Australian Sky & Telescope - May 2018

62 AUSTRALIAN SKY & TELESCOPE May | June 2018

SLIGHT TRAILS: SEAN WALKER /

S&T;

ADAM BLOCK / MOUNT LEMMON SKYCENTER / UNIVERSITY OF ARIZONA

photos should be corrected, with the exception of reddish or

greenishnaturalairglow.Likewise,ahigh-resolutionimageof

aspiralgalaxyshouldstillallowyoureyetodeterminewhere

thegalaxyendsandtheskybackgroundbegins.Determining

this boundary in a sprawling nebulous field can be tricky but

still manageable.

Even tri-colour narrowband images of deep sky objects,

whichrelyonvariousfalse-colourpalettesorcombinations,

stillrequireapleasingcolourbalanceinordertopresentthe

variedhuesthatoccurwhenionisedelementsinteract.Ifwe

were to be truly accurate, most tri-colour narrowband images

wouldbedominatedbythecolourthathydrogen-alphais

assignedtoinanimage(oftengreen),becauseitisthemost

pervasive element in the universe, compared to sulfur II or

oxygenIII.Buttobestdisplaythecolourfulinteractionsof

theseelements,imagersequalisetheexposuresinanimage.

Planetaryphotographyalsobenefitsfromapleasingcolour

balance.Forinstance,whileMarsisatawny-orangecolour

(thoughnotsaturated),blueandwhiteareoftenseeninits

thincloudsneartheplanetslimb,anditsoccasionaldust

stormshaveadistinctiveyellowhue.Arelatedaspectof

colour iscontrast.Astrophotosaretrickytoestablishproper

contrastin,becausemosttargetsareseenagainstadark

sky. You’d think that simply making the background black

wouldbetheeasieststep,butactuallythat’snotthecase.For

onething,thereisnotruly‘black’skyasseenfromEarth’s

surface. There are very few places where some artificial light

pollutionisn’tvisiblesomewhereinthesky.Butourplanet’s

atmosphere itself emits a small amount of light calledairglow.

This is commonly seen in nightscape photos taken from

darksites,thoughairglowcansometimesbeverynoticeable

SNOT BLACKEstablishing good contrast in a deep sky photo doesn’t

simplymeanmakingtheskyblackandthestarswhite.Settingthe

backgroundskytoblackwouldhavelostallofthebeautifuldustinthis

exquisite image of NGC 7497.

IMAGING AESTHETICS

even from rural locations. I’ve recorded skyglow in backyard

nightscape images taken just a few kilometres from my

nearest large city.

Given these caveats, deep sky and nightscape imagers

should strive to establish a dark, neutral background sky, but

it shouldn’t be completely black, so that faint nebulosity or

the outer extent of spiral galaxies will still be visible in your

image. Setting the background to pure black during image

processing (known as clipping) often erases these subtle

features from an otherwise excellent image.

The only exceptions to this ‘dark but not black sky’

rule in astronomical photography would be lunar and

planetary images. Shadows in lunar craters really will be

black, particularly since the Moon lacks any appreciable

atmosphere to scatter light. Planet images generally appear to

be surrounded by inky black skies because of the high focal

lengths required to magnify the target enough to resolve

details; planetary imagers often shoot at focal ratios of

f/20 or more, which translates into highly magnified views,

reducing the background sky brightness.

This brings us to saturation. While we all marvel at the

colourful images of galaxies, nebulae, and star clusters in the

pages of this magazine and elsewhere, the colours of most of

these objects are not the blazing magentas or electric blues

that are often depicted. The colours of deep sky objects are

actually more pastel and subdued (watch for an article on

this subject in an upcoming issue). The most colourful things

we can see through a typical telescope are the stars, planets

and the brightest planetary nebulae.

We are all guilty of pumping up the saturation for

dramatic effect. That doesn’t make it wrong, but there’s a

S SLIGHT TRAILS While deep sky pictures require perfectly tracked

images that produce round stars, nightscape photos aren’t as

stringent. This wide-angle, 30-second photo looks quite nice even with

slightly trailed stars.