MOS Terminology

Multi-object spectroscopy (MOS), for ground- and space-based instruments, has its own set of common terminology.

Table 1. MOS terminology




Also called 'bandpass'. A range of wavelengths over which the instrument mode is operable. NIRSpec MOS bands are described in NIRSpec Multi-Object Spectroscopy.


A list of candidates for observation. For NIRSpec MOS, the list should include TA reference stars and all other objects in the field for spectral contamination checking.


Sources whose spectra may inadvertently contaminate planned object spectra due to their proximity to target sources or failed open shutters.


The quantity, in units of delta wavelength per pixel, that describes how the spectrum is sampled on the detector. NIRSpec MOS dispersion is plotted in NIRSpec Multi-Object Spectroscopy


Moderate repositioning of the telescope between exposures to place the targets into a different location on the detector or detectors.

Filler Candidates or Secondaries

A subset of sources in the Catalog with lower scientific priority than the Primary candidates that can be used to increase the observing efficiency or multiplexing in an exposure.


Field of View of an instrument.


Line Spread Function. This is the characteristic shape of an unresolved spectral feature. The FWHM of the LSF defines the spectral resolution


The NIRSpec instrument's Micro-Shutter Assembly for performing MOS spectroscopy

MSA configuration

The file which contains the planned MSA shutter status of the NIRSpec Micro-Shutter Assembly.  This file is used onboard the JWST telescope to instruct the instrument which shutters should be used to view targets at a given pointing/exposure.


The efficiency with which the instrument can observe multiple targets at one pointing/exposure.


Repositioning the telescope slightly between exposures to place the targets into different positions within the slit. Nodding is a specific type of dithering and it implies pairwise subtraction of exposures in pipeline processing.


 A pixel refers to a physical detector pixel that is read out by some series of electronics.


The practice of obtaining imaging for improving source astrometry prior to spectroscopic observation. For JWST NIRSpec MOS spectroscopy, pre-imaging is done with NIRCam.

Primaries or Primary Candidates

These are the most important sources for observation defined by the observer.  For many instruments including the NIRSpec MSA, target weighting of the Primary candidates is also supported to help deliver these targets as products of the planning software.


Point Spread Function. Typically refers to the spatial intensity profile of an unresolved point source either incident upon the instrument optics (i.e., the telescope PSF), upon the aperture slit, or in the calibrated data.

Resolving Power

A measure of the disperser's ability to separate spectral lines at an average wavelength. NIRSpec disperser resolving power is plotted in NIRSpec Dispersers and Filters.


An element of the Micro-Shutter Assembly (which contains nearly 1/4 of a million shutters) that can be configured open or closed on a target or background in the FoV. Several may be joined in the cross-dispersion direction to form a slitlet.


An aperture through which the filtered target signal is passed.


A slit in the MSA created from opening one or more adjacent shutters in the cross-dispersion direction.

Slit Loss

The loss of light due to the slit size compared to the observed size of the source.

Slit Mask

In many ground-based MOS spectrographs, a plate consisting of tiny holes or slits milled out to pass only the light from the targets of interest in the FoV.

Source Centering

How close the source is from the center of a slit or shutter.  This depends on the pointing accuracy of the catalog and the telescope with or without target acquisition. Source centering constraints are applied in the MSA Planning Tool.

Spectral Resolution

The minimum separation in wavelengths that can be resolved by an instrument unambiguously. This quantity depends on both the resolving power of the disperser and the slit aperture of the instrument mode. See LSF.



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