Comparison of different colour calibration methods


The above image of M5 shows the result of four different colour calibration methods. The images are very well comparable, because I did the same simple processing steps for all images:

  1. Blur of R and G to get a similar FWHM value for R, G and B

  2. Multiplication of R, G and B with the colour weights

  3. Neutral Background

  4. DDP

  5. Increase of colour saturation by 50%


The top left image uses the original exposure ratio of R:G:B=1:1:1.

The top right image is a result of a G2 star calibration.

The bottom left image is a result of a B-V calibration using 5 NOMAD1 stars.

The bottom right image is a result of a u-g calibration using 4 SDSS stars.


Description of the used colour calibration methods:


The unmodified exposure ratio gives a greenish colour. The G2 star calibration did not work very well, because of a not perfect transparency. The B-V calibration is too blue. This can often be observed using the not very good photometry of NOMAD1 stars. The best result can be achieved with u-g using SDSS data. This is the preferred method, whenever SDSS data are available.