NIRISS-Specific Time-Series Observations
With the single object slitless spectroscopy mode ("SOSS"), NIRISS provides a dedicated observing mode for near-infrared (0.6–2.8 µm) high precision spectro-photometry. In the sections below we provide some strategic background and suggestions for time series observations (TSOs) with this mode.
Typical SOSS TSO observing strategies
The SOSS mode is enabled by a grism spectrometer coupled with a cylindrical lens, which disperses the light in the cross-dispersion direction. This configuration was designed to minimize the saturation and maximize the temporal stability for bright targets; it is particularly suited to time-series observations. The grism/lens combination (GR700XD) sits in the pupil wheel and is coupled with the CLEAR filter from the NIRISS filter wheel. The GR700XD has a nominal resolving power of R ~700 at 1.4 µm.
For SOSS, the target's spectrum is projected onto one of two dedicated subarrays, or full frame, that place the first order on the same physical pixels
- FULL: The full frame has 2048 × 2048 pixels and has a frame time of 10.737 s.
- SUBSTRIP256: this subarray has 256 × 2048 pixels and has a frame time of 5.491 s, approximately half the read time as a FULL array.
- SUBSTRIP96: this subarray has 96 × 2048 pixels and has a frame time of 2.213 s, approximately 5 times faster than a FULL array.
Because the light is cross-dispersed, the saturation limit of SOSS is approximately J = 5.95 (Vega) – with SUBSTRIP96 subarray. The wide cross-dispersion profile is a benefit in terms of saturation limits and stability. However, it becomes a complication when nearby sources are within ~1.0" of the target source, and the wings of the cross-dispersed PSFs contaminate each other.
SOSS mode provides 3 spectral orders (1, 2, 3); only orders 1 & 2 are considered scientifically viable. Order 1 includes wavelengths from 0.9 to 2.8 μm; Order 2 includes wavelengths from 0.7 to 1.4 μm. The dispersion profile of SOSS is characterized by a strong order overlap at wavelengths above 2.4 µm. Because the 1st order trace is considered the work-horse for the SOSS mode, the overlap of the 2nd order trace, above 2.4 μm, is considered to "contaminate" the longer wavelengths of the 1st order trace. Note that the SUBSTRIP96 subarray only includes the pixel range for the 1st order trace; while SUBSTRIP256 contains both the 1st and 2nd order traces. Therefore, it is more difficult to mitigate the order overlap with SUBSTRIP96.
The main advantages of the SOSS mode for TSOs compared with NIRSpec, which has similar wavelength coverage, are:
- The absence of the slit avoids pointing-related flux variations.
- SOSS captures wavelengths from 0.6 to 2.8 µm simultaneously. Accounting for the spectral overlap between the 1st and 2nd orders above 2.4 μm, and the steep fall off in sensitivity below 1.0 μm, SOSS has robust sensitivity between 1.0 and 2.4 µm. In contrast, NIRSpec would require 2 visits to achieve the same wavelength coverage.
- SOSS slitless configuration and cross-dispersed properties minimize both flat fielding and intra-pixel gain variation issues.
The main disadvantages of the SOSS mode for TSOs compared with NIRSpec are:
- The spectral trace is dispersed in the cross-dispersion direction, which poses an issue for spectral extraction.
- SOSS has both order overlap issues and significant nearby target contamination issues.
- The order overlap issues may be calibrated to mitigate spectral confusion uncertainties.
The nearby target overlap issue can be minimized by using the ExoPlanet Characterization Tool Kit - Contamination & Visibility Calculator or the NIRISS SOSS planning tool from the Université de Montréal to plan for the appropriate position angle that minimizes contamination.
SOSS contamination considerations
Main article: NIRISS SOSS Recommended Strategies
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 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.
SOSS APT template
Main article: NIRISS SOSS Template APT Guide
SOSS observations use the single object slitless spectroscopy template in APT. The choice between FULL, SUBSTRIP96, and SUBSTRIP256 is made in the selection of the subarray. SOSS observations will be carried out with specific TSO mode requirements:
- No dithering allowed
- Waives the 10,000 second exposure time limit.
Main article: NIRISS Target Acquisition
Given the importance of target placement for TSOs, target acquisition (TA) is mandatory for SOSS observations in subarray modes (SUBSTRIP96 or SUBSTRIP256); TA is suggested for all TSO operations. TA is performed on the same aperture location as the science observations, except with a 64x64 subarray with readout cadence of 0.050s (SUBTASOSS). TA requires between 3 to 19 groups (3 <= NGROUPS <= 19). TA observations are taken with the F480M filter in either the CLEAR (SOSSFAINT) or non-redundant mask (NRM) (SOSSBRIGHT) pupil, depending on the brightness of the target:
- The F480M filter was chosen because the longer wavelengths are expected to saturate less quickly.
- Coupiled with the CLEAR pupil, the F480M filter provides a saturation limit of M ~6.1.
- If the science target is brighter than M ~6.1, the user may select the NRM pupil element to perform the TA (the SOSSBRIGHT mode).
- SOSSBRIGHT mode places the target behind the NRM pupil aperture and couples it with the F480M filter to center on a phase defocused observation.
The science target will remain at the center of the SUBTASOSS subarray while the GR700XD pupil is placed in the path of light. This will project the light from the defined "sweet spot" to the defined spectroscopic locations for all SOSS observations.
SOSS-TA is carried out by default in NISRAPID readout mode, on the science target. However, NIS readout is available in the "SOSSFAINT" TA mode; NIS readout should be selected for targets fainter than M ~14. Users are strongly recommended to perform TA calculations in the JWST Exposure Time Calculator (ETC) to ensure successful and non-saturated TA exposures. The minimum number of groups required for the TA centroiding algorithm is 3; and the recommended minimum SNR for TA is 30. Users are however encouraged to achieve higher SNR for the TA exposure, since centroiding accuracy does increase with SNR (see TA article). Whilst the NRM is provided for TA on very bright targets, the NRM centering position and the CLEAR (no NRM) centering position will differ because the path of the light will differ. This offset is expected to be measureable and calibrated, but it will remain unknown until commissioning.
Following TA and placement of the target to the nominal pointing position in the subarray, the procedure is configured such that an exposure will be taken through the TA filter, prior to the filter wheel move, as verification of the target placement before beginning the science exposure. This image can be used to fine-tune the calibration during the data analysis.
In general, NIRISS has a number of readout patterns, subarrays, and exposure settings to select from for its various observing modes. For SOSS, the choices are fairly constrained:
- Readout pattern is either NIS or NISRAPID
- Subarray is FULL, SUBSTRIP96, or SUBSTRIP256
To define the exposure time using the MULTIACCUM readout mode parameters, Number of Groups (NGroups), and Number of Integrations (NInts), the following rules of thumb should be considered:\
- Use of the STScI package Exoplanet Characterization Toolkit can provide the optimal NGroups and NInts for TSO observations. It is a sub-set of the PandExo package to provide spectrscopic SNR and phase considerations.
- For well-calibrated data, we recommend NGROUPS > 3. For very bright targets, NGROUPS = 1 or 2 are permitted, but non-linearity and cosmic rays may not be properly calibrated; although for TSO cosmic rays may be identified by comparing previous and subsequent integrations.
Special care should be taken to confirm that the position angle of all observations minimize overlap from nearby targets — especially if the nearby target is at a similar brightness as the science target.