  The Advanced Camera for Surveys on-board HST is equipped with a set of one
grism and three prisms for low-resolution, slitless spectroscopy in the range
1150 Ang. to 10500 Ang. The G800L grism provides optical spectroscopy between
5500 Ang. and 1 micron with a mean dispersion of 39 Ang./pix and 24 Ang./pix
(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 narrow band 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 to the treatment
of the field dependence of the grism flat-field, wavelength solution and
sensitivity in both Channels.
