We report the first extrasolar planet observations from the 10.4-m Gran
Telescopio Canarias (GTC), currently the world's largest, fully steerable,
single-aperture optical telescope. We used the OSIRIS tunable filter imager on
the GTC to acquire high-precision, narrow-band photometry of the transits of
the giant exoplanets, TrES-2b and TrES-3b. We obtained near-simultaneous
observations in two near-infrared (NIR) wavebands (790.2 and 794.4 +/- 2.0 nm)
specifically chosen to avoid water vapor absorption and skyglow so as to
minimize the atmospheric effects that often limit the precision of ground-based
photometry. Our results demonstrate a very-high photometric precision with
minimal atmospheric contamination despite relatively poor atmospheric
conditions and some technical problems with the telescope. We find the
photometric precision for the TrES-2 observations to be 0.343 and 0.412 mmag
for the 790.2 and 794.4 nm light curves, and the precision of the TrES-3
observations was found to be 0.470 and 0.424 mmag for the 790.2 and 794.4 nm
light curves. We also discuss how future follow-up observations of transiting
planets with this novel technique can contribute to the characterization of
Neptune- and super-Earth-size planets to be discovered by space-based missions
like CoRoT and Kepler, as well as measure atmospheric properties of giant
planets, such as the strength of atmospheric absorption features.Comment: 9 pages, including 3 figures and 2 tables; accepted for publication
in MNRA