  WFIRST will be capable of providing Hubble-quality imaging performance over
several thousand square degrees of the sky. The wide-area, high spatial
resolution survey data from WFIRST will be unsurpassed for many decades into
the future. With the current baseline design, the WFIRST filter complement will
extend from the bluest wavelength allowed by the optical design to a reddest
filter (F184W) that has a red cutoff at 2.0 microns. In this white paper, we
outline some of the science advantages for adding a Ks filter with a 2.15
micron central wavelength in order to extend the wavelength coverage for WFIRST
as far to the red as the possible given the thermal performance of the
observatory and the sensitivity of the detectors.
