brighter_fatter_validation

Effect: Brighter-Fatter

"Brighter-Fatter" is an effect of increase of PSF size for brighter objects, attributed to the repulsion of incoming electrons by the charge already collected inside the pixel.

Useful introduction may be found in Walter (2015) and Lage et al.(2017).

Contact person(s) if any:

Craig Lage, Andrei Nomerotski/SAWG

Reference Material:

Current efforts of BF simulation in GalSim are described in Craig Lage talk: 1

Also, Craig' recent talk on BF simulation in his Poisson solver: link

Data Provenance:

The model uses vertex data produced with Poisson CCD22 solver by Craig Lage.

Model Details:

GalSim models the effect as a linear superposition of pixel edge displacements caused by charges in all nearby pixels, with amplitudes linearly scaled according to their collected charges. Such displaced pixel boundaries are used to determine the landing pixel for every produced electron, which is then used for iterative re-calculation of displacements during the collection of image.

This approach allows to accurately simulate the electric fields from a large charge collected in a single pixel, compute the displacements it causes in all nearby pixels, and use it as an input kernel for the model.

The displacement kernels are pre-computed using Poisson CCD22 solver for both E2V and ITL chips, using both 8 and 32 vertices per edge. Resulting vertex maps and corresponding model parameters are stored in lsst_{e2v,itl}_{8,32}.{dat,cfg} files in GalSim/share/sensors/.

They are read by GalSim and passed to low-level code in Silicon.cpp which scales and co-adds them according to collected charges in pixels (Silicon::updatePixelDistortions). Then the code in Silicon::insidePixel scales the edge displacements non-linearly according to electron conversion depth, and uses the edges to determine its landing pixel.

Validation Criteria:

The following criteria are used for validation of GalSim model:

• Inter-pixel correlations inside flat fields

• Direct measurements of spot sizes at different fluxes

Validation Results:

Relevant Project Team for input if any:

Camera

Release and approval log:

07/20/18 - Initial Version - Sergey Karpov