  Santa Cruz Array of Lenslets for Exoplanet Spectroscopy (SCALES) is an
instrument being designed for direct imaging of exoplanets in the infrared with
the Adaptive Optics System of the W.M. Keck Observatory. The performance of
SCALES will be largely affected by thermal transmission and emission from
various sources, including the adaptive optics and instrument structures. The
placement of a cold stop and a Lyot stop can preserve maximal and stable
throughput while limiting the emission of instrument structures such as primary
mirror segment gaps, secondary structures, and spider arms. Here we propose and
compare three cold stops, a circular inner mask paired with circular,
hexagonal, and serrated outer masks, as well as one Lyot stop design. Taking
into account the pupil nutation and mirror emissivity, we model the throughput
and the background emission for all designs to optimize the dimensions of the
cold stop and the Lyot stop.
