12“ Newton + WAM650move

Report
Colour Calibration of RGB Images
Advantages and disadvantages of different
colour calibration methodes
Bernhard Hubl
CEDIC 2009
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 Calibration
• Procedure
• Disadvantages

B-V Calibration
• Workflow
• Experiences


Alternative Methodes
Summary
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 Calibration
G2 Star Calibration
• Procedure
• Disadvantages

B-V Calibration
• Workflow
• Experiences


Alternative Methodes
Summary
Extinction
Extinction
Workflow G2 Star Calibration
Measurement of
bright G2 star
Exposure of object in
specific altitude
Calculate extinction
RGB weights
in zenith
Corrected
RGBweights
When does the G2 calibration fail?
Measurement of
bright G2 star
Exposure of object in
specific altitude
Calculate extinction
RGB weights
in zenith



No measurement of a G2 star
Very low altitude of the object
Bad transparency (clouds, fog,..)
Corrected
RGBweights
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 Calibration
G2 Star Calibration
• Procedure
• Disadvantages

B-V Calibration
• Workflow
• Experiences


Alternative Methodes
Summary
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 steps
1.
2.
3.
4.
Load DSS image
Load photometric catalogue
Load filter
Measure brightness of selected stars
Step1: Load DSS image
DSS = Digitized Sky Survey
Aladin: http://aladin.u-strasbg.fr/aladin.gml
Step1: DSS
Step2: Load Photometric Catalogue
NOMAD1 = Naval Observatory Merged Astrometric Dataset
Advantage: 100% sky coverage
Disadvantage: 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 Calibration
G2 Star Calibration
• Procedure
• Disadvantages

B-V Calibration
• Workflow
• Experiences


Alternative Methodes
Summary
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
CCD-DVD Observatory Gahberg

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25 photographers
2200 images
780 different
objects
price 14.-
Thank you very much for your attention!

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