Colour Calibration of RGB Images

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

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

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

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

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

8. Extinction

9. Extinction

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

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

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. Field Stars Calibration G2 Star Calibration Procedure Disadvantages B-V Calibration Workflow Experiences Alternative Methodes Summary 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. U – Ultraviolet B – Blue V – Visual (Green) R – Red I – Infrared UBVRI Filter

16. Colour Index • = difference of the brightness measured through two different filters • e.g: B-V, V-R

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 • Load DSS image • Load photometric catalogue • Load filter • 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

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

33. CCD-DVD Observatory Gahberg • 25 photographers • 2200 images • 780 different objects • price 14.-

34. Thank you very much for your attention!