http://www.skyandtelescope.com.au 65of each colour, which you then use when combining them in
your preferred image processing program.
Calibrating an imaging system
The other wayeXcalibratorcanbeusedistodeterminethe
exposure times for each filter taken with your particular
equipmentundernear-idealconditions.Thisisaccomplished
bytakingtwoorthree5-minuteexposureswitheachcolour
filterwithyourscopepointednearthezenith.Youcalibrate,
registerandstackthoseresults,andthenruntheresulting
images through theeXcalibratorprocess.
The correction factors to adjust the red,
greenandblueexposuretimesshouldbe
used when shooting future images. For
example, calibrating one of my imaging
systemsproducedRGBcolourratiosof1.0,
1.12, and 1.20, respectively. So I shoot my
individualR,GandBexposuresat10,11
and12minutes.Usingthesecalibrated
systemvalues,myimagesshouldhavea
well-balanced natural colour appearance.
By imaging under good transparency and
taking care to avoid atmospheric extinction
SEXCALIBRATING Using eXcalibrator is easy. Plate-solve one of
your colour 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.
XGOLDEN 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 colour image
calibrated by assuming the core of the cluster is
white, while the version at right used eXcalibrator to
determine the cluster’s colour based on the spectral
properties of known stars in the ield.
NEBULOUS FIELD Using the stars as colour balance calibration
sources makes short work out of images filled with emission
nebulosity, such as this shot of NGC 2261 in Monoceros.due to the altitude of your target, the final eXcalibrator R, G
and B correction factors should usually be close to 1:1:1.
Once you’ve calibrated your system, it’s easy to integrate
eXcalibrator into your own imaging routine. Here’s my typical
astrophotography workflow:
»Acquirethedataformyimagingtarget.
»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 colour adjustment.
»Usetheadjustmentweightstoassemblethecolourimage.
»Continue with stretching and other processes to produce
the final colour image.
Even with a calibrated system, eXcalibrator will produce
slightly varying channel weights in different data sets, because
it takes into account atmospheric extinction, transparency and
other variables from when the images were acquired.
Using different camera and filter pairings,
it’s possible to adjust the colour with
eXcalibrator so that white stars appear white
— an excellent way to consistently obtain
good colour in the initial tri-colour 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 colour in
your astro-images, then hitch your wagon
to eXcalibrator and take it for a ride. Colour
balance in astrophotography is ultimately
a matter of personal taste. But being
reasonably accurate from the start 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.