JWST ETC MSA Shutter Photometry Strategy

The MSA Shutter Photometry strategy is used for the NIRSpec MOS Confirmation Imaging mode in the JWST ETC.

NIRSpec MOS Confirmation Imaging is an imaging mode that allows photometry after the science observations are executed, through an MSA shutter within a user selected Slitlet Shape and a selected science filter (F070LP, F100LP, F170LP, F290LP, CLEAR). The selection of Slitlet Shape and filter follows the selection of the associated MOS science observation. This imaging strategy is designed to allow the target to be offset within the source shutter and performs full shutter photometry with optional background subtraction. Detector readout patterns are limited to either NRS or NRSRAPID. An exposure time is specified through the number of Groups per integration, which allows for only a single-ramp observation. This is the ETC equivalent of the Confirmation Images option on the MOS template in the JWST Astronomer's Proposal Tool (APT).

Figure 1. Layout of the Strategy Tab for MSA Shutter Photometry

The following parameters are available for the MSA Shutter Photometry strategy:

  • Centered on source - From the drop-down menu in the Scene tab, select a source within the scene to use for the calculation. "Centered" refers to the center of an MSA shutter.
  • Source offset from shutter center – The location of the source within the shutter can be specified by providing the X- and Y-offset from the shutter center. This moves the selected source off the shutter center. Throughput is dependent on the exact location in the shutter and will affect the signal. The number of "fractional shutters" that an X- or Y-offset corresponds to is reported below each input box. Note that "1.00 fractional shutters" equates to one shutter half-width or half-height; the source is now on the edge of the shutter. Equivalently, "2.00 fractional shutters" equates to a full shutter width or height; the source is now in the center of an adjacent shutter.
  • Perform Background Subtraction Using - The background subtraction can be done using background estimated from the background region, or by subtracting the noiseless sky background. If the option to use the noiseless sky background is selected, the count rate corresponding to the sky background chosen under the Backgrounds tab is used for background subtraction. This option may be preferred when the scene has other sources that may contribute a non-uniform background to the aperture. If the background region option is selected, then the background is estimated from shutters in the slitlet that are adjacent to the shutter where the source is located. For example, in the instrument setup case of a slitlet having 3 shutters (-1,0,1), if the source is located in shutter 0, then the background is estimated from shutters -1 and 1. For the selection of a Slitlet Shape of 1 shutter, background subtraction is treated as noiseless sky background.




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