Comparison of different colour calibration methods |
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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:
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: Interpretation: 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.
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