JWST ETC MSA Full Shutter Extraction Strategy

The MSA Full Shutter Extraction strategy is only available for the JWST NIRSpec multi-object spectroscopy mode.

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The NIRSpec multi-object spectroscopy mode has 2 strategies that can be used for determining signal to noise in the ETC calculations; (1) Aperture Spectral Extraction and (2) MSA Full Shutter Extraction. The MSA Full Shutter Extraction strategy is useful when a source fits entirely into a single shutter. 
Figure 1. Layout of the Strategy tab for MSA Full Shutter Extraction

 The MSA Full Shutter Extraction strategy is setup using the following parameters:

  • Centered on source:The extraction region can be centered on a source that is selected from the sources available in the drop-down list.

  • Source offset from shutter center: The extraction region may be specified by providing the X- and Y-offset from the shutter center. 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. MPT allows the user to choose source centering constraint (to constrain the positioning of the source within the shutter to help reduce slit losses). The total slit losses are dependent on location in the shutter and as a function of wavelength (because the FWHM of the PSF increases with wavelength). The following table shows the arcsec offset in the ETC that corresponds to the available source centering constraints for a perfectly centered point source anchored at 2.95 μm (column 2). More information can be found at MSA Source Centering Constraint and NIRSpec Slit Losses.

    Table 1. Shutter extraction region offsets from shutter center

    Source centering constraintMinimum relative flux transmission at 2.95 μmX offset (arcsec)Y offset (arcsec)
    entire open shutter area30%0.0620.187
    tightly constrained85%0.0090.134

  • 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. More details can be found at NIRSpec Background Recommended Strategies

  • Wavelength of Interest: The selected wavelength of interest/wavelength of slice must be within the wavelength range for the chosen instrument setup. It specifies the wavelength at which the scalar quantities are reported in the Reports pane.

  • Angular units: This is fixed as arcseconds and determines the units for the Source offset from shutter center.

Notable updates
    Updated Aperture Spectral Extraction link, Figure 1 image, added slit offsets for MOS centering constraints and link to NRS background strategies.

    Added clarification on what "fractional shutters" refers to.
Originally published