PSF/PRF Fitting

Fitting for Source Shapes

Fitting for the shapes of point sources (either their PSF or PRF) and an overall scaling constant is one of the methods of extracting photometry supported by AstroWISP. This is accomplished using the astrowisp.fit_star_shape module. Currently only piecewise bi-cubic PSF/PRF models are supported, with the shape constrained to depend smoothly on image position and any other user-defined parameters, possibly accross multiple images simultaneously and the amplitudes (fluxes) of sources being independent of each other. Fitting is done by constructing an instance of astrowisp.FitStarShape and calling it on a collection of frames to be fit simultaneously and a list of all the sources in each frame. For details on how to specify fitting parameters and source and frame listts, see the documentation of astrowisp.FitStarShape.

An example of fitting for the PSF in an image can be found here

PSF Map utilities

Smooth dependence of PSF/PRF on parameters is enforced by modeling the PSF/PRF parameters as low order polynomials of user specified quantities. AstroWISP defines a language for specifying the dependence (see astrowisp.FitStarShape.shape_terms for the definition of the language). The astrowisp.SmoothDependence class offers tools for parsing the language, and generating the various terms involved in the dependence.