PSF modelling notes

[sfarrens]

The objective of this issue is to gather notes on the various methods for modelling a PSF discussed during the reading group.

[sfarrens]

@sfarrens notes:

• PSFEx (for HSC)
  • pixel-based PSF from single expsoures
  • Co-add PSFs
  • Pixels of postage stamp used as a PSF model
  • What are basis vectors for pixel-based PSF?
    • Features of the PSF that you learn
• LensFIT
  • PSFEx only works on independent images (or CCDs)
  • LensFITS can handle the full field of view
  • Similar basis to PSFEx
  • Weighting of stars
  • Intra-pixel shift
• PIFF (for DES)
  • Similar to PSFEx
  • Modelling in sky coordinates instead of pixel coordinates
    • because of astrometric distortions
    • smoother variations in sky coordinates?
  • Basis-function polynomial for interpolation of PSF model
  • Delayed solution
    • don't need to solve for the PSF at the position of each star
    • use common information for all single exposures to improve model
  • Tree ring pattern
    • strong for some CCDs, but not for others
  • Gaussian processes not implemented for Y3
    • potential best solution for atmospheric distortion
  • PIFF has a modular implementation
• MCCD
  • Aim to capture global and local features
  • Denoising terms
  • Radial Basis Function (RBF) interpolation of weights to galaxy positions
• Discussion
  • Hard to make pixel grid that makes sense for entire focal plane
    • sky coordinate grid potentially a better solution, but this is on a sphere
    • CCD-specific models important for discontinuities

[sfarrens]

Proposed Subsections

• Break sections into rough approaches
  • Can mention specific codes under each approach with some method specific details
  • References to codes and papers
• PIFF image coordinates/world coordinates in separate section (what name?)
• Interpolation separate or included with approach?
  • Plan A to keep interpolation integrated with the modelling approach