  AstroSat/UVIT carries two gratings in the FUV channel and a single grating in
the NUV channel. These gratings are useful for low resolution, slitless
spectroscopy in the far and near UV bands of a variety of cosmic sources such
as hot stars, interacting binaries, active galactic nuclei, etc. We present the
calibration of these gratings using observations of UV standards NGC40 and HZ4.
We perform wavelength and flux calibration and derive effective areas for
different grating orders. We find peak effective areas of 18.7cm^2 at 2325
Angstrom for the -1 order of NUV-Grating, 4.5cm^2 at 1390 Angstrom for the -2
order of FUV-Grating1, and 4.3cm^2 at 1500 Angstrom for the -2 order of
FUV-Grating2. The FWHM spectral resolution of the FUV gratings is 14.6 Angstrom
in the -2 order. The -1 order of NUV grating has an FWHM resolution of 33
Angstrom. We find excellent agreement in flux measurements between the FUV/NUV
gratings and all broadband filters. We have generated spectral responses of the
UVIT gratings and broadband filters that can directly be used in the spectral
fitting packages such as XSPEC, Sherpa, and ISIS, thus allowing spectral
analysis of UVIT data either separately or jointly with X-ray data from
AstroSat or other missions.
