Sky and Telescope - June 2018 - 30
by Bob Franke
Is that Star
s a child on car trips with my family, I often heard
my parents ask this question about car colors: Is that
blue or green? While driving my parents to a restaurant nearly 40 years later, from the back seat came the same
question, "Is that blue or green?" I turned to my wife and
said, "I can't believe they're still doing that." Today I engage
in discussions about color relative to stars, galaxies, and
nebulae instead of automobiles.
Several years ago, I shared an image of the open cluster
M67 on an online imaging forum. A more experienced astrophotographer commented that the stars in the photo were
too yellow, as open clusters usually have younger bluish stars.
As the discussion continued, another imager, Wolfgang Renz,
noted that M67 is a very old cluster, no longer dominated by
blue stars. Using data from the Naval Observatory Merged
Astrometric Dataset (NOMAD) catalog, he showed that the
stars are mostly white or yellowish white.
This started my quest for a repeatable process to obtain
consistent and reasonably accurate color balance in my
J U N E 2 018 * SK Y & TELESCOPE
deep-sky astrophotos. The journey eventually culminated
in the development of the freeware program eXcalibrator for
Color Is Complicated
When imaging the night sky, several factors affect the color
of your results. First, the spectral sensitivity of different
CCD and CMOS detectors varies greatly. Some are more
sensitive to blue light, while others respond better to red or
green light. This is true with both a monochrome camera
(used with individual color ﬁlters) or a one-shot color camera
(which incorporates tiny red, green, and blue ﬁlters over
p TOO BLUE There are many techniques to achieve a natural color balance in deep-sky photography, though most are subjective. This image
of M31 in Andromeda was captured by the author and processed two
different ways. The right side is balanced based on comparison to other
images found online, while the left side uses eXcalibrator to establish
color balance based on known star colors in the image.
UNLESS OTHERWISE NOTED, ALL PHOTOS COURTESY OF AU THOR
The eXcalibrator freeware program helps to
take the guesswork out of color-balancing
your deep-sky images.