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JWST's NIRISS Wide Field Slitless Spectroscopy (WFSS) imaging template, available in the Astronomer’s Proposal Tool (APT ), has parameters for specifying and scheduling observations.

Introduction

The NIRISS Wide Field Slitless Spectroscopy (WFSS) template is used for the WFSS mode of NIRISS, and this article describes allowed values for all parameters. Advice on how to choose values which optimize your science can be found in the NIRISS Wide Field Slitless Spectroscopy Template APT Guide. 

The NIRISS WFSS template consists of the following parameters:

FieldDetailsValues Notes
General Information   
Observation Numberassign observation number

number

 

 
Observation Label provide observation labeltextoptional
Observation Commentsprovide observation commentstextoptional
Target Nameselect Target Namechoose from listfrom Target List

ETC Wkbk. Calc. ID

specify ETC Workbook Calculation IDnumberfor science exposures (optional) and target acquisition exposures
Mosaic Propertiesspecify mosaic parametersnumberif needed
Special Requirementsspecify special requirementschoose from listif needed
Template Specific Information   
Dither Specification

 Image Dithers    
 

select number of dither points

choose from list

 
Dither Pattern Sizeselect size of dither stepschoose from list 
Science Exposure
select grism name

choose from list

also applies to Direct Image
select filter name

choose from list

also applies to Direct Image
select readout pattern

choose from list

 
specify number of groups/integration

number

 
specify number of integrations/exposure

number

 
Direct Image

select readout pattern for direct image

choose from list

 
specify number of groups/integration for direct image

number

 
specify number of integrations/exposure for direct image

number

 

General Information 

The following parameters are generic to all templates, and are not discussed in this article:  Observation Number,  Observation Label, Observations Comments, Target Name, ETC Wkbk. Calc. ID, Mosaic Properties, and Special Requirements.

Note for developer: This template should have an implicit NO PARALLEL specified.

Science (Grism) Exposure 

Dither Pattern 

The following parameters define the dither pattern for NIRISS Wide Field Slitless Spectroscopy.

Image Dithers

IMAGE DITHERS [DITHER PATTERN TYPE] = 2, 3, 4 (default), 6, 8, 12, 16

This field specifies the number of dither points. The dither points are distributed around the nominal pointing location. The first dither position is the nominal pointing location. At each dither point, science exposure(s) will be taken with the grism(s) specified in the GRISM parameter.

Notes for developer: The associated parameter values in the code are WFSS2PT, WFSS3PT, WFSS4PT, WFSS6PT, and WFSS8PT.

Notes for developer: The associated parameter in the OSS scripts is PRIMARY_DITHERS.

Note for developers: see NIRISS Dithering Details  for dithering details.

Dither Pattern Size 

DITHER PATTERN SIZE [PATTERN SIZE] = SMALL, MEDIUM [default], LARGE

This field specifies the size or amplitude of the dither steps. “Small” dither steps are ~0.3 arcsec each, “Medium” dither steps are ~0.6 arcsec each, and “Large” dither steps are ~1.2 arcsec each.

 Exposure Sequences 

The following parameters define exposure sequences in the NIRISS WFSS mode. Each such sequence encompasses dithered grism exposures along with 2 direct images, one taken at the first dither position and one at the last dither position. Examples of the cadence of WFSS exposure sequences are shown in Table 1.

Table 1. Cadence of WFSS exposure sequences taken with IMAGE DITHERS = 2. The two columns on the left indicate the order in which exposures are taken during an exposure sequence with GRISM = GR150R, while the two columns on the right do so for an exposure sequence with GRISM = BOTH. 

GRISM = GR150R exposure sequence GRISM = BOTH exposure sequence
Exposure TypeDither Position Exposure TypeDither Position

1. Direct Image

1

 

1. Direct Image

1

2. GR150R Image

1

 

2. GR150R Image

1

3. GR150R Image

2

 

3. GR150R Image

2

4. Direct Image

2

 

4. Direct Image

2

 

 

 

5. Direct Image

1a*

 

 

 

4. GR150C Image

1a*

 

 

 

5. GR150C Image

2a*

 

 

 

6. Direct Image

2a*

* Positions 1a and 2a are offset slightly from positions 1 and 2, respectively (see Grism).

Grism  

GRISM [GRISM] = GR150R, GR150C, BOTH

This field specifies the grism(s) to be used for the grism exposures in the sequence. WFSS exposure sequences will involve both direct image exposures and grism exposures. If BOTH is specified, then two WFSS exposure sequences will be acquired, one with GR150R as the grism followed by one with GR150C as the grism; a small dither offset (2.5 pixels in X and 2.0 pixels in Y) is applied between those two sequences. The cadence of WFSS exposure sequences with GRISM = GR150R and GRISM = BOTH is shown in Table 1 for the case of IMAGE DITHERS = 2. For more details on the Grisms, see  NIRISS GR150 Grisms.

Note for developer: Note that the “secondary” dither offset of (2.5, 2.0) pixels (see above) shall only be applied if GRISM = BOTH.

Filter 

FILTER [FILTER] = F090W, F115W, F140M, F150W, F158M, F200W

This field specifies the filter to be used for both the WFSS grism exposures and the direct image exposuresin an exposure sequence. For the filter bandpasses, see NIRISS Filters.

Exposure Duration  

For each exposure, specify the  READOUT PATTERN, NUMBER OF GROUPS/INTEGRATION,  and the NUMBER OF INTEGRATIONS/EXPOSURE. For more information on defining an exposure specification, see  Sequences .

Readout Pattern  

READOUT PATTERN [READOUT PATTERN] = NIS (default) , NISRAPID

This field specifies the readout pattern to be used to obtain the WFSS science data.

Number of Groups/Integraton 

NUMBER OF GROUPS/INTEGRATION [NGROUPS] specifies the number of groups/integration in each WFSS integration. (warning)For readout pattern NIS, the range is 1-200, while for NISRAPID the range is 1-30.

Number of Integrations/Exposure 

NUMBER OF INTEGRATIONS/EXPOSURE [NINTS] field specifies the number of times the WFSS integration is repeated within an exposure. (warning)The range is 1-10.

Direct Image Exposures  

One direct image will be acquired before the WFSS grism exposures beginOne additional direct image will be acquired at the last dither position (i.e. after the grism exposures are completed). The filters to be used for the direct image exposures are inherited from the filters used for the grism exposures specified above.

Exposure Duration  

For each exposure, specify the  READOUT PATTERN, NUMBER OF GROUPS/INTEGRATION,  and the NUMBER OF INTEGRATIONS/EXPOSURE. For more information on defining an exposure specification, see Sequences .

Direct Image Readout Pattern  

DIRECT IMAGE READOUT PATTERN [PATTERN] = NIS (default) , NISRAPID

 This field specifies the readout pattern to be used to obtain the direct image data.

Direct Image Number of Groups/Integration 

DIRECT IMAGE NUMBER OF GROUPS/INTEGRATION [NGROUPS] specifies the number of groups in a direct image integration. (warning)For readout pattern NIS, the range is 1-200, while for NISRAPID the range is 1-30.

Direct Image Number of Integrations/Exposure 

DIRECT IMAGE NUMBER OF INTEGRATIONS/EXPOSURE [NINTS] field specifies the number of times the direct image integration is repeated within an exposure. (warning)The range is 1-10.

Note for developers: The direct image flag should be set to YES for each direct image exposure specification in the database to distinguish between the required WFSS image and the direct image. The order of operation is described in Table 1. Since the same fields in the database are used for both WFSS and Direct images, APT needs to populate the exposures in this order.

Keywords

JWST NIRISS Wide Field Slitless Spectroscopy Template

JWST NIRISS Wide Field Slitless Spectroscopy template parameters

Format definitions

Boldface italics type indicates the name of an APT parameter or a value for a parameter.

(warning)Red text indicates restrictions on a parameter.

Brown text indicates notes for the developers.

Green text indicates the name of the parameter used by Commanding.

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JWST Proposal Parameters