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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 'confirmation images' have three available acquisition options and are observed immediately prior to the MOS spectroscopy.


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Introduction

Parent articles: NIRSpec Operations  NIRSpec MOS Operations

NIRSpec is the first space-based astronomical instrument capable of multi-object spectroscopy (MOS). 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 simultaneously obtain spectra of 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 to acquire 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 Astronomers Proposal Tool. This option should be selected after observations are created in the template by using the MSA Planning Tool (MPT)

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. 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 1 (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 NIRSpec target acquisition (TA) activity 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. Note: for this option, the grating wheel will move to the imaging mirror to take confirmation images in between science spectra. When the grating wheel is reconfigured to the dispersive element, it might not return to the exact same science position used for prior exposures. As a result, small spectral shifts can occur between multiple MOS spectra acquired with a confirmation image in between exposures.

Tip

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).

Warning

Confirmation images are currently not possible when the NIRSpec observations have attached NIRCam parallels. This will be fixed in APT 26.0.

Comment

NOTE: Right now we have no interface in the ETC that can do imaging calculations with any option for NGROUPS. We also have no option to calculate brightnesses in the science long pass filters. May need to add some text here describing the situation - or perhaps a knowledgeable article. Also will clearly state that users will be able to update time for confirmation images after the cycle 1 proposal deadline.

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References

JWST technical documents

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Last updated

Published September 27, 2017


 

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Published March 02, 2017