9.2 FGS TRANS Mode

Observations with HST's FGS instrument can be done in TRANS Mode to obtain binary star positions or sizes of extended targets, and the Astronomer’s Proposal Tool (APT) has parameters for specifying these observations.

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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.

Mode=TRANS, Config=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° 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 1, 2, 3

Requests use of a specific FGS (see Figure 9.3.1 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 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 FGS Instrument Handbook for more information.

Optional Parameters


=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. 


=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) × STEP-SIZE between samples (since the motion is always at 45° 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:

  • Texp = [SCANS * t(samp) * L(axis)] / STEP–SIZE, where
  • SCANS = the number of individual scan lines,
  • t(samp) = 0.025 sec, and
  • L(axis) = the length of each scan in milliarcseconds as projected onto the x or y axis (see the FGS Instrument Handbook for recommendations and minimum length)

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


=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 (in seconds) 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 9.3.1 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.

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9.1 FGS POS Mode

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Version Cycle 32 May 2024

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