  Eclipsing planetary systems give us an important window on extrasolar planet
atmospheres. By measuring the depth of the secondary eclipse, when the planet
moves behind the star, we can estimate the strength of the thermal emission
from the day side of the planet. Attaining a ground-based detection of one of
these eclipses has proven to be a significant challenge, as time-dependent
variations in instrument throughput and atmospheric seeing and absorption
overwhelm the small signal of the eclipse at infrared wavelengths. We gathered
a series of simultaneous L grism spectra of the transiting planet system TrES-1
and a nearby comparison star of comparable brightness, allowing us to correct
for these effects in principle. Combining the data from two eclipses, we
demonstrate a detection sensitivity of 0.15% in the eclipse depth relative to
the stellar flux. This approaches the sensitivity required to detect the
planetary emission, which theoretical models predict should lie between
0.05-0.1% of the stellar flux in our 2.9-4.3 micron bandpass. We explore the
factors that ultimately limit the precision of this technique, and discuss
potential avenues for future improvements.
