NIRISS Single Object Slitless Spectroscopy

The single object slitless spectroscopy (SOSS) mode of JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS) enables medium-resolution (R ≈ 700) spectroscopy at 0.6–2.8 μm, in three cross-dispersed orders for a single bright target. The SOSS mode is optimized to carry out time-series observations (TSOs).

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See also: NIRISS Single-Object Slitless Spectroscopy APT Template, NIRISS SOSS Recommended Strategies, NIRISS SOSS Science Use Case, NIRISS-Specific Time-Series Observations

The single object slitless spectroscopy (SOSS) mode of NIRISS uses the GR700XD NIRISS grism to produce three orders of cross-dispersed spectra of bright targets in the wavelength range from 0.6–2.8 μm. The grism has a resolving power of R ≈ 700 at 1.25 μm in 1st order, and at 0.63 μm in 2nd order. The 3rd order will generally be too weak to be useful.

The SOSS mode is the time-series observation (TSO) mode for NIRISS and is thus optimized for spectroscopic applications requiring extremely high precision and spectro-photometric stability.  It was especially designed to obtain spectra of transiting exoplanet systems around stars with J-band Vega magnitudes between 7 and 15. Instrumental stability is demanded because the spectrum of the exoplanet atmosphere must be disentangled from the spectrum of the host star by subtracting or dividing spectra obtained at different orbital phases. 

To achieve the levels of spectro-photometric precision and stability required by this sort of "differential spectroscopy," the GR700XD optical element incorporates a cylindrical lens that defocuses the spectral orders in the cross-dispersion direction. This smearing in the spatial dimension mitigates the effects of flat field errors and pointing jitter, while also allowing brighter objects to be observed without saturating the detector.

Figure 1. Elements in the NIRISS pupil and filter wheel used by the SOSS mode

 Elements in the NIRISS pupil and filter wheel used by the SOSS mode

Optical elements available to the NIRISS SOSS mode are shown by the green dashed circles. The SOSS mode is enabled by using the GR700XD grism from the pupil wheel in combination with the CLEAR position in the filter wheel. 


Spectral traces for SOSS

Figure 2. Layout of GR700XD spectra on the detector

 Layout of GR700XD spectra on the detector

The locations of spectral traces for the three orders produced by the GR700XD grism are shown in this schematic representation of the detector in the "raw" detector coordinate system (note that the "science" coordinate system used in pipeline output files is mirrored across the line from (2048,1) to (1,2048) in this plot). Nominal wavelengths along the spectral traces are indicated for each order. For 1st order, wavelengths from 0.9–2.8 μm fall on the detector, while the 2nd order includes wavelengths between 0.6 and 1.4 μm. The 3rd order will generally be very weak. The subarray used for target acquisition is centered on the location labeled "acquisition sweet spot." As indicated, the 0th order for a target placed at the "sweet spot" falls well off the detector.


SOSS exposure sequence

Main article: NIRISS SOSS Template APT Guide
See also: NIRISS Target Acquisition, NIRISS Detector Subarrays, NIRISS GR700XD Grism

SOSS observations proceed in two steps:

  1. A target acquisition (TA) is performed to ensure that the spectroscopic traces fall on the desired region of the detector.  
  2. NIRISS is configured for a SOSS observation and a (potentially long) set of integrations is obtained.

For the TA, NIRISS is configured with the CLEARP (for fainter targets) or NRM (for brighter targets) in the pupil wheel and the F480M filter in the filter wheel. Short integrations are taken, and the TA procedure autonomously determines the centroid of the object in the TA subarray. Accurate knowledge of the position of the source within this subarray is used to command a small slew that places it precisely at the "sweet spot," which ensures that the spectral traces for each order are located appropriately on the detector.

For the science observation, NIRISS is configured with the GR700XD element in the pupil wheel and the CLEAR aperture in the filter wheel, and a specific subarray is selected. A typical SOSS observation consists of staring at a single target for the duration of a scientifically interesting event like an exoplanet transit or even an entire orbit of an exoplanet.  Since the exoplanet hosts are usually bright stars, individual integrations may be short; and many integrations may be required to cover the duration of the event. More details on the SOSS operations can be found on the NIRISS operations page.



SOSS subarrays

Main article: NIRISS Detector Subarrays

The SOSS mode has different subarray options to accommodate targets with a range of brightnesses: 

  • the "nominal" SUBSTRIP256 1 subarray is 2048 × 256 pixels, which is big enough to capture the 1st and 2nd orders; the 3rd order may be too weak to be useful. Stars as bright as J ≈ 8.5 or 7.2 (Vega magnitudes) can be observed without saturating in 1st or 2nd order, respectively.
  • the "bright" SUBSTRIP96 subarray is 2048 x 96 pixels, and only captures the 1st order. Stars as bright as J ≈ 7.5 (Vega magnitudes) can be observed without saturating.

The "bright" subarray is only intended for observations of the brightest stars (J7.5 in the Vega system) for which the additional wavelength coverage provided by the 2nd order is not essential.

The target acquisition for SOSS mode is performed in a 64 × 64 subarray centered on the "sweet spot."

As an option for more general applications of the SOSS mode, the entire detector can also be read out.

Figure 3. SOSS subarrays

The boundaries of the "nominal" (2048 × 256) and "bright" (2048 × 96) subarrays are indicated on this deep SOSS integration of a continuum source, which was obtained during the third Integrated Science Instrument Module (ISIM) cryo-vacuum test campaign at the Goddard Space Flight Center. Wavelengths in μm are indicated for each of the orders. More details on the SOSS operations subarrays are on the NIRISS subarrays page.

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SOSS dither patterns (or lack thereof)

Main article: NIRISS Dithers

The level of spectro-photometric stability and precision required by exoplanet transit spectroscopy can only be achieved if the same pixels are illuminated for the duration of an observation. Consequently, SOSS observations are not dithered.

However, to recover some of the usual benefits of dithering, the GR700XD grism incorporates a cylindrical lens on its "light input" surface that defocuses spectral traces in the spatial direction by ≈25 pixels. This additional smearing over many pixels in the spatial direction reduces the effect of any small shifts (due, e.g., to the rotation of the field of view during a long "staring" observation) that might cause different pixels to be illuminated. It also reduces the effect of intra-pixel sensitivity variations and detector blemishes.  Redistributing dispersed light over many pixels has the additional benefit of allowing brighter targets to be observed before the onset of saturation.

Figure 4. Simulation of the SOSS PSF in the cross-dispersed direction for different wavelengths

Simulation of the SOSS PSF in the cross-dispersed direction for different wavelengths

The top panel shows simulated PSFs produced by the GR700XD in the cross-dispersed direction at selected wavelengths. A cylindrical lens on one surface of the grism broadens the PSF by ≈25 pixels. For comparison, the bottom panel shows simulated PSFs without the lens. Defocusing in the spatial direction increases the tolerance of SOSS observations to pointing jitter and detector blemishes. Redistribution of light also allows brighter stars to be observed without saturating the detector.


References

Goudfrooij, P., Albert, L., Doyon, R., 2015, STScI Newsletter, Volume 32, Issue 01 
The Single-Object Slitless Spectroscopy Mode of Webb's NIRISS Instrument




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