1. Bernhard Hubl CEDIC 2009 Colour Calibration of RGB Images Advantages and disadvantages of different colour calibration methodes

2. Contents  Field Stars Calibration  G2 Star Calibration Procedure Disadvantages  B-V Calibration Workflow Experiences  Alternative Methodes  Summary

3. Why Colour Calibration?   Atmosphere Zenith distance Transparency   Telescope   Filters   Camera

4. White Balance  Average of field stars = white  Sunlight = white Stars with sun-like spectral type (G2) Stars with sun-like photometry (B-V)

5. Contents

6. Field Stars Calibration  Assumption Average colour of bright field stars is white  Advantages Easy (e.g. AIP4Win) Independent of atmospheric extinction Works fine for galaxy fields  Disadvantages Open star clusters Interstellar extinction Bright nebulae

7. Contents

8. Extinction

10. Workflow G2 Star Calibration Measurement of bright G2 star RGB weights in zenith Exposure of object in specific altitude Calculate extinction Corrected RGB- weights

11. When does the G2 calibration fail?  No measurement of a G2 star  Very low altitude of the object  Bad transparency (clouds, fog,..) Measurement of bright G2 star RGB weights in zenith Exposure of object in specific altitude Calculate extinction Corrected RGB- weights

12. Object exposures for G2 star calibration  Idea: Search for not saturated G2 stars  Two problems: Spectral type of faint stars is often unknown Reddening of G2 strars by interstellar dust.

13. Contents

14. B-V Calibration  Idea: Peter Riepe & Harald Tomsik VdS Journal 25+26  Calibration uses the object image  Instead of spectral type -> photometry Comprehensive photometric data of faint stars Johnson-Cousins UBVRI filter system.

15. UBVRI Filter   U – Ultraviolet   B – Blue   V – Visual (Green)   R – Red   I – Infrared

16. Colour Index  = difference of the brightness measured through two different filters  e.g: B-V, V-R Sp. typeColourB-V [mag] SpicaB1Blue-0.23 DenebA2Blue-White+0.09 SunG2White+0.65 BetelgeuseM0Orange-Red+1.50

17. HR Diagram

19. Idea  Colour index of a G2 star without interstellar extinction: B-V=0.65 mag V-R=0.36 mag  Search for white stars inside the image field: Not saturated 0.6 mag < B-V < 0.7 mag 0.2 mag < V-R < 0.6 mag  Measurement of the brightness of selected white stars in Red, Green and Blue.

20. B-V Workflow  Preprocessing Dark, Flat Registration Averaged R, G and B image  Aladin B-V steps 1.Load DSS image 2.Load photometric catalogue 3.Load filter 4.Measure brightness of selected stars

21. Step1: Load DSS image DSS = Digitized Sky Survey Aladin: http://aladin.u-strasbg.fr/aladin.gml

22. Step1: DSS

23. Step2: Load Photometric Catalogue NOMAD1 = Naval Observatory Merged Astrometric Dataset Advantage: 100% sky coverage Disadvantage: Accuracy

24. Step3: Load Filter Select white stars 0.6 < B-V < 0.7 0.2 < V-R < 0.6 Save Wolfgang Piracher‘s filter on your local disk

25. Step3: Filter

27. Aladin - Filter

28. Step4: Measurement  Aperture photometry of standard image processing software (e.g. AstroArt)  Result: 3 ADU values (R,G and B)  Example: Red 48298 / Green 56917 / Blue 51635 -> Green and Blue are too bright -> Red 1,00 / Green 0,85 / Blue 0,94.

29. Experiences with B-V  Methode is laborious  NOMAD1 catalogue Inhomogeneous Is based partly on plate scans The fainter the reference stars, the more unreliable  -> at least 5 stars are necessary (outliers)  The blue weight is often too high  Average between G2 and B-V.

30. Contents

31. Alternative Methodes  Regim by Andreas Rörig Up to 5 stars; manual selection  Theli by Mischa Schirmer LINUX software Astrometric registration Fully automated B-V methode: Usage of a big number of stars, which are found by the software (Vizier data base).  SDSS Much better photometric data Filter system ugriz (u-g=1.43mag; g-r=0.44mag) At the moment far away from full sky coverage.

32. Summary  New catalogues In several years much better catalogues compared with NOMAD1  Fully automated B-V methode is possible E.g. Theli  B-V methode as standard The B-V calibration works in all thinkable situations (bad transparency, Milky Way fields,..), provided that the used catalogue is enough accurate.  Best procedure at the moment: Combination of G2 and B-V

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34. Thank you very much for your attention!