Jayke Nguyen and Rebecca Zhang

From the calendar

Mid-IR wavelengths are of particular interest to exoplanet science due to the fact they can extend the searchable parameter space to planets that are older and/or colder. However, a significant source of uncertainty at mid-IR wavelengths on ground-based telescopes is the thermal background. This background comes from blackbody radiation in the atmosphere and telescope and is therefore dependent on instrument design and atmospheric conditions. When performing imaging observations, this background manifests as a slowly varying, inhomogeneous signal throughout the image, underlying our data. Photometry at mid-IR can greatly constrain atmospheric models but existing data are usually scarce or have significant error bars due to the difficulty of subtracting the background. Using M-band direct imaging observing sequences on NIRC2, we evaluate the thermal background of the Keck II telescope and attempt to subtract the background in a more comprehensive way. For our primary science target, the forming protoplanet AB Aur b, we present a contrast upper limit of 2*10^-4 in M-band and address the limiting factors in our observation due to the thermal background. We determine that the origin of the systematic components of the thermal background comes from the K-mirror and find that the thermal background is also strongly influenced by emission from the secondary spiders on Keck II.

Polarimetric imaging isolates faint light from disks and substellar companions by observing the polarization produced by scattered starlight and polarized thermal emission. The visible-light polarimeter SCExAO/VAMPIRES and the new near-infrared NIRC2 polarimetry mode are two polarimeters behind adaptive optics that can resolve disk structures at small angular separations, detect polarized signals from self-luminous companions, and characterize scattering phase functions to infer grain properties. I will present Mueller matrix models developed for SCExAO/VAMPIRES using on-sky and internal measurements that enable quantitative polarimetry. Installed in August 2025, the NIRC2 polarimetry mode enables coronagraphic imaging, spectropolarimetry, and laser-guide star observations from J–L′. I will present our progress on validating the different observational modes for NIRC2 polarimetry internally and on-sky. Together, the two instruments offer complementary observations, with VAMPIRES resolving fine visible-light structure in the disk surface and NIRC2 extending to redder wavelengths where scattered light from cooler dust and thermal emission from young companions become accessible.