STScI

HST Phase II Proposal Instructions for Cycle 11

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Chapter 11: 
Fine Guidance Sensors (FGS)

In this chapter . . .

 

Tables and Figures

Table 11.1: Supported Instrument Parameters for the FGS

Table 11.2: Spectral Elements for the FGS

figure 11.1 The FGS POS TARG and interferometer coordinate systems.

Introduction to the FGS

The Instrument Configurations and Operating Modes described in the following section are used to specify exposures on the RPS2 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 entry as an Optional Parameter on 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 11.1. You may wish to discuss the use of these parameters with your Contact Scientist.

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 11.

table 11.1 lists the permitted Instrument Configuration, Operating Modes, Fields of View, Spectral Elements, and Optional Parameters for the FGS.

Table 11.1: Supported Instrument Parameters for the FGS
Config Mode Field of View Spectral Elements Optional Parameters
FGS POS
Section 		PRIME,
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
Section 		PRIME,
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

 

The sections on the following pages provide further details of the entries to be made on the Visit and Exposure Specifications when a particular FGS Mode/Configuration is chosen.

Mode = POS
Configuration = FGS

This is the basic single-star positional Operating Mode of the FGS. It may be used to measure the relative positions of fixed or slowly moving (<= 0.080 arcsec/sec) targets with a precision of 0.001 to 0.002 arcsec per single observation. To measure the relative position of a target with respect to field stars, several reference targets must be observed in the same visit. In addition, a few stars (reference and/or target) should be observed multiple times during a visit to map any positional drift of the field due to HST pointing characteristics (see the FGS Instrument Handbook). In this Operating Mode the program star or asteroid is first acquired and then held in fine lock for the specified exposure time. This procedure is repeated for the other targets.

The default values of the Optional Parameters are set in this Mode to provide acceptable data for single, zero angular diameter stars. The default acquisition consists of a spiral search for a target within a radius of 15 arcsec, followed by fine-lock tracking of the target. In this default mode star selectors are moved to null the Fine Error Signal and their positions are recorded every 0.025 sec.

Aperture or FOV

PRIME

FGS 1 is designated the "prime" FGS and is used for astrometry.

1, 2, 3

Requests use of a specific FGS (see figure 11.1 The FGS POS TARG and interferometer coordinate systems.).

Spectral Element

The available spectral elements are listed in Table 11.2: Spectral Elements for the FGS. Only one filter can be used at a time. The F583W is the recommended element for all observations requiring the Optical Field Angle Distortion (OFAD) calibration. F583W cannot be used if the target magnitude m < 8.0. F5ND must be used if m < 8.0. An F5ND/F583W cross-filter calibration is supported.

Optional Parameters

NULL
=YES (default), NO

NULL determines whether the Star Selectors will be repositioned immediately after the FES-TIME or after a small delay. Consult with your Contact Scientist before specifying the non-default value; most astrometry programs require NULL=YES.

NULL=YES (the default) means that the next fine-error-averaging time (FEAT) interval will not begin until the star selectors have completed the repositioning of the FGS instantaneous field of view (IFOV) to the null position determined by the just-completed FEAT. This is the recommended procedure for all fixed targets.

NULL=NO means that the next FEAT begins immediately after the previous FEAT, implying that the star selectors will not necessarily remain fixed during the FEAT. If the FES-TIME = 0.025 sec, NULL=NO has no effect. NULL=NO is ordinarily useful only for observations of moving targets.

ACQ-DIST
=DEF (default); 0.0 - 90.0

USE WITH CAUTION-This Optional Parameter determines the size (in arcsec) of the acquisition search region. The default value is 15 arcsec. A larger search radius takes more time and is generally not necessary given HST's pointing performance.

COUNT
=DEF (default); 1 - 2621400

USE WITH CAUTION-The default count rate will be determined from the target V magnitude, and the filter and FGS, using simple scaling rules. The FGS Instrument Handbook should be consulted for the default background rates. These Optional Parameters are used to verify the value of FES-TIME. If a non-default value is needed, enter the expected target and sky count rates (in counts per second) for the FGS (see the FGS Instrument Handbook and consult with your Contact Scientist).

FES-TIME
=DEF (default); 0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2

USE WITH CAUTION-This optional parameter sets the averaging time (in seconds) for the Fine Error Signal, and consequently, the interval at which the star selectors are adjusted to null the Fine Error Signal.

For POS mode, the default value DEF will be calculated from the default or specified COUNT optional parameter value and the selected FGS and FILTER. For TRANSfer mode, the default (and required) value is 0.025 sec, regardless of target brightness. See the FGS Instrument Handbook for details.

It is necessary that the proposer include the target V magnitude for all targets. We recommend including the B-V index so your Contact Scientist can assess your calibration needs.

Special Requirements

Enter the ORIENTation visit-level Special Requirement (see ORIENTation <angle1> TO <angle2>) if the arrangement of the target or reference stars requires a particular orientation of the FGS field of view. ORIENTation may be required to keep the targets in the field of view when more than one target is observed.

The POSition TARGet <X-value>, <Y-value> special requirement can be used to offset a target from the aperture reference position. For the FGS, the aperture reference frame is centered near the geometric center of the field of view, and is not equivalent to the FGS detector reference frame. The two frames have different parities and origins (see figure 11.1 The FGS POS TARG and interferometer coordinate systems. and the FGS Instrument Handbook).

The SAME POSition AS <exposure> special requirement should be used to keep the position of the spacecraft held constant over the course of all observations within a visit. To ensure that all exposures within a visit are scheduled within the same orbit, use the SEQuence <exposure-list> NON-INTerruptible special requirement.

Time Per Exposure

The value to be entered is the photon-collecting time for measuring the position of the target. See the FGS Instrument Handbook for exposure time calculations.

Mode = TRANS
Configuration = FGS

This Operating Mode is useful for the measurement of the relative positions of close binary stars or the sizes of extended targets. In this Mode the interference fringe pattern (or "S-curve") is measured by scanning the FGS instantaneous FOV across the target at 45 degrees with respect to the interferometer axes and recording the Fine Error Signal (FES) every 0.025 sec.

Acquisition is accomplished with a nominal 15 arcsec-radius spiral search. The S-curve is measured by a series of continuous sweeps of the field of view across the target.

Moving targets can be observed if they are listed as a series of fixed targets. This observing strategy is discussed in the FGS Instrument Handbook.

Aperture or FOV

PRIME

FGS 1 is designated the "prime" FGS and is used for astrometry.

1, 2, 3

Requests use of a specific FGS (see figure 11.1 The FGS POS TARG and interferometer coordinate systems.).

Spectral Element

The available spectral elements are listed in Table 11.2: Spectral Elements for the FGS. Only one filter can be used at a time. The F583W is the recommended element for programs requiring the use of POS and TRANS modes combined because of the Optical Field Angle Distortion (OFAD) calibration, which is available only for F583W. F583W cannot be used if the target magnitude m<8.0. F5ND must be used if m<8.0. Unless special S-curve calibration observations are requested, only certain combinations of filter and target color will be calibrated. Please refer to the Instrument Handbook for more information.

Optional Parameters

SCANS
=1 (default) - 200

The value entered is the number of individual scans to make through the target, with alternate scan lines taken in the same direction.

STEP-SIZE
=1.0 (default); 0.3 - 10.0

The resolution by which the S-curve is sampled (in milliarcseconds) is specified by this Optional Parameter. The instantaneous field of view moves SQRT(2) x STEP-SIZE between samples (since the motion is always at 45 degrees with respect to the interferometer axes). STEP-SIZE values of 0.6 or 1.0 are recommended (see the FGS Instrument Handbook).

The exposure time is calculated as follows:

The minimum scan rate is 0.035 arcsec/sec along the diagonal.

ACQ-DIST
=0.0 - 90.0

USE WITH CAUTION-This Optional Parameter determines the size (in arcsec) of the acquisition search region. The default value is 15 arcsec. A larger search radius takes more time and is generally not necessary given HST's pointing performance.

Time Per Exposure

The exposure time (Texp) is the total photon-collecting time for all Nscan scans. The time per individual scan line is Texp/Nscan. See the discussion in STEP-SIZE above and the FGS Instrument Handbook.

Special Requirements

Enter the ORIENTation visit-level Special Requirement (see ORIENTation <angle1> TO <angle2>) if the arrangement of the target or reference stars requires a particular orientation of the FGS field of view. ORIENTation may be required to keep the targets in the field of view when more than one target is observed.

The POSition TARGet <X-value>, <Y-value> special requirement can be used to offset a target from the aperture reference position. For the FGS, the aperture reference frame is centered near the geometric center of the field of view, and is not equivalent to the FGS detector reference frame. The two frames have different parities and origins (see figure 11.1 The FGS POS TARG and interferometer coordinate systems. and the FGS Instrument Handbook).

The SAME POSition AS <exposure> special requirement should be used to keep the position of the spacecraft held constant over the course of all observations within a visit. To ensure that all exposures within a visit are scheduled within the same orbit, use the SEQuence <exposure-list> NON-INTerruptible special requirement.

Tables and Illustrations

Table 11.2: Spectral Elements for the FGS
Name Comments Effective Wavelength (Å) Full Width at Half Maximum (Å)
F583W "Clear" filter 5830 2340
F5ND Neutral Density (5 mag) ..... .....
F605W1 "Astrometry Clear" filter 6050 1900
F650W2 "Red" filter 6500 750
F550W "Yellow" filter 5500 750
PUPIL Pupil stop 5830 2340
1 F605W is to be specified with FGS3 and FGS1.

2 F650W is to be specified with FGS2 only.

 
 Figure 11.1: The FGS POS TARG and interferometer coordinate systems.
 

The POS TARG reference frame must be used to specify Special Requirements POS TARG and ORIENTation. Note that the FGS interferometer axes and POS TARG axes have different origins and parities. The interferometer reference frame is used in pipeline processing only.


 
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