AVAILABLE We propose to observe transits of the newly-discovered extrasolar planet TRES-1 using (1) ACS/HRC to obtain precise time-series photometry of the transit, and (2) NICMOS to measure the strength of water vapor absorption in the planetary atmosphere. The visible light curve will permit an accurate estimate of the planet's diameter by resolving uncertainty concerning the diameter of the parent star, and the water vapor observation can be used to test models of the structure, clouds and composition in the planet's upper atmosphere.

We will observe TRES-1 during three visits on each of ACS/HRC using the grism G800L. Observations will be obtained at extremely high S/N in a time-series mode with a cadence of 2.1 minutes on ACS. The first two orbits of each visit will be phased to preceed a transit of TrES-1, orbits 3 and 4 will be in transit, and the 5th and final orbit will provide out-of-transit normalization in combination with orbit 2. (We have found conditions in the first orbit are unstable and it is best not to include it in the analysis. The science goals for the ACS observations closely parallel the prime goal of the "original" HST program on HD 209458, the GO/DD 8789 to provide a much cleaner transit light curve than possible from the ground, in order to firm up system parameters. For this prime goal 8789 used STIS with the G750M grating in place as a "light bucket", by dispersing the light over many pixels it was possible to count over $10^8$ e- per exposure, and thus reach time-series S/N of 10, 000. Given the $V$ = 7.64 magnitude of HD 209458, even the 570 \AA range of G750M at low-intrinsic efficiency provided 1.55 $\times \, 10^8$ e- per 60 s integration. Using the ACS/HRC grism to disperse the light over about 200 pixels allows collection of 5 $\times 10^7$ e- per exposure while staying 30\% under saturation. Given the much higher efficiency of the G800L on ACS than the medium resolution STIS grating, and that TrES-1 is 4.15 ($V$) magnitudes fainter than HD 209458 it makes perfect sense to switch from STIS to ACS (independent of the STIS loss). Although fainter by $\times$40 in the relevant bandpass, the count rate with HRC/G800L is only $\times$4.9 lower than our original STIS observations. Adopting an exposure time of 95 s, using the 512 square subarray nearest the readout Amp for HRC provides an overhead of 35 s per read, and still encompasses the full zeroth, first and second orders from G800L. With a 130s cycle time it is possible to maintain 73\% observing efficiency on TrES-1, which compares quite favorably to the 75\% we had with HD 209458. The S/N per HST orbit provided by ACS/HRC will be a factor of $\times$2.2 times lower than that realized for the much brighter HD 209458. Thus, the overall light curve we will obtain on TrES-1 will still be stunning, with some 330 exposures in total over the three, five-orbit visits with each exposure at a S/N of 7, 150. The original STIS observations also carried a more risky goal of detecting the planet atmosphere through increased depth of the Sodium D line during transit. At a resolution of 140 compared to 5, 500 for the STIS medium resolution grating, we do not expect to be able to do this component of the science. Nor is it likely that the redder parts of the grism spectrum, where the S/N per resolution element will be down, and fringing will exist, can provide atmospheric results via molecular features. The broad bandpass coverage will provide extra constraints via quantifying color dependence of limb darkening. The scheduling for these visits will be tight. The three visits each have a 20 minute phase window, offset from each other by 20 minute centers. GSC02652-01324 2MASS19040985+3637574 STAR K V-IV Extrasolar planet host star. J2000 B-V = 0.78 POS TARG to center 0th order and end of first order G800L within subarray. starting spectral time series POS TARG to center 0th order and end of first order G800L within subarray. starting spectral time series POS TARG to center 0th order and end of first order G800L within subarray. starting spectral time series