astrowisp 2.0.1

PSF and PRF fitting

Fit for the distribution of light from a star on the detector (usually constrained to vary smoothly across stars) and an individual amplitude for each star giving a measure of the flux.

Point Spread Function, or PSF, refers to the distribution of light hitting the detector as a function of the offset from the source center. In order to predict the value that a particular pixel should have given the PSF one needs to integrate over the pixel the product of the PSF and the sensitivity of the pixel at each position within the pixel.

Pixel Response Function, or PRF, incorporates the effect of the detector. The value of the PRF at a given offset from the source center gives the value a pixel centered at that location should have.

AstroWISP allows for both PSF and PRF fitting, imposing a requirement that either function depends smoothly on the properties of the star being fit. The dependence is parametrized as an arbitrary polynomial of functions of the relevant parameters. Which properties it is allowed to depend on is entirely up to the user. Typically at least the position of the source center on the detector is included, but other properties can be included as well (e.g. the color of the source, temperature of the telescope tube, etc.). Multiple images can be fit simultaneously imposing the smooth dependence both within and across images.

Aperture photometry

Sum-up the flux in a circular aperture centered around each source. AstroWISP handles pixels that span the aperture boundary by properly integrating the product of th PSF and the sub-pixel sensitivity.