Pixels, slits and facets for a MEMS-type spectrograph for NGST

This report addresses the question of the choice of MEMS and detector element (facet and pixel respectively) sizes for the NGST near-infrared multi-object spectrograph NIRSpec. Using a specially modified spectroscopic exposure time calculator, we address both the ‘efficiency’ of observing single sources and the ‘speed’ of observing a field of sources with a space density typical for galaxies near the NGST magnitude limit. Because of the different relative importance of sky and detector noise sources, the conclusions differ somewhat between spectral resolutions of R = 1000 and R = 100. We believe, however, that a reasonable compromise can be reached by using a detector pixel scale close to 0.1 arcsec/pixel. A rectangular facet — with the long dimension perpendicular to the dispersion — is preferred. This analysis assumes an ideal generic MEMS device (MSA or MMA). The actual properties of the selected MEMS device, the detector performance and the achievable PSF may require some future refinement of this analysis.