  Up to 2020, the Chandra ACIS gain has been calibrated using the External
Calibration Source (ECS). The ECS consists of an Fe-55 radioactive source and
is placed in the ACIS housing such that all chips are fully illuminated. Since
the radioactive source decays over time with a half-life of 2.7 years, count
rates are becoming too low for gain calibration. Instead, astrophysical
calibration sources will be needed, which do not fill and illuminate the entire
field of view. Here, we determine the dominant spatial components of the gain
maps through principal component analysis (PCA). We find that, given the noise
levels observed today, all ACIS gain maps can be sufficiently described by just
a few (often only one) spatial components. We conclude that illuminating a
small area is sufficient for gain calibration. We apply this to observations of
the astrophysical source Cassiopeia A. The resulting calibration is found to be
accurate to 0.6% in at least 68% of the chip area, following the same
definition for the calibration accuracy that has been used since launch.
