Australian Sky Telescope MayJune 2017

64 AUSTRALIAN SKY & TELESCOPE May | June 2017

adjustthislater,too.Also,ifyou’veacquiredafewimagesof

thestarfieldwithbroadbandred,greenandbluefilters,you

canreplacethenarrowbandstarswiththenaturallycoloured

ones. Before leaving SHO-AIP and addressing such matters,

let’slookatanotherabilityofthescript.

Synthetic luminance

Youmightalreadybefamiliarwiththeconceptofanimage

called ‘pseudo luminance,’ also known assynthetic luminance.

Thisisagreyscaleversioncreatedfromthelightness

properties of a colour image. Comprised of the combined

signalofallthreecolourchannels,asyntheticluminanceis

usedtocomplementthecolourimage,muchinthewaythata

trueluminanceaddscontrastanddetailtoanRGBimage.The

SHO-AIP script can produce a synthetic luminance from the

colourimage,andyoucandosoimmediatelyaftercreating

thecolourimagebyfirstclickingtheNewPicturebutton.

Toproceed,expandtheMixingLuminancesection.Here

youcanorderthemastersofSII,Hα,OIII,butthesequence

doesn’tmattersincethepercentageslidercontrolsatright

controlthemix.Ifyouwanttocreateasyntheticluminance

comprised of all three master channels, click ‘Create L With

master1+2+3’.Or,ifonlytwochannelscontainthemajority

ofthedesiredsignal,choose‘CreateLwithmaster1+2’.The

blend method can be either Lighten, or possibly Screen when

abrighterresultisdesired.Onceyourluminanceisgenerated,

clickOKatthebottomright.Whilethescriptisalsocapable

of combining the synthetic luminance and colour image, this

is best done later after both have been non-linearly stretched.

From this point, linear post-processing of the image

mightinvolveusingtheBackgroundModelisationprocessto

eliminateunevenilluminationacrossthefield.Youcanalso

usedeconvolutiontosharpendetailandtightenthestars,and

linearnoisereductionbeforeanonlinearstretchisapplied.

Earlier,wenotedthegreenbiasofyourimage.While

PixInsightoffers several tools for adjusting colour, another

script is particularly useful for this task. The ColourMask

scriptbyAustralia’sRickStevensonisnothingshortof

terrific.AlthoughnotyetincludedinPixInsight’s st andard

release,youcandownloadandinstallitfromthePIForum

(tinyurl.com/zfkuzgz). Select your colour image and open

SCORRECTED COLOURThe ColourMask script helps to correct the

green bias that’s typical in a colour narrowband image.

WSTAR MASKS

Using the

StarMask

process enables

you to isolate

the stars in your

image (far left) to

correct magenta

halos often

seen in colour

narrowband

images.

ASTROPHOTOGRAPHY