Assessing the scientific value of different options for NGST spectrographs is a complex process involving a matching between the anticipated observing programmes and the predicted performance characteristics of the instruments. Although not strictly a scientific issue, the implications for the NGST operations and risk management also need to be considered at some level.
The principal metric for assessing instrument performance is, of course, the ability to carry out the DRM. It is important, however, to realise that the programmes represented in the DRM are expected to fill only approximately half of the available NGST observing time and are, by their nature, weighted towards the larger surveys.
While the Huchra committee is charged with the task of assessing all the spectrograph options, the current study is restricted to the integral field and the multi-object manifestations of grating (or prism) dispersed spectrographs and so we do not consider the possibilities of Fourier Transform or Fabry-Perot designs. This broader trade-off has already been addressed in a pdf document by Shobita Satyapal and Matt Greenhouse.
The main part of the current study is a direct comparison between the performance of the MOS and IFS designs (as far as they can be quantified) for the low spectral resolution observations of faint (KAB > 20) galaxies in the field. This is, of course, the science programme which would be expected to favour the MOS over the IFS but it is, nonetheless, instructive to examine the factors involved in the comparison and to assess the sensitivity of the result to variations of them.