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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The 1st and 2nd orders of all SOSS spectra overlap spatially on the detector, with the overlap becoming more pronounced towards the longest wavelengths of each order. As a result, flux from different wavelengths will fall on the same detector pixels, complicating any efforts to extract the different spectral orders independently.  The JWST pipeline can account for this cross-contamination, though its effectiveness is still under investigation.

A SOSS observation with the F277W filter isolates the longest wavelengths above ~2.4 µm of the order 1 spectrum, and removes the order 2 spectrum entirely. As such, performing an additional F277W+GR700XD exposure and comparing it to the science exposure in the CLEAR+GR700XD position may facilitate better constraints on the spectral traces, profiles, and wavelength solutions for a particular observation. It is important to note that 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. Notably, they will be relatively fainter than the CLEAR exposure due to the lower throughput, and their precise brightness will depend on their stellar parameters. In any event, the F277W exposure can also be used to model these sources in isolation, and account for them in the science exposures.

Users have the option to include an exposure using the F277W filter crossed with the GR700XD grism to help identify any 0th order sources and to potentially improve spectral extraction. This exposure would be taken after the standard GR700XD exposure with the CLEAR filter, since covering the exoplanet transit is not required. 

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 a Transiting Exoplanet

The NIRISS SOSS mode is optimized to obtain spectra of transiting exoplanets. Specify JWST exposures 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 (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, 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). 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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Adding optional SOSS background exposures to your observing program

Science cases that require background estimation precisions better than 2%–3% (e.g., those that target faint stars and/or those requiring precise flux calibrated spectra) are strongly 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. The background exposures are obtained in FULL mode instead of SUBSTRIP256 or SUBSTRIP96. This makes the observations more efficient in terms of photon-collecting time. 
   
   
2. The additional background exposures use, at the very least, the same number of groups as the science TSO exposures. Experience by the NIRISS/SOSS team obtaining background observations suggests such a setup will produce adequate signal-to-noise ratios when used to correct the background signal from the science TSO exposures.
   
   
3. The background exposures are composed of, at a minimum, 4 exposures that should have Y-offsets from each other—each having at least 10 integrations. The Y-offsets of each of the exposures are critical to move both the target and nearby contaminating sources out of the portion of the detector hosting the SOSS subarray. This can be accomplished by the "Offset" option in APT's special requirements. The 10 integration-per-offset minimum, in turn, has been shown to give adequate signal-to-noise ratios when it comes to infer the rates of the SOSS background. 
   
   
4. The Y-offsets (which correspond to offsets in the cross-dispersion direction) should be larger than about 30 arcseconds. This avoids contamination from the wings of the TSO target's spectral profile, but care must be taken to not contaminate the background in the portion of the detector hosting the SOSS subarrays with nearby bright stars.

A successful set of background exposures using most of the guidelines described above have already been obtained as part of PID 2113 (PI: Espinoza). This program can be used as a template for designing background exposures to be added to a given scientific program. 

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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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