  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
H\alpha\ 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
H\alpha\ 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 3\sigma\ 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 H\alpha\ 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
H\alpha\ 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 H\alpha\ luminosity function. These "narrow-deep" FourStar
observations have been obtained as part of the larger NewH\alpha\ 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]
