NIRISS Single-Object Slitless Spectroscopy APT Template
Instructions for filling out the JWST NIRISS single object slitless spectroscopy template, using the Astronomers' Proposal Tool (APT), are provided in this article.
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See also: NIRISS Single Object Slitless Spectroscopy, JWST Time-Series Observations Roadmap, NIRISS SOSS Recommended Strategies, NIRISS SOSS Science Use Case, NIRISS GR700XD Grism
NIRISS single object slitless spectroscopy is one of 4 observing modes available with the Near Infrared Imager and Slitless Spectrograph (NIRISS). The SOSS mode uses a grism to generate 3 orders of cross-dispersed spectra of a single target in the wavelength range from 0.6 to 2.8 μm. Spectral orders 1 and 2 are currently calibrated. Order 1 covers the wavelength range 0.9 to 2.8 μm at a resolving power (R) of 700. Order 2 covers the wavelength range 0.6 to 1.4 μm at R = 1,400, while order 3 peaks at 0.6 μm, largely outside the sensitivity range of the detector. The SOSS mode is optimized for spectroscopy of transiting exoplanet systems around nearby stars. A built-in lens provides mild defocus in the cross-dispersion direction, effectively raising the brightness limit for saturation.
A typical SOSS observation will consist of a single target acquisition (TA) followed by a (typically long) spectroscopic exposure with the GR700XD grism. Optionally, an additional exposure can be taken through the F277W filter to enable the isolation of the 1st order spectrum in the region where the 1st and 2nd order overlap on the detector. A TA is required if a subarray is used in the exposure and strongly encouraged for full frame readout.
The observer will have control over: (1) target acquisition mode and exposure time (via number of groups), (2) choice of full frame readout or subarray for the science exposures with the GR700XD grism, (3) choice of an additional exposure with the F277W filter, and (4) exposure Readout Pattern and exposure time (via number of groups and integrations).
Step-by-step APT instructions are provided below.
Generic parameters
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
NIRISS Single-Object Slitless Spectroscopy tab
Target acquisition parameters
Target ACQ
See also: NIRISS Target Acquisition, JWST ETC NIRISS Target Acquisition
Target acquisition (TA) positions the centroid of the brightest object in the TA field of view on the location of the SOSS aperture used for science exposures. An observer can specify one of the 3 options for Acq Target: NONE, SAME TARGET AS OBSERVATION, or any source in the target list whose position is within 30"-80" of the science target (where the maximum distance depends on Galactic latitude). TA is performed with the F480M filter in the filter wheel.
TA is required if using a subarray, and strongly encouraged for full frame readout. SOSS data will be processed properly in the JWST calibration pipeline only if a TA was performed. The option of using SOSS in full frame readout without a TA will require self-calibration of the data, which is strongly discouraged in cycle 1. This option is only useful in attempting to obtain spectra of a few sources at once, and users who wish to attempt this are encouraged to coordinate with the JWST helpdesk to ensure that their observations are feasible.
There are 2 options for Acq Mode, based on the M-band magnitude of the target as specified in the SOSS Template Parameters page: SOSSBRIGHT and SOSSFAINT.
Acq Exposure Time
See also: NIRISS Detector Readout Patterns
The only permitted Acq Readout Pattern for SOSSBRIGHT is NISRAPID. Both NIS and NISRAPID are available readout patterns when the Acq Mode is SOSSFAINT. The observer can specify the Acq Groups/Int (3, 5, 7, 9, 11, 13, 15, 17, 19), where a group is defined by the number of frames in an integration (for NISRAPID) or by the number of frames in an integration divided by 4 (for NIS). The ETC workbook identification number used to determine the exposure parameters for the TA can be entered in the ETC Wkbk. Calc ID field. Though this input is optional, it is strongly recommended to provide this information so the TA can be assessed during technical reviews to ensure the observation can be executed correctly.
SOSS parameters
Subarray
See also: NIRISS Detector Subarrays
The Subarray options are:
FULL: full 2048 × 2048 array is read out, achieving higher S/N for faint targets.
- SUBSTRIP256: 256 × 2048 subarray which captures all 3 spectral orders.
- SUBSTRIP96: 96 × 2048 subarray which only covers the first spectral order. This option allows faster readouts and thus has shorter integration times, enabling observations of brighter targets and those for which the science requires a high time resolution.
SOSS Exposure
See also: NIRISS Detector Readout Patterns, Understanding JWST Exposure Times
Specify:
- Readout Pattern:
- NISRAPID: all frames are read and stored. Generally this can be used if the number of frames per integration is small and the data volume is not an issue. It is the only available option when using a subarray.
- NIS: four frames are averaged. This is used for producing a lower data rate for longer integrations of faint targets and only supported for full frame readout.
- Groups/Int: the number of groups during an integration, equal to the number of frames read per integration (for NISRAPID), or the number of frames read out divided by 4 (for NIS).
- Integrations/Exp: the number of integrations during an exposure, where an integration is the time between detector resets.
- ETC Wkbk. Calc ID: the accompanying ETC workbook ID number for the calculation (optional).
Include Short First Exposure and F277W Exposure?
See also: NIRISS SOSS Recommended Strategies
Checking the Include Short First Exposure and F277W Exposure? box opens an interface for an optional additional exposure with the F277W filter. The typical use case for such an exposure is to isolate the 1st order spectrum in the region where the 1st and 2nd order overlap on the detector. Sometimes a second exposure with the same pupil and filter wheel elements as a first exposure can have a slightly different pupil wheel position than the first exposure, which can complicate analysis as the spectral trace can then be in slightly different regions of the detector between the first and second exposure. This problem is avoided when a short first exposure is taken prior to the science exposures. When opting to take an F277W exposure, the APT template includes two exposure specifications, the first being for the short first exposure and the second for the F277W exposure. The short first exposure does not have any exposure parameters that can be adjusted by the user as these are held fixed at default values of 1 Groups/Int and 1 Integrations/Exposure. The user specifies the exposure parameters for the F277W exposure:
- Readout Pattern:
- NISRAPID: all frames are read and stored. Generally this can be used if the number of frames per integration is small and the data volume is not an issue. It is the only available option when using a subarray.
- NIS: four frames are averaged. This is used for producing a lower data rate for longer integrations of faint targets and is only supported for full frame readout.
- Groups/Int: the number of groups during an integration, equal to the number of frames read per integration (for NISRAPID), or the number of frames read out divided by 4 (for NIS).
- Integrations/Exp: the number of integrations during an exposure, where an integration is the time between detector resets.
- ETC Wkbk. Calc ID: the accompanying ETC workbook ID number for the calculation (optional).
The JWST Exposure Time Calculator (ETC) should be used to determine the combination of Groups/Int and Integrations/Exp necessary to achieve the required signal-to-noise ratio.
Other tabs
Special Requirements
By default, APT assigns a Time Series Observation (TSO) option in the Special Requirements tab when using the NIRISS SOSS template. If using NIRISS SOSS for non-TSOs, click Remove on this option in the Special Requirements tab.
A variety of other observatory level special requirements may be chosen.