JWST ETC Aperture Spectral Extraction Strategy

The JWST Exposure Time Calculator (ETC) Aperture Spectral Extraction strategy is used for spectroscopic modes, including slit spectroscopy and slitless spectroscopy modes.

Slit spectroscopy modes are: MIRI low resolution spectroscopy (LRS) slit, NIRSpec fixed slit, NIRSpec multi-object spectroscopy, and NIRSpec bright object time series. The slitless modes are: MIRI low resolution spectroscopy (LRS) slitless, NIRCam wide field slitless spectroscopy, NIRCam LW grism time series, and NIRISS WFSS.

Figure 1. Layout of the Strategy tab for spectroscopy calculations

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

The following setup parameters are available for the Aperture Spectral Extraction strategy:

  • Aperture location:
    1. Centered on source: The aperture may be centered on any desired source within the scene used for the slitless spectroscopy calculation. The drop-down menu is populated with a list of the sources available in the scene from which the selection can be made. For fixed slit spectroscopy, if there are multiple sources located within the slit along the cross-dispersion direction, they may be selected from the drop-down menu. 
    2. Specify offsets in scene: The aperture can be chosen to be at any desired location within the slit along the cross-dispersion direction by specifying the Y-offset in arcsec from the slit center.

There is currently no option to provide an X-offset when the Specify offsets in scene option is selected. This prevents proper manual placement of the extraction aperture for slitless spectroscopy when the source is offset within the scene. For this reason, it is highly recommended that you select the Centered on source option, otherwise the wavelength grid may be shifted with respect to the spectrum. The Centered on source option is currently the default option for NIRISS WFSS, but Specify offsets in scene is the default for MIRI low resolution spectroscopy (LRS) slit, MIRI low resolution spectroscopy (LRS) slitless, NIRCam LW grism time series, NIRCam wide field slitless spectroscopy, NIRSpec multi-object spectroscopy, NIRSpec fixed slits, and NIRSpec bright object time series.

  • Aperture Full-Height: This corresponds to the full-height of the extraction aperture in the cross-dispersion direction. The spectral extraction is performed over the full extent of the specified extraction aperture.

  • Perform Background Subtraction Using: As in the case of imaging, there is an option to perform background subtraction using the background estimated from a background region specified by the user, or using a noiseless sky background. The latter may be preferred when the scene has other sources that may contribute a non-uniform background to the aperture. The effect of non-uniform backgrounds may be estimated by performing two calculations; with and without the background contamination from within the other sources in the scene.

    The Sky background sample region is specified by the Inner edge and Outer edge parameters. These parameters correspond to the inner and outer bounds of the sky region in the cross-dispersion direction, as measured from the center-line of the aperture in both the positive and negative directions. 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. Note that when a background region is specified, it also includes contamination from other overlapping regions when multiple sources are present, extended emission is present, or extended profiles that contribute to the background extraction aperture. When the background subtraction is performed, the contamination and sky contribution are subtracted. When the noiseless sky background is opted for background subtraction, the contamination is not subtracted from the extracted source flux.

  • Wavelength of Interest: This 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 Aperture location, Aperture Full-Height, and Sky background sample region.

Latest updates
    Updated for ETC 3.0.

    Updated for ETC 2.0.
    Updated for ETC 1.6.

  • Added warnings about current default options.

  • Updated for ETC 1.5.

  • Updated for ETC 1.3.
Originally published