Australian Sky & Telescope - June 2018

http://www.skyandtelescope.com.au 63

SUN: SOLAR DYNAMICS OBSERVATORY / NASA

Anothervariableisthecombinationofred,greenandblue

filtersusedinalldigitalcamerastomakethecolourresult.

Whetheryou’reusingamonochromeorone-shotcolour

camera,thesefiltersvaryintheirtransmissioncurvesand

cutoffs.Insomecases,thered,greenandbluepassbands

overlap. Other filter sets have distinct transmission gaps

intended to reduce the effects of light pollution.

Additionally,severalvariablesbeyondyourequipmentcan

affect the colour of your images. These includeatmospheric

extinctionandtransparency.Theloweryourtargetisinthesky,

themoreatmosphereitslighttravelsthrough,whichblocks

an increasing amount of blue light. Hazy skies also block bluer

wavelengthsmorethanred,skewingthecolourinyourresult.

Othereffectsareduetoextraterrestrialfactorssuchasdustin

ourgalaxy,andevenintergalacticgasanddustbetweenyour

targetandyourcamera.Finally,inconsistentimageprocessing

choices, such as the normalisation (the equalisation of

individual exposures) before stacking, can skew the result.

Colour correcting

Astrophotographersoftenrelyonseveralmethodstocorrect

thecolourbalance.Somearebetterthanothers.Often,an

imager will look online to compare their image with those

ofothers.Thisisperhapstheleastreliable way of achieving

accuratedeepskycolour!DoawebsearchforM31,andyou’ll

bepresentedwithdozensofimagesofthisgalaxy,somebluer,

othersreddish.‘Eyeballing’thecolourinyourimagesthisway

mightproduceaprettyresult,butitisn’tveryaccurate.

Onereasonablewaytocolourbalanceagalaxyimageisto

assumethattheintegratedlightofaface-onspiralgalaxyis

white.Thisapproachshowsagalaxywithitsintrinsiccolour.

However,manygalaxies,forexample,IC342,areseenthrough

interveningdustwithintheMilkyWay,whichreddensits

overallappearance.Sousingthisintrinsiccolourassumption

TDUSTY VEIL One common technique used to colour-balance galaxy photos is to assume their integrated light should be white. But some

galaxies, such as the face-on spiral galaxy IC 342 seen below, are viewed through dust within the Milky Way. The integrated light balance

technique (left) produces a nicely coloured galaxy image. Using G2V-like stars as calibration sources in eXcalibrator results in an image of the

galaxy reddened by dust, which blocks bluer wavelengths (right).

for IC 342 makes the foreground

stars too blue.

Another common technique

is to set the background colour

in your image to a neutral

grey. This works well for some

images, though not if the

field is filled with emission

nebulosity or dust.

Some imagers even use the

cumulative light of all the stars

in a picture as a white-point

average. This does work with

some objects, if there is no

intervening galactic extinction.

For instance, the core of a

globular cluster often makes a good white-point reference.

Of all the techniques mentioned, using the colour of stars

in your images is a step in the right direction to achieve

reasonably good colour balance. But star colours vary greatly,

depending on which direction you look. Stars in the arms of

our Milky Way tend to be young and therefore blue. Looking

to the galaxy’s halo and bulge, you see more reddish stars. In

fact, the general stellar population is mostly comprised of red

dwarfs, so the true average colour of stars skews toward the

red end of the spectrum.

Solar analogue

Many amateurs, in their pursuit of an accurate colour

calibration method, rely on a technique that uses G2V

spectral-class stars as a white-balance reference. Our Sun

is a G2V star, and its light appears white to our eyes. Using

this approach, you adjust the red, green and blue exposures

so that the G2V stars in an image appear white — if there are

SWHITE STAR While our Sun

is informally referred to as a

yellow dwarf, it is really a white

G-type main-sequence star

(G2V). Amateur astronomers

use other solar analogue G2V

stars as reliable white-point

calibration targets.