The Advanced Camera for Surveys (ACS), installed in HST in SM3B,
is fitted with a grism and three prisms for slitless spectrometry.
The Wide Field Camera (WFC), with a field of 3.4x3.4' and
0.05" pixels, allows slitless grism spectrometry at a dispersion of 40Å per pixel over the range 0.55 to 1.05 μm. The High Resolution Camera (HRC) provides optimally sampled imaging above 0.6μm over a field of 26x29" (0.026" pixels), uses the same grism for slitless spectrometry at 24ÅA/pixel, and a prism covering the 1600-3300Å range with a 4maximum dispersion of 5Å/pixel. The Solar Blind Channel (SBC), with pixels of 0.030x0.034" (field size 31x35"), also has two prisms giving a maximum dispersion of about 2Å/pixel and covering the 1200-1800Å range; one prism excludes the geocoronal Lyman-α for lower background.
The ground and in-flight calibration and astrophysical use of the grism and prism modes of ACS is described. Particular problems for slitless spectrometry include the dependence of spectral resolution on image size, determination of wavelength zero points and removal of flat field. The in-flight calibrations have been derived to extend and strengthen the ground calibrations taken using line and continuum lamps which will not be available in orbit. A software package is available to extract ACS spectra, based on a catalogue of images in a direct image, and has been extensively tested with a
dedicated simulator. The wide range of astrophysical programs,
for which the ACS spectral modes are well suited, will be outlined, with examples from accepted programs.