TO: 2MASS Team 1/30/95 FROM: C. Beichman SUBJECT: Limitations to Photometric Accuracy
The undersampled nature of the 2MASS data leads to a variety of photometric errors. Two sources of error are described here as due to: 1) areas of low or zero responsivity around each pixel; 2) completely dead pixels. The physical basis of problem (1) was described in a note from Mike Skrutskie. Mike described spot measurements that indicated that as much as 30% of the physical area of each pixel could be non-responsive to incident light. Typical science-grade NICMOS arrays have between 0.2-1% of the pixels which are completely dead. The first problem results in photometry with a dispersion of 1% under most seeing conditions. The proper choice of algorithm can eliminate most effects of dead pixels from the final photometry. Deriving the photometry from the coadded images, instead of from the raw frames, would result in a 2-5% amplitude bias for 10% of the sources.