DAOPHOT - A computer program for crowded-field stellar photometry

The tasks of the DAOPHOT program, developed to exploit the capability of photometrically linear image detectors to perform stellar photometry in crowded fields, are discussed. Raw CCD images are prepared prior to analysis, and following the obtaining of an initial star list with the FIND program, synthetic aperture photometry is performed on the detected objects with the PHOT routine. A local sky brightness and a magnitude are computed for each star in each of the specified stellar apertures, and for crowded fields, the empirical point-spread function must then be obtained for each data frame. The GROUP routine divides the star list for a given frame into optimum subgroups, and then the NSTAR routine is used to obtain photometry for all the stars in the frame by means of least- squares profile fits. The process is illustrated with images of stars in a crowded field, and shortcomings and possible improvements of the program are considered.