Calibrating astrophotos: lights, darks, flats and bias

Light Frames – The Actual Images of the Night Sky

Light frames are the actual exposures of a celestial object. They contain the light collected from stars, nebulae, galaxies and other astronomical targets.A deep-sky imaging session often produces several hundred light frames. Depending on the imaging software and camera settings, each exposure may last 10, 20 or 30 seconds, for example.The combined exposure time of all usable light frames determines the total integration time.

These individual exposures are particularly valuable because they can later be reprocessed in Siril or PixInsight.

Dark Frames – Dark Current and Sensor Defects

Dark frames are exposures taken without allowing any light to reach the camera sensor.They are used to record certain sensor-related disturbances, including dark current, hot pixels and other recurring patterns.Ideally, dark frames should be captured using the same exposure time and gain settings as the light frames, with the sensor temperature kept as similar as possible.During calibration, the corresponding unwanted signal components are mathematically subtracted from the actual astronomical exposures.Dark frames can therefore improve the quality of subsequent image processing. However, they do not eliminate the random photon noise produced when collecting light.

Flat Frames – Dust and Uneven Illumination

Flat frames correct differences in illumination across the camera’s field of view. These differences can be caused by vignetting, dust on optical surfaces or other characteristics of the optical system. To capture flat frames, an evenly illuminated surface is photographed. The resulting data helps compensate for brightness variations in the light frames. It is particularly important to keep the optical configuration as unchanged as possible between capturing the light frames and the flat frames.

Changing filters, modifying the optical setup or moving dust particles can make existing flat frames less suitable for accurate calibration.

Bias Frames – The Sensor’s Readout Offset

Bias frames are extremely short exposures taken without allowing any light to reach the camera sensor. They record the electronic offset and recurring components of the readout signal. Bias frames can play a role in calibrating dark frames and flat frames.

Whether separate bias frames are actually required depends on the camera system and the chosen calibration workflow. With some CMOS cameras and certain calibration methods, suitable dark frames or dark flats are used instead.

Seestar in Practice: Which Data Matters Most?

The Seestar S50 Pro already handles various imaging and processing steps through its integrated software. The exact calibration and dithering functions available depend on the device and its firmware. For more advanced image processing, the saved individual exposures are particularly valuable. Processing these files yourself in Siril or PixInsight gives you greater control over light-frame selection, alignment, stacking and subsequent image processing. However, it is important to consider whether the Seestar has already calibrated the saved exposures. Applying unsuitable calibration frames a second time can actually make the result worse.

For Foto-Gorilla, we will therefore distinguish clearly between image acquisition, calibration, stacking and subsequent image processing in our practical tutorials. This creates a transparent workflow in which every processing step has a clearly defined purpose.