Bernhard Hubl CEDIC 2009

Colour Calibration of RGB Images

Advantages and disadvantages of different colour calibration methodes

Contents

Field Stars Calibration G2 Star Calibration

• Procedure• Disadvantages

B-V Calibration• Workflow• Experiences

Alternative Methodes Summary

Why Colour Calibration?

Atmosphere• Zenith distance• Transparency

Telescope Filters Camera

White Balance

Average of field stars = white Sunlight = white

• Stars with sun-like spectral type (G2)• Stars with sun-like photometry (B-V)

Contents

Field Stars Calibration G2 Star CalibrationG2 Star Calibration

• ProcedureProcedure• DisadvantagesDisadvantages

B-V CalibrationB-V Calibration• WorkflowWorkflow• ExperiencesExperiences

Alternative MethodesAlternative Methodes SummarySummary

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

Contents

Field Stars CalibrationField Stars Calibration G2 Star Calibration

• ProcedureProcedure• DisadvantagesDisadvantages

B-V CalibrationB-V Calibration• WorkflowWorkflow• ExperiencesExperiences

Alternative MethodesAlternative Methodes SummarySummary

Extinction

Extinction

Workflow G2 Star Calibration

Measurement of bright G2 star

RGB weightsin 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 weightsin 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.

Contents

Field Stars CalibrationField Stars Calibration G2 Star CalibrationG2 Star Calibration

• ProcedureProcedure• DisadvantagesDisadvantages

B-V Calibration• WorkflowWorkflow• ExperiencesExperiences

Alternative MethodesAlternative Methodes SummarySummary

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.

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. type Colour B-V [mag]

Spica B1 Blue -0.23

Deneb A2 Blue-White +0.09

Sun G2 White +0.65

Betelgeuse M0 Orange-Red +1.50

HR Diagram

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 steps1. Load DSS image2. Load photometric catalogue3. Load filter4. Measure brightness of selected stars

Step1: Load DSS image

DSS = Digitized Sky SurveyAladin: http://aladin.u-strasbg.fr/aladin.gml

Step1: DSS

Step2: Load Photometric Catalogue

NOMAD1 = Naval Observatory Merged Astrometric DatasetAdvantage: 100% sky coverageDisadvantage: Accuracy

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

Step3: Filter

Step3: Filter

Aladin - Filter

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.

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.

Contents

Field Stars CalibrationField Stars Calibration G2 Star CalibrationG2 Star Calibration

• ProcedureProcedure• DisadvantagesDisadvantages

B-V CalibrationB-V Calibration• WorkflowWorkflow• ExperiencesExperiences

Alternative Methodes SummarySummary

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

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