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E. Bertin DES Munich meeting 05/2010 1
Deblending
Deblending in DESDMDeblending in DESDM
E.Bertin (IAP)E.Bertin (IAP)
E. Bertin DES Munich meeting 05/2010 2
Deblending
Deblending
• Detecting sub-components• Recovering objects from the
sub-components• Forthcoming developments
E. Bertin DES Munich meeting 05/2010 3
Deblending
How sources are detected in SExtractor
• 4 steps:– Sky background
modeling and subtraction
– Image filtering at the PSF scale (matched filter)
– Thresholding and image segmentation
– Merging and/or splitting of detections
E. Bertin DES Munich meeting 05/2010 4
Deblending
Detecting sub-components
• SExtractor (or COSMOS): Multithresholding– Removal of noise
peaks based on local constrast ratio
• Photo (or DAOPhot): peak detection
• IMCat: multiscale peak detection
• SExtractor _PSF parameters: multiple PSF fitting with proximity constraints
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E. Bertin DES Munich meeting 05/2010 5
Deblending
IMCAT empircal multiscale approach
Kaiser et al. 1995
E. Bertin DES Munich meeting 05/2010 6
Deblending
Wavelet analysis
Starck et al. 2000Starck et al. 2000
• Extend the benefit of filtering from point-sources to very extended objects• Wavelet analysis: a data cube w( x,a) is obtained by correlating the image with the basis functions
is localized, isotropic, and has zero mean.•The last difficult (yet unsolved) step is to connect the detections done at each scale to reconstruct the final object (Bijaoui & Rué 1995).• pyramidal median transform is an alternative to wavelet decomposition (Starck et al. 1995)
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E. Bertin DES Munich meeting 05/2010 7
Deblending
Recovering objects
• Sextractor: 1 pixel « belongs » to one object only– Pixels lying close to
boundaries are reassociated to an object on a statistical basis (dithering)
• Photo: flux fractions reassociated based on fits of « symmetrized » templates
Lupton 2005
E. Bertin DES Munich meeting 05/2010 8
Deblending
Image segmentation in SExtractor
E. Bertin DES Munich meeting 05/2010 9
Deblending
Suggested improvements
• Drop the assumption: 1 source per pixel– Still looking for a way to do that in the cleanest way– Allow to do multiple source fits?
• Try to deblend source blends that show no saddle in their profiles?
• Multichannel deblending?• Metrics to measure deblending performance?
– Cluster simulation in SkyMaker