The impact of bright sources on the operation of dynamic slit mask mechanisms in NGST Near-Infrared Multi-object Spectrograph

Current designs for the Near-Infrared Spectrograph (NIRSPEC) for NGST include a configurable multi-object option in which a micro-electric mechanical system (MEMS) is used to form a dynamic slit mask. MEMS elements are turned “on” where the targets of interest are placed, and “off” elsewhere. The ability of MEMS devices to reject unwanted light, that is, light from portions of the field of view that have been turned off, is measured by their contrast, which is defined as the ratio between the transmission in “on” and “off” pixels. Light filtering through from “off” pixels can increase the background, and thus the effective noise, in the measured spectra. Regan (2001) has estimated the impact of the diffuse background on the expected noise per pixel as a function of MEMS contrast and of detector properties, and finds that a contrast requirement of 400 (goal 1000) is adequate to ensure that the unrejected background light does not dominate the noise budget. Allington-Smith (2001) has then pointed out that unrejected light from discrete bright sources (hereafter called spoilers) can cause a localized increase in the background, which in turn increases the effective noise for part of the field of view. Jakobsen (2001) further points out that, since the unrejected light from bright sources is dispersed by the spectrograph just as the light from the desired targets, it contaminates the spectrum of the target. Therefore it is this additional light contribution that must be limited, rather than just the additional noise it causes. Jakobsen’s considerations are based on a 10%/10% requirement: no more than 10% of the targets can be contaminated at the 10% level. These considerations can have a significant impact on the requirements that MEMS devices must satisfy, and therefore it is important to quantify them as well as possible. The discussion that follows is restricted to the R=1000 case, where the requirements appear most stringent.