Flat
ConceptFlat⚓
An optical instrument does not illuminate the sensor evenly. In addition, dust on the sensor can lead to the appearance of spots on the images. To correct these effects, we need to divide each image by a masterFlat. The masterFlat is obtained from the median (or average with deviant pixels rejected) combination of images of a uniform light area (usually the background sky at dusk or dawn), taken with a short exposure time
Note
Remember that each flat image is always composed of the true flat signal and the bias signal. You must then subtract the masterBias from each flat to get the true masterFlat. In addition, if the exposure time of the flats is too long, they also contain a darkFlat signal that must be removed.
Warning
Siril automatically evaluates a normalization coefficient which is used during the division of the master FLAT. However, this value can be set manually.
Advice
The masterFlat should be created by stacking the flats with the median algorithm (or Winsorized, checking that the rejection rate does not exceed 0.5%), and choosing Multiplicative Normalization
It is calculated from 9 images with the Winsorized Sigma Clipping stack.
Note
Note that the vignetting and dusting are strongly visible. You can also see the typical Bayer matrix structure.