Sky and Telescope - June 2018 - 33
NEBULOUS FIELD Using the stars as color balance calibration
sources makes short work out of images ﬁlled with emission
nebulosity, such as this shot of NGC 2261 in Monoceros.
p EXCALIBRATING Using eXcalibrator is easy. Plate-solve one of your
color images and enter it into the WCS File section at upper right. Next,
select each of your red, green, and blue frames in their respective lines
at top left. Then simply click the Calibrate Image button at the bottom
right, and in a few moments the scaled weight of each channel will be
presented next to Avg at the bottom left of the screen.
Calibrating an Imaging System
The other way eXcalibrator can be used is to determine the
exposure times for each ﬁlter taken with your particular
equipment under near-ideal conditions. This is accomplished
by taking two or three 5-minute exposures with each color
ﬁlter with your scope pointed near the zenith. You calibrate,
register, and stack those results, and then run the resulting
images through the eXcalibrator process. The correction factors to adjust the red, green, and blue exposure times should
be used when shooting future images. For example, calibrating one of my imaging systems produced
RGB color ratios of 1.0, 1.12, and 1.20,
respectively. So I shoot my individual R, G,
and B exposures at 10, 11, and 12 minutes.
Using these calibrated system values, my
images should have a well-balanced natural
By imaging under good transparency and
taking care to avoid atmospheric extinction
due to the altitude of your target, the ﬁnal
eXcalibrator R, G, and B correction factors
should usually be close to 1:1:1.
u GOLDEN CLUSTER Globular clusters are another target that is thought to be a good white-point
calibration object. But many of these star clusters
are dominated by older, redder stars and are also
seen through intervening dust in the Milky Way. The
image of M10 at upper right presents a color image
calibrated by assuming the core of the cluster is
white, while the version at right used eXcalibrator to
determine the cluster's color based on the spectral
properties of known stars in the field.
Once you've calibrated your system, it's easy to integrate
eXcalibrator into your own imaging routine. Here's my typical
» Acquire the data for my imaging target.
» Calibrate, register, and combine the R, G, and B exposures.
» Plate-solve one of the stacked results.
» Run the R, G, and B images through the eXcalibrator process
to determine the ﬁnal color adjustment.
» Use the adjustment weights to assemble the color image.
» Continue with stretching and other processes to produce
the ﬁnal color image.
Even with a calibrated system, eXcalibrator will produce
slightly varying channel weights in different data sets,
because the program takes into account atmospheric extinction, transparency, and other variables that occurred when
the images were acquired.
Using different camera and ﬁlter pairings, it's possible to adjust the color with
eXcalibrator so that white stars appear white
- an excellent way to consistently obtain
good color in the initial tri-color assembly.
By following through with consistent image
processing, you can accurately compare different images and say, for example, "Galaxy
A is bluer than Galaxy B."
If you agree that making white stars
white will produce reasonable color in your
astro-images, then hitch your wagon to
eXcalibrator and take it for a ride. Color
balance in astrophotography is ultimately a
matter of personal taste. But being reasonably accurate from the get-go will add
consistency to your results and make your
later processing decisions easier.
¢ BOB FRANKE is a retired software developer and avid astrophotographer. See more
of his images at bf-astro.com/index.htm.
sk yandtelescope.com * J U N E 2 018