Grapes, Grism spectroscopy of the Hubble Ultra Deep Field:: Description and data reduction

We present deep unbiased spectroscopy of the Hubble Ultra Deep Field (UDF) carried out using the slitless grism spectroscopy mode of the Advanced Camera for Surveys on the Hubble Space Telescope (HST). The Grism ACS Program for Extragalactic Science (GRAPES) achieves continuum detection as faint as z(AB) 27.2 using 40 orbits (9.2 x 10(4) s) on HST. The data were taken at four orientation angles to correct for the overlap of spectra. GRAPES data provide a unique, uninterrupted, low-resolution (R 100) spectral coverage for 5500 Angstrom < lambda < 10500 Angstrom and allow us to detect high-redshift galaxies at 4 < z < 7 whether they have Lyalpha lines or just show the Lyman break, as well as find low-luminosity active galactic nuclei in an unbiased fashion. This paper describes in detail the observations and the data reduction and examines the quality of the extracted spectra. Subsequent papers will deal with the analysis of the data. The extracted and calibrated GRAPES spectra will be available from MAST at STScI.