FGS External Calibration

JWST's FGS External Calibration template, available in the Astronomer’s Proposal Tool (APT), has parameters for specifying and scheduling observations.

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 FGS External Calibration template is to be used for all observations using the external calibration mode of the FGS, and this article describes allowed values for all parameters.  (warning) If this template will be used for Multi-Instrument Multi-Field (MIMF) observations, you will need to add the Special Requirement WAVEFRONT SENSING   with a value of SENSING_ONLY.

The FGS External Calibration template consists of the following parameters:

FieldDetailsValues Notes
General Information   
Observation Numberassign observation number


Observation Labelprovide observation labeltextoptional
Observation Commentsprovide observation commentstextoptional
Target Nameselect Target Namechoose from listfrom Target list
Mosaic Propertiesspecify mosaic parametersnumberonly available if TARGET TYPE is IMAGE
Special Requirementsspecify special requirementschoose from listif needed

Template Specific Information  

Target Nameselect Target Namechoose from listfrom Target list
Calibration Exposures
Pointing Typeselect Pointing TypePRIME, PARALLEL
Target Typeselect target typeIMAGE, DARK, SKYFLAT
Primary Dithersspecify number of steps1-25
Subpixel Positionsselect number of secondary dither positions1, 4

Detectorselect detectorGUIDER1, GUIDER2
Readout Patternselect readout patternFGS, FGSRAPID
Number of Groups/Integrationspecify number of groups per integrationnumber
Number of Integrations/Exposurespecify number of integrations per exposurenumber

General Information 

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

Target Name   

 Select the TARGET NAME [TBD] from the list of targets previously entered (see list of targets).

Calibration Exposures

Pointing Type


This parameter enables the user to control whether the data will be obtained in parallel like other calibrations, or as a primary observation like regular science.

Note for developers:

  • This field is not saved directly into the database.
  • If PRIME, make TARGET NAME required, remove the NONE option, and make the default "None Selected." If a PARALLEL special requirement is present, remove it.
  • If PARALLEL, set TARGET NAME = NONE (not changeable) and hide the Dither fields. If a PARALLEL special requirement is not present, add it.

Target Type 


This field specifies the how the ground system should process the data.  Mosaics are available when Target Type is IMAGE.


Note for developers: Dither options are not available when used as a parallel.

Primary Dithers  

PRIMARY DITHERS [PRIMARY_DITHERS] = 1, 2, 3, 4, 5, 6, 7, 8, . . .23, 24, 25

This parameter specifies the number of primary dither points.

Subpixel Positions 


This parameter specifies the secondary dither points used for every primary dither point.

Note that selecting a value of 1 for both PRIMARY DITHERS and SUBPIXEL POSITIONS will result in a single image with no offset images.

Note for developers: see FGS Dither Parameters for dithering details.



This field specifies the detector to be used to obtain the data.

For each exposure to be executed at each position in a dither pattern, specify the READOUT PATTERN, NUMBER OF GROUPS/INTEGRATION,  and the NUMBER OF INTEGRATIONS/EXPOSURE.

Note for developers: A Guide Star special requirement is required and is added when DETECTOR is selected. The special requirement may not use the same guider as the DETECTOR.

Readout Pattern 


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

Number of Groups/Integration 

NUMBER OF GROUPS/INTEGRATION [NGROUPS] specifies the number of groups in an integration.

Number of Integrations/Exposure 

NUMBER OF INTEGRATIONS/EXPOSURE [NINTS] field specifies the number of times the integration is repeated.

Change log

Version 5 (April 19, 2019)

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

Version 4 (December 20, 2018)

  1. PROPINSTJWST-91218 - added Pointing Type section

Version 3 (July 13, 2018)

  1. PR 85821 - added developer notes under "Dither Patterns" and "Detector" sections.

Version 2 (September 18, 2017)

  1. PR 87895 - updated Exposure field labels to Groups/Integration and Integrations/Exposure

Version 1 (May 8, 2017)

This is the converted Word file from Chapter 59.