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 \arcsec .
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 .