Warm Spitzer IRAC photometry: dependencies on observing mode and exposure time

Abstract

We investigate differences in Spitzer/IRAC 3.6 and 4.5  μm photometry that depend on observing strategy. Using archival calibration data, we perform an in-depth examination of the measured flux densities (fluxes) of 10 calibration stars, observed with all the possible observing strategies. We then quantify differences in the measured fluxes as a function of (1) array mode (full or subarray), (2) exposure time, and (3) dithering versus staring observations. We find that the median fluxes measured for sources observed using the full array are 1.6% and 1% lower than those observed with the subarray at [3.6] and [4.5], respectively. In addition, we found a dependence on the exposure time such that for [3.6] observations, the long frame times are measured to be lower than the short frame times by a median value of 3.4% in full array and 2.9% in subarray. For [4.5] observations, the longer frame times are 0.6% and 1.5% in full and subarray, respectively. These very small variations will likely only affect science users who require high-precision photometry from multiple different observing modes. We find no statistically significant difference for fluxes obtained with dithered and staring modes. When considering all stars in the sample, the fractional well depth of the pixel is correlated with the different observed fluxes. We speculate the cause to be a small nonlinearity in the pixels at the lowest well depths where deviations from linearity were previously assumed to be negligible.