9.1 FGS POS Mode

Observations with HST's FGS instrument can be done in POS Mode to obtain single star positions, and the Astronomer’s Proposal Tool (APT) has parameters for specifying these observations.

Expand/Collapse all...

On This Page

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.

Mode=POS, Config=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

1, 2, 3

Requests use of a specific FGS (see Figure 9.1: FGS POS TARG and Interferometer Coordinate Systems.

Spectral Element

The available spectral elements are listed in Table 9.2: 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 or an Instrument Scientist ( at https://stsci.service-now.com/hst ) 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.


=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. 


=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 or an Instrument Scientist). 


=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 that the Instrument Scientist who reviews your Phase II program can assess your calibration needs.


=FINE (default); COARSE; 0.0 - 1.0

Tracking of the target in fine lock (FINE) or coarse track (COARSE) may be selected with this Optional Parameter. The fraction of the exposure time in fine lock may be specified by entering a value between 0.0 and 1.0. If 0.0 is entered, fine lock will be tested but no measurements will be made. LOCK=FINE and LOCK=1.0 produce identical results. If LOCK=COARSE, the values of several other Optional Parameters are overridden: FES-TIME and NULL will be ignored.

Time Per Exposure

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

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 9.1: 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.

Expand/Collapse all...

Related Links

9.2 FGS TRANS Mode

Table of Contents

Change Log

Version Cycle 32 May 2024

      PROPINST-91407- Improve the links to other documents.