Australian Sky & Telescope - June 2018

64 AUSTRALIAN SKY & TELESCOPE July 2018

anyG2V stars in the field.

Usingthissingularcalibration

methodstillhasproblems.First,

asmentionedearlier,thereisno

correction for the altitude of the

target as it is imaged throughout

anightoroverseveralnights.It’s

particularly problematic when the

target falls below about 40° above

thehorizon.Atmosphericdust

scatters green light more than red,

andbluemorethangreen.

Secondly,G2V calibration

doesn’taccountforthesky’s

variabletransparency.Asthenight

progresses,thincloudsmightcome

andgo.Thiscancompromisethe

dataforjustoneofthethreecolour

filters,throwingameticulousG2V

calibrationschemeoutthewindow.

Still, this technique at least heads

youintherightdirection.

Continued research led to the

work of amateur Bernhard Hubl

andMishcaSchirmeroftheMax

Planck Institute for Astronomy.

TheyusestarsfromtheSloan

Digital Sky Survey (SDSS) database

as white-balance reference points.

They identified white (Sun-like)

starsintheirimagestodetermine

colour correction.

This seemed like a reasonable

solution. I liked combining a

G2V-like calibration method with

informationfromtheimageitself

to colour-balance. Additionally,

thistechniqueusescataloguedata

acquiredbyaprofessionalsurvey.

And so, with help from Hubl and

Schirmer,eXcalibratorwas born.

Withthehelpofamateur

Bruce Waddington, the program

incorporates a linear regression (LR) routine to allow the use

of stars of any colour as calibration sources that reference

the SDSS data. By obtaining nearly identical results, the LR

routine reinforces the white-star colour-balancing concept.

The program also incorporates the AAVSO Photometric All-

Sky Survey (APASS) data, fully automating colour calibration

in deep sky images recorded with monochrome cameras and

colour filters.

Putting eXcalibrator to work

Contrary to Superman legend, the Sun is not a yellow star.

When viewed near the zenith or

from space, the Sun looks and

photographs white. Measuring the

colour balance of the Sun with

SDSS’s u- (ultraviolet through

blue), g- and r-filter values yields

(u–g) = 1.43 and (g–r) = 0.44. So

eXcalibrator searches an image for

stars with SDSS (u–g) and (g–r)

values similar to the Sun’s. The

program uses the average values of

the identified stars to determine

the proper red, green and blue

scaling ratios for colour correction.

With the LR routine, eXcalibrator

only incorporates the green and red

values from the SDSS or APASS data

(most filters for amateur cameras

block ultraviolet light). The program

then computes a colour correction

by comparing the stars’ colours in

the image with their known colour

values from these databases.

There are two ways to use

eXcalibrator. The first is to colour-

balance your data. To start, you

first need to plate-solve one

of your calibrated and stacked

monochrome FITS images shot

through a red, green or blue

filter, which can be accomplished

with the Image Link function in

TheSkyX (bisque.com) or online

at nova.astrometry.net. This adds

World Coordinate System (WCS)

information to the FITS header

that the program will then use

to find suitable calibration stars

in the image. The program also

works on one-shot colour images

that have been split into their

respective colour channels and

saved as FITS files.

Once one of the images has been plate-solved, load the

red, green and blue images into their respective boxes and

also select the plate-solved image in the WCS File box. The

program then employs the FITS WCS data to determine the

centre of the field of view, image size, scale and rotation.

Click Calibrate Image, and the program downloads SDSS or

APASS data from the VizieR Catalogue Service. EXcalibrator

then identifies stars that should be white and calculates the

colour adjustment. To 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 GREGG DINDERMAN /

S&T

(3)

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100%

0

500

Wavelength (nanometers)

Transmittance

600 700

400

1.0

0.8

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Wavelength (nanometers)

Relative Visual Sensitivity

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Quantum Eficiency

600 700 800 900 1,000

S SPECTRAL VARIABLES While many outside factors

can contribute to the colour of your deep sky astrophotos,

none is as inluential as the camera and ilter you use to

shoot through. The spectra above show the red, green and

blue spectral response curve of a KAI 11002 CCD detector

(top) and the passbands of AstroDon Gen2 colour ilters.

The bottom graph displays the spectral response of the

human eye. Each camera and ilter combination will produce

different exposure times to achieve a natural colour balance.

IMAGE PROCESSING