What is a dark frame in astrophotography?

BBC Sky at Night Magazine's Scope Doctor Steve Richards solves your practical astronomy ailments.

The Rosette Nebula in hydrogen-alpha, before dark frame calibration (left) and with reduced thermal noise after it has been applied (right). Credit: Steve Richards

What is a dark frame in astrophotography?

Steve says: “During the long exposures required for many deep-sky images, the sensor warms up, generating ‘thermal noise’ in the images. This noise tricks the sensor into believing that it has received some additional photons.

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Among other unwanted artefacts, this can result in a peppering of white pixels across the image looking rather like tiny stars.

Dark frames are images taken with the telescope capped, using exposures of the same length and camera temperature as your main images.

They contain just the thermal noise as no actual light has reached the sensor. Subtracting these special images from your normal images using software like DeekSkyStacker or Photoshop removes the sensor noise.”

Read our complete guide to using dark frames.

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Email your astronomy queries to Steve at scopedoctor@skyatnightmagazine.com and they could get answered in a future issue of BBC Sky at Night Magazine.