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Infrared observations of metastable 2$^3$S helium absorption with ground- andspace-based spectroscopy are rapidly maturing, as this species is a uniqueprobe of exoplanet atmospheres. Specifically, the transit depth in the tripletfeature (with vacuum wavelengths near 1083.3 nm) can be used to constrain thetemperature and mass loss rate of an exoplanet's upper atmosphere. Here, wepresent a new photometric technique to measure metastable 2$^3$S heliumabsorption using an ultra-narrowband filter (full-width at half-maximum of0.635 nm) coupled to a beam-shaping diffuser installed in the Wide-fieldInfrared Camera (WIRC) on the 200-inch Hale Telescope at Palomar Observatory.We use telluric OH lines and a helium arc lamp to characterize refractiveeffects through the filter and to confirm our understanding of the filtertransmission profile. We benchmark our new technique by observing a transit ofWASP-69b and detect an excess absorption of $0.498\pm0.045$% (11.1$\sigma$),consistent with previous measurements after considering our bandpass. Then, weuse this method to study the inflated gas giant WASP-52b and place a95th-percentile upper limit on excess absorption in our helium bandpass of0.47%. Using an atmospheric escape model, we constrain the mass loss rate forWASP-69b to be $5.25^{+0.65}_{-0.46}\times10^{-4}~M_\mathrm{J}/\mathrm{Gyr}$($3.32^{+0.67}_{-0.56}\times10^{-3}~M_\mathrm{J}/\mathrm{Gyr}$) at 7,000 K(12,000 K). Additionally, we set an upper limit on the mass loss rate ofWASP-52b at these temperatures of $2.1\times10^{-4}~M_\mathrm{J}/\mathrm{Gyr}$($2.1\times10^{-3}~M_\mathrm{J}/\mathrm{Gyr}$). These results show thatultra-narrowband photometry can reliably quantify absorption in the metastablehelium feature.

Constraints on Metastable Helium in the Atmospheres of WASP-69b and WASP-52b with Ultra-Narrowband Photometry