Differential NICMOS Spectrophotometry at High S/N

Transiting extrasolar planets present an opportunity for probing atmospheric conditions and constituents by taking advantage of different apparent radii, hence transit depth as a function of wavelength. Strong near-IR bands should support detection of water vapor via G141 spectroscopy of the bright star HD 209458 (H=6.13) by comparing in- and out-of-transit ratios of in- and out-of-band spectral intensity ratios. The reduction and analysis of science observations in which the goal is to support 1 part in 10,000, or better, development of spectral diagnostics using NICMOS grism-based spectroscopy is discussed.