White Balance Average of field stars = white Sunlight = white Stars with sun-like spectral type (G2) Stars with sun-like photometry (B-V)
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
Workflow G2 Star Calibration Measurement of bright G2 star RGB weights in zenith Exposure of object in specific altitude Calculate extinction Corrected RGB- weights
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
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.
B-V Calibration Idea: Peter Riepe & Harald Tomsik VdS Journal Calibration uses the object image Instead of spectral type -> photometry Comprehensive photometric data of faint stars Johnson-Cousins UBVRI filter system.
UBVRI Filter U – Ultraviolet B – Blue V – Visual (Green) R – Red I – Infrared
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
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.
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
Step3: Load Filter Select white stars 0.6 < B-V < < V-R < 0.6 Save Wolfgang Piracher‘s filter on your local disk
Aladin - Filter
Step4: Measurement Aperture photometry of standard image processing software (e.g. AstroArt) Result: 3 ADU values (R,G and B) Example: Red / Green / Blue > Green and Blue are too bright -> Red 1,00 / Green 0,85 / Blue 0,94.
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.
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.
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