He-filter observations of WASP-69b & WASP-52b

Specifically, the transit depth in the triplet feature (with vacuum wavelengths near 1083.3nm) can be used to constrain the temperature and mass-loss rate of an exoplanet's upper atmosphere. Here, we present a new photometric technique to measure metastable 23S helium absorption using an ultranarrowband filter (FWHM 0.635nm) coupled to a beam-shaping diffuser installed in the Wide-field Infrared Camera on the 200inch Hale Telescope at Palomar Observatory.

We use telluric OH lines and a helium arc lamp to characterize refractive effects through the filter and to confirm our understanding of the filter transmission profile. We benchmark our new technique by observing a transit of WASP-69b and detect an excess absorption of 0.498%{+/-}0.045% (11.1{sigma}), consistent with previous measurements after considering our bandpass. We then use this method to study the inflated gas giant WASP-52b and place a 95th percentile upper limit on excess absorption in our helium bandpass of 0.47%.