This image shows a series
of X-ray images collected during the calibration of the ACIS instrument.
These data were collected with the AXAF telescope (HRMA) image deliberately
out-of-focus, so that the X-rays from the four concentric telescope mirrors
appear as four concentric rings. Because the outer mirrors focus only low
energy X-rays and the inner mirrors focus all energies up to about 10 keV,
the outer rings disappear as the energy of the X-ray beam is increased.