NIRISS Single-Object Slitless Spectroscopy APT Template

This article provides instructions for filling out the NIRISS single object slitless spectroscopy APT template.

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Main article: NIRISS Single Object Slitless Spectroscopy
See also TSO RoadmapNIRISS SOSS Recommended StrategiesNIRISS 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 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 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

Generic

The following parameters are generic to all templates, and are not discussed in this article: observation Number, observation Labelobservations CommentsTarget name, ETC Wkbk. Calc ID (in the Filters dialog box), Mosaic Properties, and Special Requirements.

Target acquisition parameters

Target ACQ

Main Article: NIRISS Target Acquisition
See Also: 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 TargetNONE, 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 PageSOSSBRIGHT and SOSSFAINT. 

Acq 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 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

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.

Exposure Time

Main Article: NIRISS Detector Readout PatternsUnderstanding 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. 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/Intthe 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/Expthe 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).

F277W Exposure

Main Article: NIRISS SOSS Recommended Strategies

Selecting the Include F277W Exposure? button 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.  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. 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/Intthe 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.

  


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Special requirements

A variety of observatory level special requirements may be chosen.




Published

 

Latest updates
  •  
    Updated to have consistent parameter headings as APT v. 27.0

  •  - Updates regarding the optional additional F277W exposure (as of APT 2020.1)

  •  Updated to have consistent block/parameter headings to APT 2020.1; update to the max. distance of offset TA targets.