Chapter 9 Fine Guidance Sensors, FGS

The Fine Guidance Sensors (FGS) are one of the scientific instruments on HST, and this article contains instructions for how to specify observations for FGS using the Astronomer's Proposal Tool (APT).

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Format definitions

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

(red star) Black text indicates an important note.

Magenta text indicates available but unsupported parameters (requires prior approval from STScI).

Red text indicates restricted parameters (for STScI use only).

Brown text indicates text file parameters.

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

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

Introduction

The Instrument Configurations and Operating Modes described in the following section are used for the Visit and Exposure Specifications. A summary of the legal Visit and Exposure Specifications entries is provided; more complete descriptions of Instrument Configurations, Modes, Apertures, Spectral Elements, etc. are available in the FGS Instrument Handbook.

Note that many of the Optional Parameters have default values; in such cases, an Optional Parameter entry in the Visit and Exposure Specifications is necessary only if it is desired to override the default value. In the STScI proposal system the physical units of Optional Parameter quantities are always implicit and should never be entered by the observer.

Optional Parameters whose descriptions begin with the phrase “USE WITH CAUTION” pose a risk to the intended science if a non-default value is used inappropriately. These parameters are italicized in Table 9.1: Supported Instrument Parameters for the FGS. You may wish to discuss the use of these parameters with your Contact Scientist, if you have one, or with an Instrument Scientist via https://stsci.service-now.com/hst.

Please note that spatial scans are not permitted with the FGSs. Also note that only FGS1 will be calibrated as a science instrument for Cycle 25.

Table 9.1: Supported Instrument Parameters for the FGS lists the permitted Instrument Configuration, Operating Modes, Fields of View, Spectral Elements, and Optional Parameters for the FGS.

Table 9.1: Supported Instrument Parameters for the FGS 

Config

Mode

Field of View

Spectral Elements

Optional Parameters

FGS

POS

1, 2, 3

FGS 1, 2, and 3:

F583W, F5ND, F550W, PUPIL

FGS 1 and 3: F605W;

FGS 2 only: F650W

NULL, ACQ-DIST,

COUNT, FES-TIME

TRANS

1, 2, 3

FGS 1, 2, and 3: F583W, F5ND, F550W, PUPIL

FGS 1 and 3: F605W;

FGS 2 only: F650W

SCANS, STEP-SIZE,

ACQ-DIST

available but unsupported parameters

Table 1: Additional Instrument Parameters for the FGS

Config.

Mode

Field of View

Spectral Elements

Optional Parameters

FGS

POS

1, 2, 3

F605W, F583W,

F5ND, F650W, F550W, PUPIL

LOCK (Restricted)

The 9.1 FGS POS Mode and 9.2 FGS TRANS Mode articles provide further details of the entries to be made on the Visit and Exposure Specifications when a particular FGS Mode/Configuration is chosen.

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