The step that corrects the light frames with darks, flats and bias frames before stacking. It removes hot pixels, vignetting and dust shadows. Done well, it makes processing the image much easier.
Related terms
- Light frame
An image of the object itself, straight from the camera. It holds the signal, but also thermal noise, vignetting, dust shadows and the sensor’s electronic offset. Calibration frames (darks, flats, bias) are used to correct these flaws.
- Dark frame
An exposure taken with the shutter closed or the tube capped, with the same duration, gain and temperature as the lights. It records thermal signal and hot pixels, which software then subtracts. Usually 20 to 50 are taken.
- Flat frame
An exposure of an evenly lit surface (light panel, twilight sky) taken without touching focus or camera orientation. It records vignetting and dust shadows, which software corrects by dividing the lights. Flats are essential for a smooth background.
- Bias frame
The shortest possible exposure, shutter closed. It measures the sensor’s electronic baseline, present in every image even without light. It is subtracted in particular from the flats.
- Stacking
Adding or averaging many images of the same object, aligned on the stars, to reduce noise. Stacking 16 frames improves the signal-to-noise ratio by a factor of 4. Software such as Siril or DeepSkyStacker does the job.
← The whole glossary