Photometric Color Calibration
Warning
The calibration of the colors by photometry must imperatively be carried out on a linear image whose histogram was not yet stretched. Otherwise, the photometric measurements will be wrong and the colors obtained without guarantee of being correct.
Another way for retrieving the colors is to compare the color of the stars in the image with their color in catalogues, to obtain the most natural color in an automatic and non-subjective way. This is the PCC (Photometric Color Calibration) tool (Ctrl + Shift + P). It can only work for images taken with a set of red, green and blue filters for colors, or on-sensor color. To identify stars within the image with those of the catalogue, an astrometric solution is required. Please see the documentation of the plate solver module.
This method is less accurate than Spectrophotometric Color Calibration explained below, however it can be performed using local catalogues and is therefore the best option when an internet connection is not available.
Note
This technique is heavily dependent on the type of filter used. Using different kinds of R, G, B filters will not make a large difference, but using a Light pollution filter or no IR-block filters will make the solution deviate significantly and not give the expected colors.
Since version 1.4, the two tools run independently: it is possible to run the
photometric analysis and color correction of the image only if the image has
been already plate solved. It also means different catalogues can be used for
PCC and astrometry. The tool is also available as the pcc command, so it can
be embedded in image post-processing scripts.
Photometric Color Calibration dialog window.
If the image was previously plate solved, turn on the annotations feature to check that catalogues align with the image. If the astrometric solution is not good enough, please redo a plate solving.
The Catalog Settings section allows you to choose which photometric catalog should be used, NOMAD, APASS or Gaia DR3, as well as the limiting magnitude.
Tip
The NOMAD catalog can be locally installed, while the APASS and GAIA catalogs require internet access to obtain their contents.
The Star Detection section allows you to manually select which stars will be used for the photometry analysis. It's better to have hundreds of them at least, so individual picking would not be ideal.
If desired, the Background Reference can be manually selected as described in Manual Color Calibration. This can be useful in the case of nebula images where the background sky parts are small.
When enough stars are found and the astrometric solution is correct, the PCC will print this kind of text in the Console tab:
Applying aperture photometry to 433 stars.
70 stars excluded from the calculation
Distribution of errors: 1146 no error, 18 not in area, 48 inner radius too small, 4 pixel out of range
Found a solution for color calibration using 363 stars. Factors:
K0: 0.843 (deviation: 0.140)
K1: 1.000 (deviation: 0.050)
K2: 0.743 (deviation: 0.130)
The photometric color correction seems to have found an imprecise solution, consider correcting the image gradient first
We can understand that 433 stars were selected from the catalogue and the image for photometric analysis, but somehow, only 363 we actually used, 70 being excluded. The line Distribution of errors explains for what reason they were excluded: 18 were not found in the expected position, 48 were too big and 4 probably saturated. It is very common to have many stars rejected because they don't meet the strict requirements for a valid photometric analysis.
We can also see that the PCC found three coefficients to apply to the color channels to correct the white balance. The deviation here, which is the average absolute deviation of the color correction for each of the star of the photometric set, is moderately high. On well calibrated images without gradient, with correct filters and without a color nebula covering the whole image, devation would get closer to 0.
Siril command line
pcc [-limitmag=[+-]] [-catalog=] [-bgtol=lower,upper]