NIRSpec MOS Operations - Confirmation Images

The JWST NIRSpec multi-object spectroscopy observing mode provides the opportunity to acquire images of science sources through their MSA open shutter slitlet configurations. These MOS observations, referred to as confirmation images, are observed immediately prior to the MOS spectroscopy.

NIRSpec is the first space-based astronomical instrument capable of multi-object spectroscopy (MOS) with slits. This is accomplished using a micro-shutter assembly (MSA), which is composed of four quadrants, each with ~62,000 shutters. The MOS observing mode provides the means to obtain spectra simultaneously for multiple objects within the MSA's 3.6' × 3.4' field of view. This observing mode also allows for users to acquire confirmation images of science sources through the open MSA shutter configuration pattern used for the spectroscopy. This article describes the options for confirmation images.

MOS confirmation images and available options

An optional feature of the MOS spectroscopy mode is the ability to acquire confirmation images of science sources using the imaging mirror of NIRSpec. These images could be used to analyze the position of science sources through the open shutter "slitlets" (open columns of MSA shutters that form spectral slits). However, because sources may not be perfectly centered, their flux can be truncated by slit losses.

Confirmation images are planned in the NIRSpec multi-object spectroscopy template in the JWST Astronomers Proposal Tool Overview. This option should be selected after observations are created in the template by using the MSA Planning Tool (MPT).

Confirmation images are not currently allowed if the NIRSpec MOS observation has attached parallels. 

These images are observed through the filter used for the spectroscopy (F070LP, F100LP, F170LP, F290LP or CLEAR), and using the same open/closed shutter configuration used to acquire the MOS observations. The allowed readout patterns follow from what has been selected for the MOS science, either traditional (NRS, NRSRAPID) or IRS2 (NRSIRS2, NRSIRS2RAPID) patterns. They are taken after the MSA shutters are configured into the open/closed science slitlet configuration, and just before the grating wheel is moved to put the disperser into the beam.

There are three available options for acquiring NIRSpec MOS confirmation images:

  • No* (default): no MOS confirmation images through the open/closed slitlet configurations are acquired at any point in the execution of science exposures.

  • After Target ACQ: MOS confirmation images are acquired following the conclusion of the MSA Target Acquisition (MSATA) in a visit, and after the MOS open shutter pattern and science filter are configured. These images can provide verification of target positioning within the first science slitlet configuration used in a visit. TA executes only once during a visit, so this confirmation image will also be taken just once.

  • After Target ACQ and New MSA Config: MOS confirmation images are acquired immediately after configuration of MSA shutters into a new open/closed slitlet pattern, every time a new MSA slitlet configuration is used.

Some objects that are safe for spectroscopy may saturate in a confirmation image. Confirmation images should not be acquired for science targets that will saturate in NIRSpec imaging mode (for example in target acquisition, see Table 2 for imaging saturation magnitudes in TA filters).  The ETC has a MOS Confirmation Imaging mode that can be used to determine appropriate exposure parameters for these observations.

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Latest updates

 Added note that these images not allowed with parallels.  Also listed the allowed readout patterns.