  We describe a concept for an imaging spectrograph for a large orbiting
observatory such as NASA's proposed Next Generation Space Telescope (NGST)
based on an imaging Fourier transform spectrograph (IFTS). An IFTS has several
important advantages which make it an ideal instrument to pursue the scientific
objectives of NGST. We review the operation of an IFTS and make a quantitative
evaluation of the signal-to-noise performance of such an instrument in the
context of NGST. We consider the relationship between pixel size, spectral
resolution, and diameter of the beamsplitter for imaging and non-imaging
Fourier transform spectrographs and give the condition required to maintain
spectral modulation efficiency over the entire field of view. We give examples
of scientific programs that could be performed with this facility.
