4. PRF/PSF Fitting

Command: wisp-fit-star-shape

Important Parameters

• photometry-catalog-max-magnitude

• subpixmap: if processing images collected with a color camera

• background-annulus

• shapefit-smoothing

• shape-grid

• shape-mode

• shape-terms-expression

• map-variables

Description

The first flux measurements AutoWISP generates for each image is one based on fitting for the shape of stars in the image and the flux (as the proportionality constant before the shape and the actual pixels belonging to that star on the image), called PSF or PRF fitting. This is accomplished by the wisp-fit-star-shape command.

Each point source once it is imaged by our observing system produces a particular distribution of light on the detector. The idea of PRF and PSF fitting is to model that distribution as some smooth parametric function centered on the projected source position that has an integral of one. For each star AutoWISP also fits for the amplitude that best scales this shape to the observed pixel values. The amplitude of course is then a measure of the flux of the source, while the parameters of the function specify its shape.

To review the terms:

• Point Spread Function or PSF: PSF(dx, dy) is the amount of light that hits the surface of the detector offset by (dx, dy) from the projected position of the source. In order to actually predict what a particular detector pixel will measure, one computes the integral of the PSF times a sub-pixel sensitivity map over the area of the pixel.

• Pixel Response Function or PRF: PRF(dx, dy) is the value that a pixel with a center offset by (dx, dy) from the projected source position will register. Note that dx and dy can be arbitrary real values and not just integers. The PRF already folds in its definition the sub-pixel sensitivity map, and other detector characteristics. Further, since the PRF is the PSF convolved with the sub-pixel sensitiity map it is generally smoother than the PSF and thus easier to model.

In this pipeline we use AstroWISP to perform PSF and PRF fitting. For the details of how this is done, see the AstroWISP documentation. Briefly, the PSF and PRF are modeled as piecewise bi-cubic functions with a number of free parameters. These parameters are in turn forced to vary smoothly as a function of source and image properties across sources and across images.

The information from PSF fitting is then used in the next step (Aperture Photometry)