Astropho­tog­ra­pher Will Gater ex­plores the use of colour in astro imag­ing and of­fers ad­vice on how to strike the right balance when shoot­ing the stars

Sky at Night Magazine - - CONTENTS -

Ad­vice to help you keep a balance – and some sci­en­tific ve­rac­ity – when it comes to colour in your as­tropho­tog­ra­phy.

A strop ho tog rap hers face many chal­lenges when cap­tur­ing views of the night sky, but one el­e­ment of the craft presents a par­tic­u­lar col­lec­tion of hur­dles all its own: colour. Cap­tur­ing, pro­cess­ing and in­ter­pre­ta­tion of colour in as­tro­nom­i­cal im­agery is ar­guably the most com­plex, and some­times con­tro­ver­sial, facet of the sub­ject.

Colour deep-sky imag­ing us­ing fil­ters re­quires a skil­ful blend of art and sci­ence to pro­duce spec­tac­u­lar re­sults. But even when tak­ing ‘sim­pler’ wide-field and nightscape im­ages, a strop ho tog rap hers must tackle is­sues such as light pol­lu­tion that can dra­mat­i­cally af­fect what’s shown.

Com­bine all this with the fact that we don’t all see colours the same, that we will view im­ages via dif­fer­ent medi­ums and – per­haps most cru­cially – that ev­ery­one has their own opin­ions on what they like to see, and you can be­gin to see why colour in as­tropho­tog­ra­phy is of­ten a source of pas­sion­ate de­bate. And that’s be­fore you even con­sider that we’re talk­ing about pic­tures your eyes could never see!

In this ar­ti­cle we’ll look at some of the prac­ti­cal ways you can ap­proach the treat­ment of colour in your own as­tropho­tog­ra­phy. While there should al­ways be room for artis­tic li­cence, our ad­vice is based on the premise that we want to rep­re­sent ce­les­tial phe­nom­ena as ac­cu­rately as pos­si­ble. The challenge here is to pro­duce not just a cap­ti­vat­ing im­age but also one where we’ve ob­jec­tively as­sessed what the scene should ac­tu­ally look like.

Nowa­days, with pow­er­ful edit­ing soft­ware that can dras­ti­cally ma­nip­u­late a digital im­age file, it is in the post-pro­cess­ing of an as­trophoto where many colour choices are made. But the mo­ment of cap­ture – par­tic­u­larly in nightscape and star-trail pho­tog­ra­phy – will al­ways be a crucial mo­ment that hugely in­flu­ences the tone and the in­ter­play of dark and light in a shot.

Con­trast ver­sus de­tail

Imag­ine you’re shoot­ing a twi­light nightscape, in­clud­ing a bright planet, us­ing a DSLR. With a short ex­po­sure you might cap­ture a darker, high­er­con­trast scene, with rich twi­light colours but lit­tle fore­ground de­tail; per­haps just black sil­hou­ettes. Us­ing a long ex­po­sure the same view would look markedly dif­fer­ent: the sky would be brighter while the over­all con­trast would likely be lower, but you could pick up more de­tail in the shad­ows. You could even aim to blend two such ex­po­sures

“Use some ba­sic as­tro­physics to guide how you tweak colours in post pro­cess­ing”

in post-pro­cess­ing. What­ever the ap­proach, the point is that in the act of se­lect­ing the ex­po­sure for an im­age, you’re al­ready mak­ing an im­por­tant de­ci­sion that will af­fect the colours in the final pic­ture and – even if in­ad­ver­tently – the feel­ings it’ll con­vey and how it will di­rect the viewer’s eye. Of course, such aes­thetic con­sid­er­a­tions are only one side of the coin. Since you’re aim­ing for a faith­ful ren­di­tion of the view de­picted, you also need to think about the sci­ence of what’s shown; and it’s this that should ul­ti­mately pro­vide the guide for the pro­cess­ing and en­hance­ment of colour in any kind of as­tropho­tog­ra­phy, from plan­e­tary imag­ing to long-ex­po­sure, deep-sky work.

Take, for ex­am­ple, the bright band of the Milky Way, which is a cen­tral el­e­ment in count­less astro im­ages. Our Gal­axy’s core is com­posed of older, more red and yel­low stars, while the spi­ral arms in its disc are in­hab­ited by hot, young, bright, blue­white stars. So in long-ex­po­sure im­ages show­ing the Milky Way stretch­ing across the sky, we would ex­pect the star fields of the Gal­axy’s cen­tral bulge, in and around the con­stel­la­tions of Sagittariu­s and Ophi­uchus, to have more of a yel­low­ish-white colour to them; while fur­ther away from the core – where the disc of the Gal­axy tra­verses Cygnus and Cas­siopeia – the Milky Way’s star fields should show more blueish-white tones. Of­ten these rich star fields also show a hint of ochre and brown in places;

this is where their light is red­dened by in­ter­stel­lar dust and gas clouds, so don’t be put off if your Milky Way im­ages have some muddy brown colours – they are real!

The challenge of achiev­ing pleas­ing and ac­cu­rate star colours crops up wher­ever there are stars de­picted in an im­age, but it’s per­haps most keenly felt in long-ex­po­sure, deep-sky work. Here again, though, you can use some ba­sic as­tro­physics to guide how you tweak colours dur­ing post-pro­cess­ing.

The colours of stars

Pro­fes­sional astronomer­s group stars into ‘spec­tral types’, which tell us about the ap­pear­ance of key chem­i­cal ‘fin­ger­prints’ in their light as well as their tem­per­a­ture. It’s the dif­fer­ing tem­per­a­tures be­tween stars that cause the colour vari­a­tions we see. For ex­am­ple, a hot ‘O-type’ star like Min­taka in Orion’s Belt ap­pears blueish, while a cooler ‘K-type’ star, such as Alde­baran, will have an or­ange tint. Among the other spec­tral types there are also ‘F-type’ stars, which tend to ap­pear closer to white. These are of par­tic­u­lar use to as­tropho­tog­ra­phers as you can use a plan­e­tar­ium pro­gramme, like Stel­lar­ium or Starry Night, to find such a star in your im­age. Then dur­ing edit­ing and pro­cess­ing, check that you have it show­ing a neu­tral white-ish colour; this way you can be con­fi­dent that the over­all colour balance of your im­age is ac­cu­rate.

When ex­am­in­ing and pro­cess­ing colour in astro im­ages it’s of­ten easy to con­cen­trate on the main fea­tures, while for­get­ting about the back­ground; do so at your peril. Back­grounds present their own com­pli­ca­tions. The night sky is not per­fectly black so, in most cases, when tweak­ing the colour of an im­age you should aim for a neu­tral, ex­tremely dark

grey back­drop. How­ever, cer­tain deep-sky im­ages can be the ex­cep­tion, as they of­ten fea­ture re­gions of colour­ful ne­bu­los­ity across the whole frame.

Some­times, though, there will be strong colour casts or gra­di­ents within an im­age that af­fect the back­ground. Oc­ca­sion­ally these will come from nat­u­ral sources, so we’d ar­gue for pre­serv­ing them and mak­ing sure they’re faith­fully por­trayed. For ex­am­ple, you’d ex­pect colour im­ages taken in deep twi­light, or when a Full Moon is high up, to show a washed-out back­ground with a blue cast – and note that when a full Moon is ris­ing it ini­tially washes the sky out with a yel­low-grey light that changes to a whiter, bluer hue as the lu­nar disc gains al­ti­tude.

Sim­i­larly, nightscape pho­tog­ra­phers imag­ing from dark-sky lo­ca­tions of­ten cap­ture the at­mo­spheric phe­nom­e­non known as ‘air­glow’, which can cre­ate greeny-blue swathes of colour across an im­age. Air­glow can re­ally throw off the pro­cess­ing of an im­age if it’s not recog­nised as be­ing present in a shot, as the pho­tog­ra­pher des­per­ately tries to cor­rect the strong, green back­ground gra­di­ents.

Learn to love lev­els

How­ever, the most com­mon cause of back­ground gra­di­ents or colour casts is usu­ally light pol­lu­tion. Spe­cial­ist fil­ters can re­duce the ef­fects of this un­wanted in­tru­sion to a cer­tain ex­tent, but some – in do­ing so – im­part their own colour cast to a photo, which has to be dealt with in pro­cess­ing later.

How, then, do you go about cor­rect­ing and tweak­ing the colour of an astro im­age in im­age- edit­ing soft­ware? One of the ways to be­gin to cor­rect or ‘balance’ the colour within an as­trophoto is to use the ‘lev­els’ tool com­monly found in soft­ware like Pho­to­shop and GIMP. Usu­ally this fea­ture is used to in­crease the bright­ness and con­trast in an im­age, to pull out de­tail. How­ever by choos­ing to ad­just in­di­vid­ual colour chan­nels, one at a time, you can also vary the colour balance within a shot. To get a feel for how this al­ters the colours in an im­age, first

se­lect a colour chan­nel (ei­ther red, green or blue) from the drop down ‘chan­nel’ menu and then move the mid­dle ‘slider’ icon in the ‘in­put lev­els’ win­dow; you’ll see the colour of your im­age in the main win­dow change, and by fur­ther ex­per­i­ment­ing with the dif­fer­ent chan­nels and other slid­ers hope­fully you’ll get close to an im­age tone you’re happy with.

Both Pho­to­shop and GIMP also have an ad­vanced ‘Colour balance’ tool that en­ables you to al­ter the balance of colour in an im­age be­tween three pairs of colours: cyan and red; ma­genta and green; and yel­low and blue. Ad­just­ments made with this fea­ture can be ap­plied – in­de­pen­dently – to the shad­ows, mid­tones and high­lights of an im­age, mean­ing it’s in­cred­i­bly use­ful for pre­ci­sion colour cor­rec­tion.

Some im­age-edit­ing pro­grams also have a fea­ture that can be used to ad­just a pic­ture’s ‘tem­per­a­ture’; in other words, how yel­low or blue the tones in the final im­age will be. This tool is fre­quently used by aurora pho­tog­ra­phers – for aes­thetic rea­sons – to give an im­age a ‘cooler’ feel, with the nat­u­ral, green colour of the auro­ral light be­com­ing a more greeny­cyan hue. How far you are will­ing to push this is a mat­ter of per­sonal taste.

And that is, of course, true of ev­ery­thing we’ve dis­cussed in this ar­ti­cle: how far, and in which chro­matic di­rec­tion, you want to take your colour bal­anc­ing and cor­rect­ing is ul­ti­mately down to you, and will likely vary from im­age to im­age de­pend­ing on a mul­ti­tude of fac­tors. But, hope­fully, armed with the ideas we’ve looked at here you’ll have some fresh ideas and new per­spec­tives to con­sider the next time you’re shoot­ing and pro­cess­ing a ce­les­tial shot of your own.

The dif­fer­ent star colours in Auriga: bright nearly-white Capella (top right), or­angey K-type Has­saleh (bot­tom right) and the blueish B-type El­nath (bot­tom left)

In Pho­to­shop you can ad­just the lev­els for each of the red, green and blue chan­nels in­di­vid­u­ally

As­tropho­tog­ra­phers tend to ‘cool’ au­ro­rae shots by adding more cyan to their nat­u­ral green us­ing the tem­per­a­ture con­trols

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