NIRISS SOSS Recommended Strategies
Recommendations for crafting a NIRISS single object slitless spectroscopy (SOSS) observing program are presented. This mode 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 1st and 2nd orders overlap spatially at their long wavelength ends, and the observer has the option to take an exposure through the F277W filter to isolate the 1st order spectrum in the overlap region.
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.
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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 NIRISS Target Acquisition article: SOSSBRIGHT for 2.9 ≤ M ≤ 6.0 and SOSSFAINT for 6.0 ≤ M ≤ 14.4 (Vega mag).
Advice about TSO capabilities for the SOSS mode is given below.
Adding an optional F277W exposure to your observing program
As of APT v. 2020.1.1, users have the option to include an exposure using the F277W filter crossed with the GR700XD grism. This exposure would be taken after the standard GR700XD exposure with the CLEAR filter, since covering the exoplanet transit is not required. The data from the GR700XD/F277W exposure can be used to isolate the 1st spectral order in the wavelength interval 2.4–2.8 μm, where the 1st and 2nd orders overlap in the standard GR700XD/CLEAR exposure.
Generally, the GR700XD/F277W exposure will have the same number of groups as the GR700XD/CLEAR exposure, with the number of integrations chosen to achieve the desired signal-to-noise ratio (SNR). Note: as of ETC v. 1.5.1, the ETC does not yet offer the GR700XD/F277W combination. The throughput of the F277W filter is about 90% that of the CLEAR filter, so users can estimate the SNR for the GR700XD/F277W exposure by multiplying the SNR of a GR700XD/CLEAR calculation in the ETC by a factor sqrt(0.9) ~ 0.95.
As a general rule of thumb, it is recommended that the GR700XD/F277W exposure covers a time span that is at least as long as the largest time bin that is planned to be used in binning up the GR700XD/CLEAR exposure, with a recommended minimum of 10 integrations per GR700XD/F277W exposure. Furthermore, such a GR700XD/F277W exposure should be obtained for every SOSS observation of a given target, because the spectral trace position can vary between observations due to the imperfect repeatability of the positioning of the grism wheel.
Minimizing contamination from nearby objects
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 affect all nearby or background sources as well. Therefore, spectroscopic contamination is a significant consideration when scheduling observations. In particular, the ExoPlanet Characterization Tool Kit (ExoCTK) Contamination & Visibility Calculator or the NIRISS SOSS contamination 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 (NGroups sat) from the Exposure Time Calculator, and choose the number of groups per integration to be NGroups sat/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" (Batalha et al. 2017) can also be used to determine exposure parameters for a SOSS observation.
Batalha, N. E., Mandell, A., Pontoppidan, K., et al. 2017, PASP, 129, 064501
PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST