record a full set of matching dark
The Cleanup Crew
frames and apply them to the ﬂats before assembling them
Three types of calibration frames are applied in the calibrainto a master ﬂat-ﬁeld image. It's far simpler to keep temperation routine to address speciﬁc issues unique to long-expoture and other camera settings consistent with your other
sure astrophotography. These are known as dark frames, bias
calibration frames. Regardless of the method used to acquire
frames, and ﬂat-ﬁeld images. Dark frames are used to correct
ﬂats, the exposure time should be set to produce a moderately
for the electronic signal that builds up over time during a
bright image, roughly 30% to 50% of the full-well capacity of
long exposure. Bias frames characterize and correct readout
your camera (the point at which a pixel saturates) in order to
noise - the noise generated when information on the sensor
be effective in correcting your light frames.
is transferred to a computer - but it is only really necesShooting ﬂat-ﬁelds can be tricky. If your camera has
sary for advanced calibration procedures when scaling dark
a mechanical shutter, you'll need to expose your ﬂats for
frames made at different temperatures than the light frames
at least several seconds in order to ensure the frame isn't
were made. Flat-ﬁeld images are then applied to correct for
affected by the brief time the shutter passes through the
uneven ﬁeld illumination, making vignetting and dustimage. Additionally, if you are using a monochrome camera
doughnuts seem to magically disappear.
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Sky & Telescope - November 2019

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