4.2 Planning Position Mode Observations

4.2.1 Target Selection Criteria

When targets are selected for FGS Position mode observations, several options and requirements should be considered. These options are described below.


The bright limit for FGS1r is V = 8.0 without the neutral density filter in place. With the F5ND filter, objects of V = 3.0 or fainter can be observed. The faint limit is V ~ 17.0.

Near Neighbors

FGS target acquisition in Position mode will be unreliable if the target has a neighbor of comparable or greater brightness within a radius of 10 arcseconds. In essence, the FGS’s IFOV—a 5" × 5" box—expects to encounter the target star within the search radius. Companions of similar brightness within this search radius may be mistakenly acquired instead of the target. However, for magnitude differences Δm > 1, companions within ~6" will not affect the acquisition of the brighter target. Note that binary stars with component separations less than about 0.5" can be successfully acquired in Position mode, regardless of the Δm. Refer to the discussion under Section 4.2.5 for further details regarding the acquisition of binary systems in Position mode.

Target Field

The target field consists of the science target and reference stars. Observations of the reference stars will be used to define the local reference frame for relative astrometry. Since the optical field angle distortions are calibrated most accurately in the central region of the FOV, the pointing of the spacecraft (via POS_TARG commands—Chapter 6 for more details) should be specified to place the target field (as much as possible) in this area.

If the visit also includes Transfer mode observations of an object, the spacecraft pointing should be chosen to place the object at the FOV center, as this is the only location calibrated for Transfer mode. If the target field geometry requires the Transfer mode observations be executed at other locations in the FOV, special calibrations will be needed. Proposers should consult STScI’s Help Desk for assistance.

Reference Stars

Ideally, reference stars should have the following characteristics:

  • Have magnitudes in the range 8 < V < 15 to avoid the need for an F5ND cross-filter calibration and to minimize exposure times.
  • Be geometrically distributed around the target.
  • Have at least 10 arcsec distance between one another and from the science target to avoid acquisition of the wrong object.
  • Should fall within the FOV for all HST orientations specified in the proposal.

Check Stars

Check stars, which are a subset of the target list, are observed several times over the course of an orbit (visit). Two or more check stars, distributed across the field, provide the information needed to characterize the drift of the FGS’s FOV on the sky (which is typically about 4 mas over the course of the visit). Each check star should be observed at least three times. The best check stars are brighter than 14th magnitude to minimize exposure time, and should include the science object for the highest accuracy astrometry.

4.2.2 Filters

Table 4.1 is a listing of the FGS1r filters, their calibration status and applicable brightness restrictions. (Refer back to Figure 2.11 for the filter transmissions as a function of wavelength.)

Table 4.1: Filters for which FGS1r will be Calibrated


Calibration Status


Target Brightness Restrictions



“Clear” filter; OFAD calibration filter; Position Mode Stability Monitor



Pos Mode Cross Filter calibration with F583W; limited to selected locations within the FOV.

Required for targets w/
3.0 < V < 8.0;



Not calibrated

V > 7.5



Not calibrated

V > 8.0



Not calibrated

V > 7.5


Only the F583W filter will be calibrated for Position mode for the full FGS FOV. Filter F5ND will be calibrated only at selected locations within the FOV

PUPIL Not Recommended for Position Mode

Occupying the fifth slot on the wheel is the PUPIL. It is not a filter but rather a 2/3 pupil stop. Use of the PUPIL significantly reduces the degrading effect of spherical aberration (which does not necessarily improve Position mode performance) but collaterally alters the field dependence of the distortions. Consequently, the OFAD calibration for the F583W filter cannot be applied to PUPIL observations. In addition, PUPIL observing attenuates the object’s apparent brightness by nearly a full magnitude, which sets the faint limiting magnitude at about V=16 while making observations of stars fainter than V = 14.5 excessively time consuming.

FESTIME and Signal-to-Noise

Photon statistics dominates the noise in the measured position of stars fainter than V ~ 13.0. To track fainter objects, the Fine Error Signal must be integrated for longer periods. Table 4.1 lists the default FESTIMES for various target magnitudes. The default FESTIMES, determined from the Phase II target magnitude, are appropriate for most observations, and are set to ensure that photon noise, when converted into the Noise Equivalent Angle (NEA), does not exceed a predefined angular error threshold. The NEA is given by the relation

\mathrm{NEA}=\Biggl(\frac{1}{1.51\times10^7}\Biggr)\times \frac{\sqrt{0.5\times C+B}}{0.5\times C\times\sqrt{t}}~.

The NEA is used by the proposal processing tool (APT) to set the default FESTIME time. The parameter C is the total count rate expected from the target summed over all four PMTs, B is the background count rate, and t is the FESTIME. The NEA is plotted as a function of magnitude and FESTIME in Figure 4.1. C as a function of filter and magnitude for FGS1r is given by:


The constant f-factor is a function of the filter and the target’s spectral color. Table 4.3 provides the f-factor for each combination of filter and color. The default FES times used by the proposal processing software for Position mode measurements are listed in Table 4.1.

Table 4.2: Default FES Times

V Magnitude














Figure 4.1: Default FESTIME as a Function of V Magnitude for F583W FGS1r: NEA as a Function of Magnitude and FESTIME

Table 4.3: F-factor Transmission Estimator for Combination of Filter and Color























4.2.3 Background

Background noise includes cosmic ray events, particle bombardment during passages through the South Atlantic Anomaly (SAA), and scattered light falling in the 5 x 5˝ IFOV. Cosmic ray events are suppressed by special circuitry and the FGS is prohibited from operating while transiting regions of heaviest impact from the SAA. Table 4.4 gives the typical dark + background counts for FGS1r in 0.025 seconds. Typically these values appear to be valid for all observations of isolated targets (suggesting that the dark counts dominate the background contribution). If the background counts for a specific observation are needed for the analysis of the observation, such as when the source is embedded in significant nebulosity or in a crowded star field, it can be obtained from the photometry gathered during the slew of the IFOV to (or away from) the target position. These data extracted by the FGS pipeline package CALFGSA from the FITS files that input are cleaned of spikes from “interloping stars” and can be used to estimate the background levels during post-observation data reduction.

Table 4.4 lists the average dark+background counts/25 msec for each of the FGS1r PMTs. These data were serendipitously gathered over a 45 minute interval from a failed science observation (the target was not acquired due to a guide star problem). These data have proved invaluable for the analysis of Transfer mode observation of faint stars (V>15).

Table 4.4: FGS1r: Dark Counts


Average Background + Dark
Counts per 0.025 sec









4.2.4 Position Mode Exposure Time Calculations

The exposure time is the minimum time that an object will be tracked in FineLock. Based the rate at which the measured location (or centroid) of a star converges (from analysis of FGS1r data) Table 4.5 lists the recommended exposure times as a function of target magnitude. We note that:

  • Exposures should be as short as possible to allow for more individual observations during the visit, but should be longer than HST’s mid-frequency oscillations (~10 seconds).
  • Usually, Position mode observations yield an additional 10 to 20 seconds of FineLock data in excess of the exposure time specified in the phase II proposal (a result of unused overheads). Hence, specifying a 10 second exposure results in 20 to 30 seconds of FineLock data.

Table 4.5: Recommended FGS1r Exposure Times


phase2 exposure time
(in sec)





4.2.5 Exposure Strategies for Special Cases

Observing Binaries and Extended Sources in Position Mode

Multiple or extended sources in the FGS’s IFOV will result in a reduction of the amplitude of the observed interferometric fringes (relative to that of a point source). This occurs because light from multiple sources in the IFOV do not interact coherently (the observed rays originate from different angles on the sky). Therefore, multiple point source fringes will be superimposed upon one another, each scaled by the relative brightness of the source and shifted by its relative angular displacement on the sky. The result is a composite Transfer Function with reduced fringe visibility.

The fringe visibility reduction for the brighter component of a binary system with an angular separation along the X or Y axis greater than about 80 mas (i.e., when the individual S-Curves are fully separate) is given by:


where fa and fb are the intensities of the brighter and fainter components, respectively. A similar expression, but with l_b in the numerator, is appropriate for the faint star S-curve (see Figure 4.3 for examples).

For projected angular separations less that 80 mas, the Transfer Function will be a blend of the merged point source S-Curves. The resultant fringe visibility will depend on the relative brightness and the angular separation of the components (i.e., Fr is more difficult to predict).

Even significant loss of fringe visibility does not pre-dispose the object from being successfully observed in Position mode. To be acquired in FineLock, an object’s Fine Error Signal (see Appendix A) must exceed a fringe detection threshold (see Figure A.2). The threshold is set on the basis of the target’s V magnitude, as entered in the proposal, to accommodate the acquisition of faint targets. (The fainter the target the more effectively the background and dark counts reduce the fringe amplitude, hence lower detection thresholds must be applied.) If the GO were to state the V magnitude of a binary system or extended source to be sufficiently faint, (regardless of its true value), then the observed fringes will exceed the (lower) detection threshold, and the FGS will successfully acquire the object. However, if a false magnitude is specified, one should also manually set the FESTIME (an optional parameter) to the value appropriate to the object’s true magnitude. Otherwise, the observation’s overheads will be excessively long.

Some binary systems are not reliably observed in Position mode, even with the adjustment to the fringe detection threshold. Objects in this category include those with components exhibiting small magnitude differences (Δm < 1) and angular separations greater than 60 mas but less than 800 mas (as projected along an interferometric axis). In these cases, either star may be acquired. There have been cases where one component was acquired on the X-axis while the other was acquired on the Y-axis. Such data are still useful, but care must be applied in the post- observation data processing.

There is a class of binary stars which cannot be observed in Position mode. In a FineLock acquisition (see Appendix A), the WalkDown to FineLock is a finite length path (approximately 0.810") beginning at a point which is “backed off” a fixed distance from the object’s photocenter. If the fringes of both stars lie outside this path, then neither will be encountered and the FineLock acquisition will fail. The condition for such a failure is the following,

where X is the location of the system’s photocenter, ra and rb are the distances from the photocenter to the fringes of the components “a” and “b” respectively, la and lb are the flux from each component, xs is the starting position of the WalkDown, and xl is the length of the WalkDown. If the position of the binary along either the X or Y axis is known to meet this failure requirement, Position mode observations of this system should not be attempted.

It is recommended that a proposer contact the STScI Help Desk for assistance with Position mode observations of binary systems.

4.2.6 Sources Against a Bright Background

For sources against bright backgrounds, the fringe visibility function is reduced by I / (I + B) where I is the point source flux and B is the background flux. The proposer should contact the STScI Help Desk for assistance with such observations.

4.2.7 Crowded Field Sources

Crowded fields create two problems for FGS observations:

  • A nearby star (< 10 arcsec away) of similar magnitude could be acquired during the search phase.
  • The background brightness in the 5 × 5 arcsec aperture may be increased by the presence of numerous faint stars or nebulosity.

The proposer should consult the STScI Help Desk for assistance with such observations.


Proposers should document—in the proposal—the logic for selecting a ­FESTIME or entering a false apparent magnitude of a target.