The purpose of this document is to present a benchmark comparison of the performance of Integral Field (IF) and Multi-Object (MO) grating or prism dispersed spectrographs for faint galaxy (survey-type) observations with NGST. Assumptions about instrument parameters have been made based on our (limited) knowledge of the studies currently being carried out in the US, in Europe and in Canada. These parameters are not necessarily optimised for the type of observation considered here and so the absolute value of the comparison - relative speed - is rather ill-determined. The study does, however, give us an opportunity to assess the dependence of the relative performance on certain key design choices.
It should be borne in mind that, while the comparison presented here is for spectroscopic observations of faint galaxies which can be identified on broadband images, the two instrument concepts produce datasets with fundamentally different content. The MOS will produce galaxy spectra from a relatively large field together with sky spectra - either from the same apertures (slits) used for the galaxies or from separately defined slits. The IFS will, however, record spectra from every spatial element within a (smaller) field: it is, therefore, potentially sensitive to sources which are beyond the reach of the imaging observations, eg, isolated emission lines.
The document has been written for the (internal) discussions within the ESA NGST-SST and IFS-MOS trade-off study team. It has been made available on the web in order to contribute to the wider community debate on this important topic.
Although MOS should should most appropriately be compared with the IFS-LR (low resolution mode), the IFS-HR (high resolution mode) is included in some calculations in order to have a more complete view of the capabilities of such a mode.