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Sky at Night Magazine - - CONTENTS - With Steve Richards

Cor­rect ex­po­sure time is de­pen­dent on many fac­tors, in­clud­ing the cho­sen ob­ject, te­le­scope aper­ture, te­le­scope fo­cal length, mount track­ing abil­ity, cam­era type and light pol­lu­tion. As you would ex­pect, brighter ob­jects re­quire shorter ex­po­sures and dim­mer ob­jects re­quire longer ex­po­sures to cap­ture de­tail, but also con­sider your cam­era’s sen­si­tiv­ity. DSLRs and one-shot-colour cam­eras are less sen­si­tive than mono­chrome CCD cam­eras.

Most deep-sky im­ages are pro­duced from mul­ti­ple im­ages that have been stacked to pro­duce a sin­gle im­age, as this is a great way of in­creas­ing the sig­nal-to-noise ra­tio in your im­ages. Lots of shorter im­ages stacked to­gether go some way to­wards match­ing a sin­gle ex­po­sure of the same to­tal time, but the sin­gle ex­po­sure will be deeper and con­tain more de­tail. How­ever, the stack of shorter ex­po­sures will be less noisy so the aim is to take the long­est ex­po­sures you can and stack as many as pos­si­ble to get the best of both worlds.

Light pol­lu­tion places a ceil­ing on ex­po­sure length as this can pro­duce ‘fog­ging’, where its or­ange glow over­whelms the im­age. It is im­por­tant to keep ex­po­sures be­low this thresh­old, which re­quires some trial and er­ror. For star clus­ters and glob­u­lar clus­ters, 120-180 sec­onds is a good start. For gal­ax­ies and neb­u­lae, 300-600 sec­onds is a good base if your track­ing will al­low it. Do re­mem­ber that these are only rough guides.

Cap­tur­ing a deep-sky im­age like this is down to a mul­ti­tude of fac­tors – light pol­lu­tion is only one

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