Pixel Centering and Photometry with NIC3
As with many other array detectors, the sensitivity of NICMOS detectors is lower near the edges of the pixels than in their centers. It is as though there were small regions of reduced sensitivity along the intra- -pixel boundaries. In practical terms this effect means that for a source whose flux changes rapidly on a size comparable with or smaller than the pixel size, the measured countrate, and therefore flux, will depend on where the center of the source lies with respect to the center of the pixel. Because this position is not known a priori, this effect introduces some uncertainty in the flux calibration for a point source.
For NICMOS detectors, this uncertainty will be largest for camera 3 at short wavelengths, in which the PSF is largely undersampled. Calibration tests show that uncertainties are indeed largest (~ 11%) in the blue filter (F110W), decrease as the wavelength increases (~ 6% for F160W) and is not measurable at long wavelengths (2 microns and beyond). The effect of the intra-pixel sensitivity in the photometry is less important for observations of extended targets and outside the NIC3 campaigns when the camera is out of focus.
For high precision photometry and to compute the amount of photometric uncertainty for a filter, in particular filters in the J- and H-band wavelength range, many measurements with subpixel dithering are recommended.
A calibration program designed to quantify the impact of the intra-pixel sensitivity in the NIC3 photometry will be executed in June 1998 during the second NIC3 campaign. The results of this program will be presented here soon after the analysis of the data are completed.