How to take calibration frames

Stop noise, dust and vignette messing up your DSLR astro images

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If you're an astrophotographer dedicating a lot of time to capturing deep-sky images, you'll want to ensure you're getting the best results from that hard-earned data. But that's not just down to the 'light' frames of your deep-sky target. It's also about collecting calibration frames.

Calibration frames help to remove any artefacts introduced by your camera and/or telescope, often in the form of noise or vignetting. By capturing a set of images that only show these artefacts, you can effectively 'subtract' these from your image during the stacking process.

The three core calibration frames are bias (or offset), dark and flat frames, and each captures a particular type of artefact. You should take fresh calibration frames for every new target you capture. Bias and dark are the easiest and require no special equipment or skills. Both help to reduce digital noise in an astrophoto, allowing you to produce cleaner results that require less work to process.

Bias, darks and flats

The purpose of bias frames is to pick up the electronic noise introduced when a sub-frame is read from the camera sensor. Bias frames are taken with the dust cap or camera lens cover fitted, and should appear completely black and show very little to no noise, especially from newer-model cameras.