  The E-310 experiment at the Facility for Advanced Accelerator Experimental
Tests II (FACET-II) at SLAC National Accelerator Laboratory aims to demonstrate
the creation of high brightness beams from a plasma photocathode. Betatron
radiation will be measured by a Compton spectrometer, currently under
development at UCLA, to provide single-shot, nondestructive beam diagnostics.
We give a brief overview of this spectrometer as well as double differential
spectrum reconstruction from the spectrometer image and beam parameter
reconstruction from this double differential spectrum. We discuss three models
for betatron radiation: an idealized particle tracking code which computes
radiation from Li\'enard-Wiechert potentials, a quasi-static particle-in-cell
(PIC) code which computes radiation from Li\'enard-Wiechert potentials, and a
full PIC code which computes radiation using a Monte Carlo QED method. Spectra
computed by the three models for a simple case are compared.
