Calibration of the Swift-UVOT ultraviolet and visible grisms

We present the calibration of the Swift UVOT grisms, of which there are two, providing low-resolution field spectroscopy in the ultraviolet and optical bands respectively. The UV grism covers the range λ1700-5000 u with a spectral resolution (λ/�λ) of 75 at λ2600 u for source magnitudes of u=10-16 mag, while the visible grism covers the range λ2850-6600 u with a spectral resolution of 100 at λ4000 u for source magnitudes of b=12-17 mag. This calibration extends over all detector positions, for all modes used during operations. The wavelength accuracy (1-sigma) is 9 u in the UV grism clocked mode, 17 u in the UV grism nominal mode and 22 u in the visible grism. The range below λ2740 u in the UV grism and λ5200 u in the visible grism never suffers from overlapping by higher spectral orders. The flux calibration of the grisms includes a correction we developed for coincidence loss in the detector. The error in the coincidence loss correction is less than 20%. The position of the spectrum on the detector only affects the effective area (sensitivity) by a few percent in the nominal modes, but varies substantially in the clocked modes. The error in the effective area is from 9% in the UV grism clocked mode to 15% in the visible grism clocked mode .