  A ground-layer adaptive optics system (GLAO) uses a single adaptive mirror to
partially correct the wavefront for atmospheric and telescope aberrations over
a wide field of view. Instead of reaching diffraction limit on a narrow field,
the idea is to provide partial improvement of the PSF over a wide field
appropriate for multi-object spectrographs or wide field imagers. We derive an
analytic formulation for the GLAO corrected PSF and then apply that in
developing a methodology for calculating sensitivity improvement for
astronomical instruments.
