Slitless grism spectroscopy with the Hubble Space Telescope advanced camera for surveys

The Advanced Camera for Surveys on board the Hubble Space Telescope is equipped with one grism and three prisms for low-resolution, slitless spectroscopy in the range 1150-10500 angstrom. The G800L grism provides optical spectroscopy between 5500 angstrom and > 1 mu m, with a mean dispersion of 39 and 24 angstrom 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