The long-term monitoring of the the gain can be accomplished without risking the deployment of the source (which is mounted on a mechanical arm that moves in and out of the focal plane). We break the monitoring into two cases: (1) monitoring the relative change in the Offset using an astrophysical source; and (2) monitoring the relative change in the Slope using high-energy background lines in the NuSTAR background. Both of these measurements are performed after the time-dependent Slope correction been applied to the data.
Use use the Kepler supernova remnant (SNR) to determine if there have been any changes in the Offset beyond what is recorded in the CALDB. This remnant is compact (only 3-acmin across), allowing us to easily integrate over the entire remnant. We concentrate on the low-energy X-ray band (2.5 -- 10 keV) where the spectrum can be describe by two thermal plasmas (modeled here using bremss) and several X-ray lines from the heated ejecta. Here we do not attempt to determine any spatial variations in the ejecta spectrum; we are primarily interested in constructing a phenomenological model that produces an adequate fit to the spectrum. We know that on the time scale of ~years the spectrum should not be changing. We can then use observation spaced out over several years to search for changes in the energy scale.