Colour Calibration of RGB Images

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Colour Calibration of RGB Images

• Advantages and disadvantages of different colour
  calibration methodes

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Contents

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

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Why Colour Calibration?

• Atmosphere
• Zenith distance
• Transparency
• Telescope
• Filters
• Camera

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White Balance

• Average of field stars white
• Sunlight white
• Stars with sun-like spectral type (G2)
• Stars with sun-like photometry (B-V)

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Contents

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

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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

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Contents

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

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

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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.

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Contents

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

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B-V Calibration

• Idea Peter Riepe Harald Tomsik
• VdS Journal 2526
• Calibration uses the object image
• Instead of spectral type -gt photometry
• Comprehensive photometric data of faint stars
• Johnson-Cousins UBVRI filter system.

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UBVRI Filter

• U Ultraviolet
• B Blue
• V Visual (Green)
• R Red
• I Infrared

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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
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HR Diagram
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Idea

• Colour index of a G2 star without interstellar
  extinction
• B-V0.65 mag
• V-R0.36 mag
• Search for white stars inside the image field
• Not saturated
• 0.6 mag lt B-V lt 0.7 mag
• 0.2 mag lt V-R lt 0.6 mag
• Measurement of the brightness of selected white
  stars in Red, Green and Blue.

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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

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Step1 Load DSS image
DSS Digitized Sky Survey Aladin
http//aladin.u-strasbg.fr/aladin.gml
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Step1 DSS
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Step2 Load Photometric Catalogue
NOMAD1 Naval Observatory Merged Astrometric
Dataset Advantage 100 sky coverage Disadvantage
Accuracy
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Step3 Load Filter

• Select white stars
• 0.6 lt B-V lt 0.7
• 0.2 lt V-R lt 0.6
• Save Wolfgang Pirachers filter on your local disk

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Step3 Filter
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Step3 Filter
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Aladin - Filter
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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
• -gt Green and Blue are too bright
• -gt Red 1,00 / Green 0,85 / Blue 0,94.

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Experiences with B-V

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

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Contents

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

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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-g1.43mag g-r0.44mag)
• At the moment far away from full sky coverage.

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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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CCD-DVD Observatory Gahberg

• 25 photographers
• 2200 images
• 780 different objects
• price 14.-

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Thank you very much for your attention!