OPTIMAL ADDITION OF IMAGES DETECTION AND PHOTOMETRY

In this paper we describe weighting techniques used for the optimal co addition of CCD frames with differing characteristics. Optimal means m aximum signal to noise (S/N) for stellar objects. We derive formulas f or four applications: (1) object detection via matched filter, (2) obj ect detection identical to DAOFIND, (3) aperture photometry, and (4) A LLSTAR profile-fitting photometry. We have included examples involving 21 frames for which either the sky brightness or image resolution var ied by a factor of 3. The gains in S/N were modest for most of the exa mples, except for DAOFIND detection with varying image resolution whic h exhibited a substantial S/N increase. Even though the only considera tion was maximizing S/N, the image resolution was seen to improve for most of the variable resolution examples. Also discussed are empirical fits for the weighting and the availability of the program, WEIGHT, u sed to generate the weighting for the individual frames. Finally, we i nclude appendices describing the effects of clipping algorithms and a scheme for star/galaxy and cosmic-ray/star discrimination.