USING IMAGE AREA TO CONTROL CCD SYSTEMATIC-ERRORS IN SPACEBORNE PHOTOMETRIC AND ASTROMETRIC TIME-SERIES MEASUREMENTS

We have investigated the effect of some systematic errors for high-precision time-series spaceborne photometry and astrometry, with a CCD as the detector. The "pixelization" of the images causes systematic error in astrometric measurements. We show that this pixelization noise scales as image radius r-3/2. Subpixel response gradients, not correctable by the "flat field", and in conjunction with telescope pointing jitter, introduce further photometric and astrometric errors. We model subpixel gradients using observed properties of real flat fields. These errors can be controlled by having an image span enough pixels. Large images are also favored by CCD dynamic range considerations. However, magnified stellar images can overlap, thus introducing another source of systematic error. An optimum image size is therefore a compromise between these competing factors.