Stellar Photometry with CCD Sub - Pixel SensitivityVariationsA

The variation of sensitivity within a pixel of a CCD can be substantial and also substantially the same for each pixel of the CCD. Variations of the order of 10%, now being found for CCDs, can lead to errors in stellar photometry on the order of a few percent, dependent on the size of the stellar image and the position of its center within the pixel. An enhanced version of Starman, a proole tting stellar photometry package, has been used to analyze the eeect of such variations. When the seeing drops below one pixel FWHM, photometry errors can rise to the 1{5 percent levels, depending on the nature of the sub-pixel variations. By using suitable software which allows for the sub-pixel pattern, the measuring errors can be reduced to the millimag level.