  The NIRC2 vortex coronagraph is an instrument on Keck II designed to directly
image exoplanets and circumstellar disks at mid-infrared bands $L^\prime$
(3.4-4.1 $\mu$m) and $M_s$ (4.55-4.8 $\mu$m). We analyze imaging data and
corresponding adaptive optics telemetry, observing conditions, and other
metadata over a three year time period to characterize the performance of the
instrument and predict the detection limits of future observations. We
systematically process images from 359 observations of 304 unique stars to
subtract residual starlight (i.e., the coronagraphic point spread function) of
the target star using two methods: angular differential imaging (ADI) and
reference star differential imaging (RDI). We find that for the typical
parallactic angle (PA) rotation of our dataset ($\sim$10$^{\circ}$), RDI
provides gains over ADI for angular separations smaller than
0.25$^{\prime\prime}$. Furthermore, we find a power-law relation between the
angular separation from the host star and the minimum PA rotation required for
ADI to outperform RDI, with a power-law index of -1.18$\pm$0.08. Finally, we
use random forest models to estimate ADI and RDI post-processed detection
limits a priori. These models, which we provide publicly on a website, explain
70%-80% of the variance in ADI detection limits and 30%-50% of the variance in
RDI detection limits. Averaged over a range of angular separations, our models
predict both ADI and RDI contrast to within a factor of 2. These results
illuminate important factors in high-contrast imaging observations with the
NIRC2 vortex coronagraph, help improve observing strategies, and inform future
upgrades to the hardware.
