  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
point-like sources observed near the optical axis. The image quality near the
optical axis is characterized by a half power diameter (HPD) of $0.^{\!\!\prime
\prime}66$ that is a convolution of the PSF of the HRMA and the CCD pixel shape
($24\,\mu$m square). By considering only corner events the image quality is
improved to $0.^{\!\!\prime \prime}56$ (HPD), 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 $0.^{\!\!\prime \prime}33$, 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.
