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.
Red text indicates restrictions on a parameter.
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:
| Field | Details | Values | Notes |
|---|---|---|---|
| General Information | |||
| Observation Number | assign observation number | number | |
| Observation Label | provide observation label | text | optional |
| Observation Comments | provide observation comments | text | optional |
| Target Name | select Target Name | choose from list | from Target List |
| specify ETC Workbook Calculation ID | number | for science exposures (optional) and target acquisition exposures | |
| Mosaic Properties | specify mosaic parameters | number | if needed |
| Special Requirements | specify special requirements | choose from list | if needed |
| Template Specific Information | |||
| Dither Specification | |||
| select number of dither points | choose from list | ||
| Dither Pattern Size | select size of dither steps | choose from list | |
| Science (Grism) Exposures | |||
| 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 Dithers | specify the number of times the direct image integration is repeated within an exposure | Yes, 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 (Grism) Exposures
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 Type | Dither Position | Exposure Type | Dither 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 | |||
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| 5. Direct Image | 1a* | |||
|
|
| 4. GR150C Image | 1a* | |||
|
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| 5. GR150C Image | 2a* | |||
|
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| 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. 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. The range is 1-100.
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. 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. The range is 1-100.
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.
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