JWST ETC SOSS Spectral Extraction Strategy

The JWST Exposure Time Calculator (ETC) SOSS Spectral Extraction strategy is only used for the NIRISS single object slitless spectroscopy mode, and allows extraction of the signal to noise for 2 spectral orders that will be used for science observations.

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NIRISS Single Object Slitless Spectroscopy (SOSS) covers the wavelength range 0.63 to 2.81 μm and has 2 spectral orders that are scientifically useful, with order 1 covering 0.83 to 2.81 μm and order 2 covering 0.63 to 1.26 μm. The SOSS Spectral Extraction strategy in the ETC provides an option to specify the order for which the signal-to-noise ratio should be reported (Figure 1)The 1st and 2nd orders overlap spatially in the long wavelength region (when default CLEAR is selected with GR700XD in the Instrument Setup tab). The Instrument Setup tab has the option to select the F277W filter to isolate the 1st order spectrum in this overlap region. An error will be returned when selecting F277W filter for order 2.

SOSS offers full frame readout and two subarray options, SUBSTRIP256, which samples both orders, and SUBSTRIP96, which captures only the 1st spectral order (an error will be returned when attempting to run a calculation with SUBSTRIP96 and order set to 2). The wavelength range covered by the 2 spectral orders are different and the wavelength at which to report the SNR should be specified accordingly.

The readout pattern and subarray options are specified in the Detector Setup tab. Both NIS and NISRAPID are supported for full frame readout, but only NISRAPID is supported when using either the SUBSTRIP256 or SUBSTRIP96 subarray. An error will be returned when selecting NIS readout and the SUBSTRIP256 or SUBSTRIP96 subarray.

Figure 1. Layout of the Strategy tab for SOSS Spectral Extraction

The extraction height of the SOSS spectrum in the cross-dispersion direction is determined by the PSF. The PSF for NIRISS SOSS is extended due to slight defocus that is built-in to allow observations of bright targets without causing saturation. The sky background is extracted for the same aperture set by the PSF, and there is no contaminating background flux in this case. The sky background is set by the options under the Backgrounds tab.

Latest updates
    Updated for ETC 3.0.

  • Updated for ETC 1.6 to include support for F277W filter.
Originally published