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. Standard SOSS observations were designed to obtain spectra of transiting exoplanet systems around stars with J-band Vega magnitudes between 7 and 15, and newer multistripe subarrays expanded this capability to ~3.5. 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 6 subarrays (2 standard, and 4 multistripe).  

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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The 1st and 2nd orders of all SOSS spectra overlap spatially on the detector, particularly at the longer wavelengths of each order, and flux from both orders falls on the same detector pixels. While the JWST pipeline can account for this cross-contamination and extract the different orders independently, its effectiveness is still under investigation. 

A F277W+GR700XD SOSS exposure isolates the longest wavelengths above ~2.4 µm of the order 1 spectrum, and removes the order 2 spectrum entirely. Comparisons to the CLEAR+GR700XD science exposure may facilitate better constraints on the spectral traces, profiles, and wavelength solutions for a particular observation, though an automatic application or modification of reference files with such an exposure is currently not included in the JWST pipeline. Additionally, as all wavelengths below ~2.4 µm are removed when using the F277W filter, many potential 0th order contaminating sources will be clearly resolved. The F277W exposure can therefore be used to model these sources in isolation, and account for them in the science exposures, however, they will be relatively fainter than the CLEAR exposure due to the lower throughput, and their precise brightness will depend on their individual stellar parameters.

When including an F277W exposure, users should consider the following:

• The exposure should be taken immediately after the standard CLEAR+GR700XD exposure. Coverage of an time-variable signal such as an exoplanet transit is not necessary. 
• Generally, the exposure should have the same number of groups as the CLEAR+GR700XD exposure. 
• As a general rule of thumb, the exposure should cover a time span at least as long as the largest time bin planned to be used for the CLEAR+GR700XD exposure.
• If very little, or no binning is expected, a minimum of 10 integrations per F277W+GR700XD exposure is recommended. If a higher level of signal-to-noise is required, users should utilize the Exposure Time Calculator (ETC) to determine an appropriate number of integrations.
• Exposures should be obtained for every SOSS observation of a given target, as the spectral trace position can vary between observations due to variations in the grism wheel positioning.

Choosing the number of groups versus number of integrations for exoplanet transits

See also: Step-by-Step ETC Guide for NIRISS SOSS Time-Series Observations of a Transiting Exoplanet

The NIRISS SOSS mode is optimized to obtain spectra of transiting exoplanets. JWST exposures are specified by number of groups and number of integrations. You 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 reaching half the saturation limit. In the context of number of groups for JWST, you 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). You 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 (primarily zodiacal light), its illumination of the pick-off mirror (POM), and its dispersion by the GR700XD element. The main characteristics include a smooth, rising 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). Background intensities vary as a function of sky position, and relative observatory pointing throughout the year, and the typical peak amplitudes vary between 2 ADU/s and 7 ADU/s (i.e. ~3–10 e^–/s). Additionally, it has been observed that the profile of the background does not vary uniformly between the left side of the break and the right side of the break - a single empirical model cannot be linearly scaled to produce a match to any given dataset. Figure 1 shows an example of a science image alongside a contemporaneous background. 

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. A background correction is incorporated into the JWST pipeline, and makes use of best-match model for a given observation as found from a library of empirical models that are scaled independently at either side of the break. A 4% larger RMSE (residual value of ~0.3 ADU/s) has been observed for this empirical correction compared to the contemporaneous background solution. Additionally, the median residuals are ~10% lower than those of a contemporaneous background (residual value ~0.2 ADU/s), suggestive of over-subtraction due to the influence of the broad PSF wings of the order 1 and 2 traces. Refinements to this correction process are currently under investigation, but meanwhile, for applications that require higher precision, dithered exposures on a nearby area of the sky to obtain contemporaneous background measurements are encouraged. Further details on the background subtraction methodology can be found in JWST-STScI-009046 (T. Baines et al. 2025).

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Adding optional SOSS background exposures to your observing program

Science cases that require the highest background estimation precisions (e.g., those that target faint stars and/or those requiring precise flux calibrated spectra) are encouraged to obtain an empirical background as part of their observational strategy. It is recommended these background exposures be obtained before the TSO science exposure(s) to avoid any possible low-level persistence from the TSO exposure. 

In general, such background exposures should be linked to the TSO via the Group/Sequence Observation link in the Astronomers' Proposal Tool's (APT) special requirements. In addition, it is recommended these background exposures are taken as close to the field of the TSO target(s) as possible. As a set of general rules of thumb, STScI recommends that:

1. Use FULL frame mode: The background exposures are obtained in FULL mode instead of the science subarray to maximize photon-collecting efficiency. 
   
2. Acquire multiple offset exposures: A minimum of 4, and ideally 6, background exposures should be obtained. Each exposure should use different Y-offsets (cross-dispersion direction), specified via APT’s Offset special requirement. Offsetting ensures that both the target and nearby sources are moved out of the detector region used for the SOSS subarray. 
3. Ensure sufficient Y-offset separation: Y-offsets should be greater than approximately 30" to avoid contamination from the wings of the TSO target’s spectral trace. However, care must be taken to prevent placing bright nearby stars in the SOSS subarray region during offset exposures.
4. Match or exceed science exposure depth: Background exposures should use at least the same number of groups as the science TSO exposures, and at least 10 integrations per offset position. This configuration has been demonstrated by the NIRISS/SOSS team to provide sufficient signal to noise for accurate background correction.

An example of a successful set of these recommendations is PID 2113 (PI: Espinoza), which can serve as a useful template when designing background exposures for new programs. 

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Considering bright limits and signal-to-noise for SOSS multistripe subarrays 

Beginning in Cycle 5 (APT 2025.5), SOSS will have a selection of new "multistripe" subarrays (SUB17/60/204/680STRIPE_SOSS) as detailed in NIRISS Detector Subarrays. While these offer improved bright limits by splitting the standard SUBSTRIP256 subarray into multiple sequentially read stripes, they can also lead to reduced signal to noise; while an integration is being performed on one stripe, photons are not being actively measured in the other stripes. As such, users may want to consider the potential trade space between the standard and multistripe subarrays when planning their observation. As a general workflow, consider the following:

• Is the target fainter than the SUBSTRIP256 bright limit of 8.4?
  • If yes, use SUBSTRIP256. 
  • If no, continue. 
• Is the target fainter than the SUBSTRIP96 bright limit of 6.3?
  • If yes, is the 0.6–1.4 μm coverage provided by order 2 important to reach your science goals?
    
    • If no, use SUBSTRIP96. 
    • If yes, continue. 
  • If no, continue.
• Are you willing to accept localized detector saturation?
  • If yes, use the ETC to explore SNR trade-offs between SUBSTRIP96/256 and multistripe subarrays. 
  • If no, use the multistripe subarray with the largest stripe size that avoids saturation. 

On-sky observations for SOSS multistripe subarrays will not be obtained prior to the Cycle 5 call for proposals. As such, the subarrays have not been fully calibrated and an empirical understanding of their relative performance compared to the standard subarrays is yet to be determined. 

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Related links

ExoPlanet Characterization Tool Kit (ExoCTK) homepage

PandExo homepage

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References

Baines, T., et al., 2025, JWST Technical Report JWST-STScI-009046, SM-12
Empirical Modeling of Zodiacal Backgrounds to Improve JWST NIRISS/SOSS Data Reduction

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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