Slitless Grism Spectroscopy with the Hubble Space TelescopeAdvanced Camera for SurveysThe Hubble Space Telescopeis a project of international cooperation between NASA and the European Space Agency (ESA). The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

The Advanced Camera for Surveys on board the Hubble Space Telescopeis equipped with one grism and three prisms for low-resolution, slitless spectroscopy in the range 1150-10500 Å. The G800L grism provides optical spectroscopy between 5500 Å and >1 ?m, with a mean dispersion of 39 and 24 Å pixel?1(in the first spectral order) when coupled with the Wide Field and the High Resolution Channels, respectively. Given the lack of any on-board calibration lamps for wavelength and narrowband flat-fielding, the G800L grism can only be calibrated using astronomical targets. In this paper, we describe the strategy used to calibrate the grism in orbit, with special attention given to the treatment of the field dependence of the grism flat field, wavelength solution, and sensitivity in both channels.