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The point response function of the WFC is dominated by the
performance of the mirrors.
Figuring errors determine the on-axis response but aberrations inherent
in the optical design dominate at the edge of the field of view.
As the energy of the incident photons increases the scattering
wings becomes more pronounced
(Fig. 6.6).
At 
and 
the on-axis half energy widths
(HEW) are 2.47' and 1.93' respectively.
However, at the edge of the field of view the HEW is 4.6'
and independent of photon energy.
The encircled energy fraction for a range of off-axis angles is shown in
Fig. 6.7.
The average HEW over the field of view is 3.5'.
Figure 6.8
shows the in-orbit PSF measured from a 
observation
of the bright white dwarf HZ 43 in the S2B filter.