NIRISS SOSS Recommended Strategies
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. The SOSS mode is optimized to carry out time series observations (TSOs).
The single object slitless spectroscopy (SOSS) mode of NIRISS uses the GR700XD NIRISS grism to produce 3 orders of cross-dispersed spectra of bright targets in the wavelength range from 0.6 to 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. For the 1st order, wavelengths from 0.9 to 2.8 μm fall on the detector, while the 2nd order includes wavelengths between 0.6 and 1.4 μm.
The SOSS mode is the time-series observation (TSO) mode for NIRISS and is thus optimized for spectroscopic applications requiring extremely high precision and spectrophotometric 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.
NIRISS SOSS observations can be readout in full frame mode or with one of two subarrays (SUBSTRIP256 or SUBSTRIP96).
A Target Acquisition (TA) is required when using a subarray and strongly recommended for full frame readout to ensure that the target is always placed on the same detector pixel. Note the recommended TA mode for SOSS observations as a function of target brightness near the bottom of the Target Acquisition article: SOSSBRIGHT for 3 ≤ M (Vega) ≤ 6.1 and SOSSFAINT for 6.1 ≤ M (Vega) ≤ 14.5.
An overview of TSO capabilities for the SOSS mode is discussed in a dedicated NIRISS-Specific TSO Observations Methods article. Additional advice is given below.
Minimizing contamination from nearby objects
Main article: NIRISS-Specific TSO Observations
The SOSS GR700XD grism disperses the photons in both the spectroscopic and spatial dimensions. In the spatial (cross-dispersion) dimension, the width of the trace can be as large as 25–30 pixels. The trace is also curved in a highly non-linear behavior. These combined features effect all nearby or background sources as well. Therefore, spectroscopic contamination is a significant consideration when scheduling observations. In particular, the ExoPlanet Characterization Tool Kit Contamination & Visibility Calculator or the NIRISS SOSS planning tool from the Université de Montréal can be used to determine the best position angle for observing the target in order to minimize contamination from background objects.
- As a rule of thumb, observations should be scheduled at a position angle where the target and background spectra are separated by >3 cross-dispersion widths (~90 pixels); 4 cross-dispersion widths is suggested (>100–120 pxiels).
- Using the tool above will provide the time of year and position angle at which any known background objects may contaminate the spectra—from specific orders and wavelength range.
Number of groups versus number of integrations for exoplanet transits
The NIRISS SOSS mode is optimized to obtain spectra of transiting exoplanets. We specify JWST exposures by number of groups and number of integrations. We want to observe a balanced number of groups per integration to maximize both temporal resolution and spectral precision. Previous experience has led the community to sample up the ramp until we reach half the saturation limit. In the context of number of groups for JWST, we will derive the number of groups corresponding to the onset of saturation (NGroupssat) from the Exposure Time Calculator, and choose the number of groups per integration to be NGroupssat/2 (rounding up). We will then choose the number of integrations that fully covers the full transit window.
The JWST Exposure Time Calculator (ETC) can be used to derive NGroups following the steps above. Alternatively, PandExo the "ETC ('Pandeia') for Exoplanets" can also be used to determine exposure parameters for a SOSS observation, as discussed in the Step-by-Step PandExo Guide for the NIRISS SOSS Science Use Case.