  We demonstrate a new approach to calibrating the spectral-spatial response of
a wide-field spectrograph using a fibre etalon comb. Conventional wide-field
instruments employed on front-line telescopes are mapped with a grid of
diffraction-limited holes cut into a focal plane mask. The aberrated grid
pattern in the image plane typically reveals n-symmetric (e.g. pincushion)
distortion patterns over the field arising from the optical train. This
approach is impractical in the presence of a dispersing element because the
diffraction-limited spots in the focal plane are imaged as an array of
overlapping spectra. Instead we propose a compact solution that builds on
recent developments in fibre-based Fabry-Perot etalons. We introduce a novel
approach to near-field illumination that exploits a 25cm commercial telescope
and the propagation of skew rays in a multimode fibre. The mapping of the
optical transfer function across the full field is represented accurately
(<0.5% rms residual) by an orthonormal set of Chebyshev moments. Thus we are
able to reconstruct the full 4Kx4K CCD image of the dispersed output from the
optical fibres using this mapping, as we demonstrate. Our method removes one of
the largest sources of systematic error in multi-object spectroscopy.
