Improvement of the spatial resolution of the ACIS using split-pixel events

The position accuracy of X-ray photons on a CCD detector is generally belie ved to be limited by the CCD pixel size. While this is true in general, the position accuracy for X-ray events which deposit charge in more than one p ixel can be better than that of the CCD pixel size. Since the position unce rtainty for corner events is much better than the pixel size, we can improv e the Chandra ACIS spatial resolution by selecting only these events. We ha ve analyzed X-ray images obtained with the Chandra ACIS for six pointlike s ources observed near the optical axis. The image quality near the optical a xis is characterized by a half-power diameter (HPD) of 0."66 that is a conv olution of the point-spread function (PSF) of the high-resolution mirror as sembly (HRMA) and the CCD pixel shape (24 mum square). By considering only corner events the image quality is improved to 0."56 (HPD) at the optical a xis, which is very close to the image quality of the HRMA alone. We estimat ed the degradation of the image quality obtained by using all events, compa red to that obtained using only corner events, to be 0."33, which coincides with that expected from the pixel size. Since the fraction of the corner e vents is relatively small, this technique requires correspondingly longer e xposure time to achieve good statistics.