Slitless Grism Spectroscopy with the Hst 1 Advanced Camera for Surveys

The Advanced Camera for Surveys on-boardHST is equipped with a set of one grism and three prisms for low-resolution, slitless spectroscopy in the range 1150 Å to 10500 Å. The G800L grism provides optical spectroscopy between 5500 Å and> 1 μm with a mean dispersion of 39 Å/pix and 24 Å/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. Subject headings: instrumentation: spectrographs, methods: data analysis, techniques: spectroscopic