Sky and Telescope - June 2018 - 31
Another variable is the combination of red, green, and
blue ﬁlters used in all digital cameras to make the color
result. Whether you're using a monochrome or one-shot
color camera, these ﬁlters vary in their transmission curves
and cutoffs. In some cases, the red, green, and blue passbands
overlap. Other ﬁlter sets have distinct transmission gaps
intended to reduce the effects of light pollution.
Additionally, several variables beyond your equipment can
affect the color of your images. These include atmospheric
extinction and transparency. The lower your target is in the
sky, the more atmosphere its light travels through, which
blocks an increasing amount of blue light. Hazy skies also
block bluer wavelengths more than red, skewing the color in
your result. Other effects are due to extraterrestrial factors
such as dust in our galaxy, and even intergalactic gas and
dust between your target and your camera. Finally, inconsistent image-processing choices, such as the normalization
(the equalization of individual exposures) before stacking,
can skew the ﬁnal result.
Astrophotographers often rely on several methods to correct the color balance. Some are better than others. Often,
an imager will look online to compare his or her image
with those of others. This is perhaps the least reliable way of
achieving accurate deep-sky color! Do a simple web search
for M31, and you'll be presented with dozens of images of
this showpiece galaxy, some bluer, others reddish. "Eyeballing" the color in your images this way might produce a pretty
result, but it isn't very accurate.
One reasonable way to color-balance a galaxy image is to
assume that the integrated light of a face-on spiral galaxy is
white. This reasonable approach shows a galaxy with its intrinsic color. However, many galaxies, for example, IC 342, are
seen through intervening dust within the Milky Way, which
reddens its overall appearance.
So using this intrinsic color
assumption for IC 342 makes
the foreground stars too blue.
Another common color-balancing technique is to set the
background color in your image
to a neutral gray. This works
well for some images, though
not if the ﬁeld is ﬁlled with
emission nebulosity or dust.
p WHITE STAR While our Sun
is informally referred to as a
Some imagers even use the
yellow dwarf, it is really a white
cumulative light of all the stars
G-type main-sequence star
in a picture as a white-point
(G2V). Amateur astronomers
average. This does work with
use other solar analog G2V
some objects, if there is no
stars as reliable white-point
intervening galactic extinction. For instance, the core of a
globular cluster often makes a good white-point reference.
Of all the techniques mentioned, using the color of stars
in your images is a step in the right direction to achieve
reasonably good color balance. But star colors vary greatly,
depending on which direction you look. Stars in the arms of
our Milky Way tend to be young and therefore blue. Looking
to the galaxy's halo and bulge, you see more reddish stars. In
fact, the general stellar population is mostly comprised of red
dwarfs, so the true average color of stars skews toward the
red end of the spectrum.
Many amateurs, in their pursuit of an accurate color calibration method, rely on a technique that uses G2V spectral-class
stars as a white-balance reference. Our Sun is a G2V star,
and its light appears white to our eyes. Using this approach,
you adjust the red, green, and blue exposures so that the G2V
SUN: SOL A R DY N A M ICS O B SERVATO RY / N ASA
q DUSTY VEIL One common technique used to color-balance galaxy photos is to assume their integrated light should be white. But some galaxies,
such as the face-on spiral galaxy IC 342 seen below, are viewed through dust within the Milky Way. The integrated light balance technique (left) produces a nicely colored galaxy image. Using G2V-like stars as calibration sources in eXcalibrator results in an image of the galaxy reddened by dust,
which blocks bluer wavelengths (right).
sk yandtele scope.com * J U N E 2 018