This project provides three tools, each addressing a a different task related to doing aperture photometry in an optimal way for the case typical of ground based transit searches for extrasolar planets. The emphasis is on correcting for the fact that the sensitivity of some detectors is not uniform over a pixel. This effect will cause different flux to be measured for the same source depending on where within a pixel its center of light falls.

The three tools are:

SubPixPhot - performs aperture photometry on a list of sources with known locations and point spread functions (PSFs), correcting for:
- pixels only partially intersected by the aperture
- non-uniform sensitivity of a pixel (a sub-pixel sensitivity map must be provided)
FitSubpix - tries to fit for the sub-pixel sensitivity from a set of images which must contain the same sources with the same brightness (at least that's the ideal).
FitPSF - fits for the PSFs of a given list of sources on an image either on a source-by-source basis, or as an ensamble under the assumption that the PSF varies only slowly over the image. If a subpixel sensitivity map is provided it is used when fitting for the PSF.

The sub-pixel sensitivity map is assumed to be constant on rectangular pieces. It is supplied as a fits image.