NIRISS Wide Field Slitless Spectroscopy (WFSS) Template Parameters

ƒJWST'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

Purple text indicates the parameter is Limited Access.

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

(warning)Red text indicates restrictions on a parameter.

(red star) Black text indicates an important note.

Brown text indicates notes for the developers.

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

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.

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 APT Template article. 

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
select number of dither points

choose from list


Dither Pattern Sizeselect size of dither stepschoose from list
Science Observation
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



Two Extra Dithersspecify the number of times the direct image integration is repeated within an exposureYes, No

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 Observation

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. 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. Additional dither patterns are available when NIRISS WFSS is prime in a coordinated parallel (see Additional Dithers for Coordinated Parallel Observations). 

Dither Pattern Size 

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

This field specifies the size or amplitude of the dither steps. Dither offsets are provided in NIRISS WFSS Dithers. When a coordinated parallel dither is selected, DITHER PATTERN SIZE is not available.

 Exposure Sequences 

Each WFSS observation consists of one or more  exposure sequences. For each sequence, APT creates a cadence of direct and grism exposures in a prescribed order which includes one or more dithered grism exposures, and two or more direct image exposures using the same filter as the grism exposures. One direct image will be taken at the first dither position of each grism exposure, and another one at the last dither position of each grism exposure.

Examples of the cadence of WFSS exposure sequences are shown in Table 1. 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. 

Table 1. Cadence of WFSS exposure sequences taken with IMAGE DITHERS = 2.  

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 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 sequences. If BOTH is specified, then two WFSS sequences will be created, one with GR150R as the grism followed by one with GR150C as the grism; a filter-dependent dither offset is applied between those two sequences in order to optimize spatial coverage between the two orthogonal grisms. Details on these filter-dependent dithers are described in NIRISS WFSS Recommended Strategies.

The cadence of WFSS exposures 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 developers: If only GRISMR or GRISMC is selected, provide the following warning: "Use of only one of GRISMR or GRISMC may result in spectral overlap from multiple sources that can't be corrected. Users should address this issue in their proposal text." 

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 exposures in 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 in the NIRISS Wide Field Slitless Spectroscopy APT Template article.

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 Exposure Parameters  

One direct image will be acquired before the WFSS grism exposures begin. One additional direct image will be acquired at the last dither position of each grism exposure (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.

Direct Images  

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 in the NIRISS Wide Field Slitless Spectroscopy APT Template article.

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.

Two Extra Dithers

TWO EXTRA DITHERS =  NO (default), YES

Specifies two additional dithers added only to the direct image exposure following each grism exposure. Dither offsets2 are described in Table 4 of NIRISS WFSS Dithers.

Footnotes

1. Additional dithers are available when NIRISS WFSS is prime in a coordinated parallel. Additional dithers for NIRISS parallels are specified in the PRD table NirissCompromiseDithers.

2. Dither offsets for Two Extra Dithers are specified in the NirissWfssSecondary PRD table.


Change log

The initial version of this chapter was provided by Lily Liu.


August 18, 2022

  1. PROPINSTJWST-91283 Added developer note to GRISM section to issue a warning if only GRISMR or GRISMC is selected.


May 3, 2022

  1. PROPINSTJWST-91373 Updates to Exposure Sequences and Two Extra Dithers section to document source of dither specifications in PRD and link to JDOX articles.
  2. Editorial change - Updates some section titles to match APT GUI.


Version 5 (August 9, 2021)

  1. Editorial change - Anchor links fixed in parameter table by removing pagename from link.


Version 4 (August 14, 2020)

  1. PROPINSTJWST-91372 - Added Two Extra Dithers section


Version 3 (April 18, 2019)

  1. Editorial change (moved change log to main article).


Version 2 (May 15, 2017)

  1. PR 79105 - major edits, especially to dithers
  2. PR 87006 - update to dither positions
  3. PR 87895 - revised names of Number of Groups and Integrations


Version 1 (March 16, 2016)

This is the converted Word File from Chapter 41.