US­ING CCD CAM­ERAS

Sky at Night Magazine - - THE DAWN MISSION -

Many of the most spec­tac­u­lar deep-sky im­ages of galax­ies, neb­u­lae and star clus­ters are taken not with one-shot colour cam­eras such as DSLRs, but with spe­cially cooled as­tro­nom­i­cal ‘CCD’ (charged-cou­ple de­vice) cam­eras with mono­chrome sen­sors. These pro­duce greyscale RAW im­ages, and in or­der to cre­ate vi­brant full-colour im­ages, as­tropho­tog­ra­phers need to use a set of fil­ters on their lenses. For ex­am­ple, to cap­ture a scene in the kind of pal­ette most peo­ple see in – where trees are green, the sky is blue, straw­ber­ries are red – the im­ager must cap­ture ex­po­sures through red, green and blue fil­ters and later com­bine this data into a full-colour – some­times called ‘true’ colour – ‘RGB’ im­age in im­age-edit­ing soft­ware. Like all astro im­ages, these pic­tures still need to be colour bal­anced care­fully as some­times fil­ters can let in more or less light of cer­tain colours, or the camera sen­sor it­self might be more sensitive to some colours.

Many high-end as­tro­nom­i­cal im­ages­tack­ing and pro­cess­ing pro­grams con­tain au­to­matic colour bal­anc­ing fea­tures or tools to re­move or neu­tralise back­ground colour casts. These are usu­ally ex­cel­lent, though can some­times pro­duce an ‘off’ re­sult, so do take time to look care­fully at your final im­age’s colour; make man­ual ad­just­ments if nec­es­sary and use any preview func­tion available to move back and forth be­tween it­er­a­tions of the im­age as you tweak the colour.

CCD cam­eras cap­ture in mono­chrome, but by us­ing red, green and blue fil­ters and com­bin­ing the re­sult­ing shots, colour im­ages are cre­ated

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