We present first results from a narrowband imaging program for intermediate redshift emission-line galaxies using the newly commissioned FourStar infrared camera at the 6.5m Magellan telescope. To enable prompt identification of Halpha emitters, a pair of custom 1% filters, which sample low-airglow atmospheric windows at 1.19 mu m and 2.10 mu m, is used to detect both Halpha and [OII]lambda 3727 emission from the same redshift volume at z=2.2. Initial observations are taken over a 130 arcmin^2 area in the CANDELS-COSMOS field. The exquisite image quality resulting from the combination of the instrument, telescope, and standard site conditions (~0.55 FWHM) allows the 1.19 mu m and 2.10 mu m data to probe 3sigma emission-line depths down to 1.0e-17 erg/s/cm^2 and 1.2e-17 erg/s/cm^2 respectively, in less than 10 hours of integration time in each narrowband. For Halpha at z=0.8 and z=2.2, these fluxes correspond to observed star formation rates of ~0.3 and ~4 Msun/yr respectively. We find 122 sources with a 1.19 mu m excess, and 136 with a 2.10 mu m excess, 41 of which show an excess in both bands. The dual narrowband technique, as implemented here, is estimated to identify about >80% of z=2.2 Halpha emitters in the narrowband excess population. With the most secure such sample obtained to-date, we compute constraints on the faint-end slope of the z=2.2 Halpha luminosity function. These narrow-deep FourStar observations have been obtained as part of the larger NewHalpha Survey, which will combine the data with wide-shallow imaging through a similar narrowband filter pair with NEWFIRM at the KPNO/CTIO 4m telescopes, to enable study of both luminous (but rare) and faint emission-line galaxies in the intermediate redshift universe. [Abridged]
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