Intrapixel response variations cause variations in single frame aperture photometry. These effects can cause photometric biases in sources saturated in some of the frames, as those frames in which the source is near the edge of a pixel are more likely to be unsaturated. This effect is most evident in the southern H band array, and the H band array in the north put in the camera during the summer 1999 monsoon season. The effect is most evident when the seeing is good, as poor seeing smooths the psf over the pixel.
To diagnose the problem, the single frame aperture photometry residuals (frame aperture photometry minus average frame aperture photometry) from the *.fex files was binned vs in-scan and cross-scan position within the pixel for selected scans in both hemispheres for a range of seeing. In the following plots, the ordinate is 0.5 when a source is at the edge of a pixel. The single frame extractions have coordinates which are integers at the center of the pixel.
971116n_107: jsh 1.03 hsh 0.98 ksh 0.97
990523n_098: jsh 1.00 hsh 1.01 ksh 1.03
000317n_017: jsh 1.08 hsh 1.05 ksh 1.05
000317n_030: jsh 0.96 hsh 0.94 ksh 0.96
980319s_023: jsh 1.02 hsh 0.99 ksh 1.01
980319s_039: jsh 0.95 hsh 0.93 ksh 0.97
990723s_038: jsh 0.95 hsh 0.93 ksh 0.97
990923s_028: jsh 0.97 hsh 0.96 ksh 0.99
990923s_016: jsh 1.02 hsh 1.01 ksh 1.02
This page last updated on May 9, 2001.
Gene Kopan - gene at ipac.caltech.edu