  In direct imaging, broad band flat-fields can easily be applied to correct
deviation from uniform sensitivity across the detector field. However for
slitless spectroscopy data the flat field is both field and wavelength
dependent. The effect of the wavelength dependent flat field for the slitless
G800L grism mode of the ACS Wide Field Channel (WFC) has been investigated from
observations of a flux calibrator at different positions in the field. The
results of various flat-fielding schemes are presented including application of
a flat field cube derived from in-orbit broad band filter flat fields.
Excellent results are reported with deviations in the extracted spectra <2%
across the WFC field.
