Title: Monitoring Telluric Absorption with CAMAL,
Abstract: Ground-based astronomical observations may be limited by telluric water vapor
absorption, which is highly variable in time and significantly complicates both
spectroscopy and photometry in the near-infrared (NIR). To achieve the
sensitivity required to detect Earth-sized exoplanets in the NIR, simultaneous
monitoring of precipitable water vapor (PWV) becomes necessary to mitigate the
impact of variable telluric lines on radial velocity measurements and transit
light curves. To address this issue, we present the Camera for the Automatic
Monitoring of Atmospheric Lines (CAMAL), a stand-alone, inexpensive six-inch
aperture telescope dedicated to measuring PWV at the Fred Lawrence Whipple
Observatory on Mount Hopkins. CAMAL utilizes three narrowband NIR filters to
trace the amount of atmospheric water vapor affecting simultaneous observations
with the MINiature Exoplanet Radial Velocity Array (MINERVA) and MINERVA-Red
telescopes. Here we present the current design of CAMAL, discuss our data
analysis methods, and show results from 11 nights of PWV measurements taken
with CAMAL. For seven nights of data, we have independent PWV measurements
extracted from high-resolution stellar spectra taken with the Tillinghast
Reflector Echelle Spectrometer (TRES) also located on Mount Hopkins. We use the
TRES spectra to calibrate the CAMAL absolute PWV scale. Comparisons between
CAMAL and TRES PWV estimates show excellent agreement, matching to within 1 mm
over a 10 mm range in PWV. Analysis of CAMAL's photometric precision propagates
to PWV measurements precise to better than 0.5 mm in dry (PWV < 4 mm)
conditions. We also find that CAMAL-derived PWVs are highly correlated with
those from a GPS-based water vapor monitor located approximately 90 km away at
Kitt Peak National Observatory, with a root mean square PWV difference of 0.8
mm.