An on-sky investigation into factors limiting the performance of Keck-NIRC2 for conducting infrared high-contrast imaging

The most common instrument used by the exoplanet/brown dwarf direct imaging community at the W.M. Keck Observatory is currently the NIRC2 near-infrared imager. We document three on-sky testing results of non-uniform effects that exist in the NIRC2 system when operating in L and M-band that can affect the performance when conducting high-contrast imaging observations. First, we report the measurements of the throughput of the vector vortex L/M coronagraph. We quantify the throughput and additional background flux penalties, noting the effects of using the VVC in M-band are greater than in L-band. Second, we utilize the recently commissioned NIRC2 electronics upgrade to measure the L/M band sky variability at sub-second speeds. We find that the background varies at timescales of less than 30s, indicating that the electronics upgrade may improve opportunities for future surveys. Third, we document the contribution of the image derotator to the spatial non-uniformity in the background flux. We conclude by giving a set of how the Keck-NIRC2 high-contrast imaging community can adapt their observing strategies to improve the sensitivity of future surveys.