NIRISS Single-Object Slitless Spectroscopy APT Template
This article provides instructions for filling out the NIRISS single object slitless spectroscopy APT template.
NIRISS Single-Object Slitless Spectroscopy is one of four observing modes available with the Near Infrared Imager and Slitless Spectrograph (NIRISS). The SOSS mode uses a grism to generate three 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 the usable orders. 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=1400, 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 continuous spectroscopic exposures with the GR700XD grism. 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 GR700XD grism, and 3) SOSS readout pattern and exposure time (via number of groups and integrations).
Step-by-Step APT instructions
Target acquisition parameters
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 TARG: NONE, SAME TARGET AS OBSERVATION, or any source within the target list that is within 60" of the science target. 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.
Acquisition exposure time
Main Article: 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 NO. OF GROUPS (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 four (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.
Main Article: NIRISS Detector Subarrays
The SUBARRAY options are:
- FULL: full 2048 × 2048 array is readout, 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.
Main Article: NIRISS Detector Readout Patterns
- 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. Only available option when using a subarray.
- NIS: four frames are averaged. Used for producing a lower data rate for longer integrations of faint targets. 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 readout 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.
A variety of observatory level special requirements may be chosen.