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).

See also: NIRISS Single Object Slitless Spectroscopy, NIRISS SOSS Template APT Guide, NIRISS SOSS Science Use Case, NIRISS GR700XD Grism

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 1^st order, and at 0.63 μm in 2^nd order. The 3^rd order will generally be too weak to be useful. For the 1^st order, wavelengths from 0.9 to 2.8 μm fall on the detector, while the 2^nd order includes wavelengths between 0.6 and 1.4 μm. The 1^st and 2^nd 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 1^st 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. 

NIRISS SOSS observations can be readout in full frame mode or with one of 2 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 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.

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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 1^st spectral order in the wavelength interval 2.4–2.8 μm, where the 1^st and 2^nd 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) as determined by the Exposure Time Calculator (ETC).

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.

Number of groups versus number of integrations for exoplanet transits

See also: Step-by-Step ETC Guide for NIRISS SOSS Time-Series Observations of WASP-39

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 (N_Groups sat) from the Exposure Time Calculator, and choose the number of groups per integration to be N_Groups 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 N_Groups 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.

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SOSS Background Observations

The sky background associated with SOSS observations has an unusual shape that results from the spectral content of the background (which is typically the zodiacal light), its illumination of the pick-off mirror (POM), and its dispersion by the GR700XD element. The main characteristics include a smooth, raising background level towards longer wavelengths, with a sharp decrease caused by the edges of the POM at about 2.1 μm (which corresponds to column 700 in the trace of order 1). Although the amplitude of the background depends on the strength of the zodiacal signal, its shape as determined during commissioning remains constant to within 2%–3%. The typical peak amplitude of the background flux on the brightest background pixels is approximately 2 ADU/s (i.e., about 3 e^–/s).

This background shape is important to remove in science cases where precise absolute and/or relative flux measurements are performed, such as in exoplanet transit spectroscopy, since the signal from the background can produce significant dilution of an exoplanet’s transit/eclipse depth as a function of wavelength. In general, transit/eclipse depths can be diluted by a factor of about 1 / (1 + FR), where FR is the flux ratio of the background flux over the target flux. Rates of 20 ADU/s from a star at the position of the brightest background pixels implies a dilution of about 90% in the measured transit/eclipse depth.

The JWST calibration pipeline does not currently have a step to remove the background flux, which must be removed manually during a post-processing stage. The NIRISS team has provided a smoothed background measurement obtained during commissioning observations for SUBSTRIP256 and for SUBSTRIP96, obtained by combining and smoothing dithered rates of a field with relatively few stars (observation 5 of program ID 1541).  During commissioning, it was found that scaling this model background frame using background pixels from a target frame, allows the background component to be removed with an accuracy of up to 2%–3% (e.g., peak background rates of 2 ADU/s are reduced to 0.04–0.06 ADU/s). This accuracy, however, might vary by a factor of a few from visit to visit given both, the possibility of zodiacal background variations and the fact that the pupil wheel position doesn't return to the same commanded position for every given visit (Martel, 2022); these effects are currently under investigation. For applications that require higher precision, dithered exposures to obtain background measurements on a nearby area of the sky are encouraged.

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Related links

ExoPlanet Characterization Tool Kit (ExoCTK) homepage

PandExo homepage

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References

Batalha, N. E., Mandell, A., Pontoppidan, K., et al. 2017, PASP, 129, 064501
PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST

Martel, A., 2022, JWST-STScI-008298
The Early Behavior of the NIRISS Pupil Wheel and Filter Wheel

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