JWST ETC MSA Shutter Photometry Strategy
The MSA Shutter Photometry strategy is used for the NIRSpec MOS confirmation imaging mode in the JWST ETC.
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This imaging strategy in the ETC 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 NRS, NRSRAPID, NRSIRS2, and NRSIRS2RAPID. An exposure time is specified through the number of Groups per integration, which allows for only a single-ramp observation.
- 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 shutters that an X- or Y-offset corresponds to is reported below each input box. A value of 0.50 shutter widths or heights places the source on the edge of the shutter. Equivalently, a value of 1.00 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.
- Angular units: This is fixed as arcseconds and determines the units for the Source offset from shutter center.