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 believed 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 pixel can be better than that of the CCD pixel size. Since the position uncertainty for corner events is much better than the pixel size, we can improve the Chandra ACIS spatial resolution by selecting only these events. We have analyzed X-ray images obtained with the Chandra ACIS for six pointlike sources observed near the optical axis. The image quality near the optical axis is characterized by a half-power diameter (HPD) of 0."66 that is a convolution of the point-spread function (PSF) of the high-resolution mirror assembly (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 axis, which is very close to the image quality of the HRMA alone. We estimated the degradation of the image quality obtained by using all events, compared 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 events is relatively small, this technique requires correspondingly longer exposure time to achieve good statistics.