Sky and Telescope - June 2018 - 32
J U N E 2 018 * SK Y & TELESCOPE
G REGG DINDER M A N / S&T (3)
Relative Visual Sensitivity
stars in an image appear white - if
and photographs white. Measur0.60
there are any G2V stars in the ﬁeld.
ing the color balance of the Sun
Using this singular calibration
with SDSS's u- (ultraviolet through
method still has problems. First,
blue), g-, and r-ﬁlter values yields
as mentioned earlier, there is no
(u-g) = 1.43 and (g-r) = 0.44. So
eXcalibrator searches an image for
correction for the altitude of the
stars with SDSS (u-g) and (g-r)
target as it is imaged throughout
values similar to the Sun's. The
a night or over several nights. It's
program uses the average values of
particularly problematic when the
the identiﬁed stars to determine
target falls below about 40° above
the proper red, green, and blue
the horizon. Atmospheric dust
scaling ratios for color correction.
scatters green light more than red,
With the LR routine, eXcalibrator
and blue more than green.
only incorporates the green and red
Secondly, G2V calibration
values from the SDSS or APASS data
doesn't account for the sky's vari60
(most ﬁlters for amateur cameras
able transparency. As the night
block ultraviolet light). The program
progresses, thin clouds might come
then computes a color correction by
and go. This can compromise the
comparing the stars' colors in the
data for just one of the three color
image with their known color values
ﬁlters, throwing a meticulous G2V
from these databases.
calibration scheme out the win350 400 450 500 550 600 650 700 750
There are two ways to utilize
dow. Still, this technique at least
eXcalibrator. The ﬁrst is to colorheads you in the right direction.
balance your data. To start, you
Continued research led to the
ﬁrst need to plate-solve one of your
work of amateur Bernhard Hubl
calibrated and stacked monoand Mishca Schirmer of the Max
chrome FITS images shot through a
Planck Institute for Astronomy.
red, green, or blue ﬁlter, which can
They use stars from the Sloan
be accomplished with the Image
Digital Sky Survey (SDSS) database
Link function in TheSkyX (bisque.
as white-balance reference points.
They identiﬁed white (Sun-like)
com) or online at nova.astrometry.
stars in their images to determine
net . This adds World Coordinate
System (WCS) information to
This seemed like a reasonthe FITS header that the program
able solution. I liked combining a
will then utilize to ﬁnd suitable
G2V-like calibration method with
calibration stars in the image. The
p SPECTRAL VARIABLES While many outside factors
information from the image itself
program also works on one-shot
can contribute to the color of your deep-sky astrophotos,
none is as influential as the camera and filter you use to
to color-balance. Additionally,
color images that have been split
shoot through. The spectra above show the red, green, and
this technique uses catalog data
into their respective color channels
blue spectral response curve of a KAI 11002 CCD detector
acquired by a professional survey.
and saved as FITS ﬁles.
(top) and the passbands of AstroDon Gen2 color filters. The
And so, with help from Hubl and
Once one of the images has
bottom graph displays the spectral response of the human
Schirmer, eXcalibrator was born.
plate-solved, load the red,
eye. Each camera and filter combination will produce different exposure times to achieve a natural color balance.
With the help of amateur
green, and blue images into their
Bruce Waddington, the program
respective boxes and also select the
incorporates a linear regression (LR) routine to allow the use
plate-solved image in the WCS File box. The program then
of stars of any color as calibration sources that reference the
employs the FITS WCS data to determine the center of the
SDSS data. By obtaining nearly identical results, the LR routine
ﬁeld of view, image size, scale, and rotation. Click Calibrate
reinforces the white-star color-balancing concept. The proImage, and the program downloads SDSS or APASS data from
gram also incorporates the AAVSO Photometric All-Sky Survey
the VizieR Catalogue Service. EXcalibrator then identiﬁes stars
(APASS) data, fully automating color calibration in deep-sky
that should be white and calculates the color adjustment. To
images recorded with monochrome cameras and color ﬁlters.
obtain a larger sample, the LR routine also selects stars that
are yellow and cyan in hue. When completed, the program
presents you with the average weight of each color, which you
Putting eXcalibrator to Work
then use when combining them in your preferred imageContrary to Superman legend, the Sun is not a yellow star.
When viewed near the zenith or from space, the Sun looks