Colour Calibration of RGB Images Advantages and disadvantages

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

Contents u u Field Stars Calibration G 2 Star Calibration • Procedure • Disadvantages u B-V Calibration • Workflow • Experiences u u Alternative Methodes Summary

Why Colour Calibration? u Atmosphere • Zenith distance • Transparency u u u Telescope Filters Camera

White Balance u u Average of field stars = white Sunlight = white • Stars with sun-like spectral type (G 2) • Stars with sun-like photometry (B-V)

Contents u u Field Stars Calibration G 2 Star Calibration • Procedure • Disadvantages u B-V Calibration • Workflow • Experiences u u Alternative Methodes Summary

Field Stars Calibration u Assumption • Average colour of bright field stars is white u Advantages • Easy (e. g. AIP 4 Win) • Independent of atmospheric extinction • Works fine for galaxy fields u Disadvantages • Open star clusters • Interstellar extinction • Bright nebulae

Contents u u Field Stars Calibration G 2 Star Calibration • Procedure • Disadvantages u B-V Calibration • Workflow • Experiences u u Alternative Methodes Summary

Extinction

Workflow G 2 Star Calibration Measurement of bright G 2 star Exposure of object in specific altitude Calculate extinction RGB weights in zenith Corrected RGBweights

When does the G 2 calibration fail? Measurement of bright G 2 star Exposure of object in specific altitude Calculate extinction RGB weights in zenith u u u No measurement of a G 2 star Very low altitude of the object Bad transparency (clouds, fog, . . ) Corrected RGBweights

Object exposures for G 2 star calibration u u Idea: Search for not saturated G 2 stars Two problems: • Spectral type of faint stars is often unknown • Reddening of G 2 strars by interstellar dust.

Contents u u Field Stars Calibration G 2 Star Calibration • Procedure • Disadvantages u B-V Calibration • Workflow • Experiences u u Alternative Methodes Summary

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

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

Colour Index u u = difference of the brightness measured through two different filters e. g: B-V, V-R Sp. type Colour B-V [mag] Spica B 1 Blue -0. 23 Deneb A 2 Blue-White +0. 09 Sun G 2 White +0. 65 Betelgeuse M 0 Orange-Red +1. 50

HR Diagram

Idea u Colour index of a G 2 star without interstellar extinction: • B-V=0. 65 mag • V-R=0. 36 mag u 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 u Measurement of the brightness of selected white stars in Red, Green and Blue.

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

Step 1: Load DSS image DSS = Digitized Sky Survey Aladin: http: //aladin. u-strasbg. fr/aladin. gml

Step 1: DSS

Step 2: Load Photometric Catalogue NOMAD 1 = Naval Observatory Merged Astrometric Dataset Advantage: 100% sky coverage Disadvantage: Accuracy

Step 3: 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

Step 3: Filter

Aladin - Filter

Step 4: Measurement u u u Aperture photometry of standard image processing software (e. g. Astro. Art) 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.

Experiences with B-V u u Methode is laborious NOMAD 1 catalogue • Inhomogeneous • Is based partly on plate scans • The fainter the reference stars, the more unreliable u u u -> at least 5 stars are necessary (outliers) The blue weight is often too high Average between G 2 and B-V.

Contents u u Field Stars Calibration G 2 Star Calibration • Procedure • Disadvantages u B-V Calibration • Workflow • Experiences u u Alternative Methodes Summary

Alternative Methodes u Regim by Andreas Rörig • Up to 5 stars; manual selection u 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). u SDSS • Much better photometric data • Filter system ugriz (u-g=1. 43 mag; g-r=0. 44 mag) • At the moment far away from full sky coverage.

Summary u New catalogues • In several years much better catalogues compared with NOMAD 1 u Fully automated B-V methode is possible • E. g. Theli u 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. u Best procedure at the moment: • Combination of G 2 and B-V

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

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