Photometric linearization of the NICMOS-3 array

The inherent non-linearity in the photometric response of NICMOS-3 HgCdTe arrays can lead to photometric calibration errors of 1-2 percent or more when using standard flattening and calibration techniques involving single flat-field exposures and mid-level exposures of standard sources for calibration. Furthermore, the useful dynamic range of the device must be restricted to achieve even this precision, since deviations from linearity are greatest at higher exposure levels. We describe a technique to digitally linearize NICMOS-3 images by interpolating within the characteristic response curve for each pixel as determined from a series of exposures of a flat screen. Using astronomical observations, we quantify the improvements that can be achieved in final photometric precision and useful dynamic range.