Space Telescope Science Institute   6.1 Phase II Proposals: Introduction  6.3 Special Requirements

6.2 Instrument Configuration


This section is designed to complement the section on FGS instrument parameters contained in the Phase II Proposal Instructions. Table 6.1 summarizes the FGS instrument parameters.

Table 6.1: FGS Instrument Parameters
Config
Aperture
Sp_Element
Opmode
Optional_Parameters
FGS
1, 2, 3
(FGS1=FGS1r)
F5ND
F550W
F583W
F605W(FGS1r,FGS3)
F650W (FGS 2)
PUPIL
POS
ACQ-DIST, ACQ-MODE, COUNT, FESTIME, NULL, LOCK
TRANS
ACQ-DIST, ACQ-MODE, SCANS, STEP-SIZE

Aperture

The aperture must be defined as either "1", "2", or "3". Only FGS1r (aperture=1) is calibrated as a science instrument. FGS2 and FGS3 should not be used for science observing, For illustrations of the FGS1r field of view in the HST focal plane, refer to Chapter 1.

Spectral Element

"Spectral Element" refers to filters, etc. chosen for the observation. For FGS1r, the available filters are: F583W, F5ND, PUPIL, F605W, and F550W. Only one filter can be in place for an exposure, though multiple filters can be used during a visit (check that calibrations are available for the configuration). Recommendations for specific mode and filter combinations are discussed in Chapters 5 and 7. Table 6.2 presents a summary of the calibrated modes and filter configurations.

Table 6.2: Summary of Calibrated Mode and Filter Combination
Mode
Filter
Status
Comments
Pos mode:
F583W
Calibrated across entire FOV
OFAD in F583W only
Pos mode
F5ND
Calibrated at the center and a small number of selected locations in the FOV
Cross-filter calibration with F583W
Trans mode:
F583W, F5ND;
Others as needed
  • All calibrations at FOV center
  • Select reference S-Curves from library of standard stars
  • If needed, propose for calibration time to observe a reference standard star at the same epoch as the science visit (see Chapter 7).
Availability of standard stars with appropriate spectral energy distributions, i.e., colors of reference star and science target should differ by no more than (B-V) ~ 0.3 for a given filter (F583W or F5ND).

Mode

Two operation modes are available: Position mode (POS) for measuring relative astrometric positions of targets in the FGS FOV and Transfer mode (TRANS) for obtaining high angular resolution interferometry.

6.2.1 Optional Parameters for FGS Exposures

A program is customized by specifying optional parameters for each exposure. Table 6.3 and Table 6.4 list the optional parameters, default values, and recommended settings for Pos and Trans modes, respectively.

Many of the optional parameters have default values. In such cases, the entry for that optional parameters may be omitted. If an observer wishes to override the default value, the parameter and its value must be specified.


Table 6.3: Pos Mode Optional Parameters  
Optional
Parameter
Allowed
Values
Comments and Recommendations
ACQ-DIST
DEF (default=10),
0.0 to 90.0

Units: arcsec
Determines the size (in arcseconds) of the search region. For ACQ-MODE=SEARCH, ACQ-DIST is the maximum radius of the search spiral.

Recommendation: Although Search radii up to 90 arcsec are allowed, larger values must be used with caution, i.e., the larger the search radius, the more potential to acquire a spurious nearby star. If the target coordinates are expressed with respect to the Guide Star System, the default value of 10 arcsec (radius) should be sufficient to find the target. Typically the target is found with 0.3 to 1.0 arcsec. Note that large search radii incur a large overhead (time) cost.

Reference: Appendix 1: Target Acquisition
ACQ-MODE
SEARCH (default)

Units: None
Determines the strategy for locating the target. The Search Phase is the initial outward spiral to find and identify the target. The sequence of Search, CoarseTrack, WalkDown, and FineLock is the standard astrometry target acquisition scenario. No other options are available for GO programs.

Requirement: Default "SEARCH"

Reference: Appendix 1: Target Acquisition
COUNT
DEF (default), 1-2621400

Units: FGS PMT counts
Expected count rate [target + background]. When the default is set, the count rate is calculated (by the proposal processing software) from the target V magnitude, filter, and aperture. The default background rate (from in-orbit measurements) is 712 counts/sec for all four PMTs.

Recommendation: Allow the system to select the default based upon the target's specified visual magnitude. The default values are calculated using data from on-orbit photometric calibrations. The value of COUNTS is also used to determine the default FESTIME.

A few special cases may require non-default values and are discussed in the text.

Reference: Chapter 7: See sections on Pos and Trans mode Signal to Noise and Exposure calculation sections.
FESTIME
DEF (default), 0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2

Units: seconds
Sets the averaging time for the Fine Error Signal (in seconds). The FESTIME also determines the rate at which the star selectors are adjusted to null the fine error signal. The default value for Pos mode is calculated from the COUNT parameter if explicitly entered, or if COUNT defaults to the V magnitude and the filter. All Trans mode observations use FESTIME = 0.025 seconds.

Recommendation: The algorithm that selects the default FESTIMEs from the specified brightness of the target is based on photon statistics and a conversion from photon noise to equivalent positional error. For fainter targets, longer integration times are supplied by the default algorithm to assure that the FGS can track the fringe and remain in FineLock.

For non-point sources, the S-Curve amplitude is reduced because of the increased contribution of the background or companion. In these cases, it may be desirable to adjust the FESTIME. Please consult with STScI.

Reference: Chapter 7: See Sections on Pos and Trans mode Signal to Noise and Exposure calculation sections.
LOCK
FINE (default), COARSE, 0.0-1.0

Units: None
The method used by the astrometer to track the science or reference target. (Note, this parameter does not specify the HST guiding mode). Because of the excessive wear to bearings inside the instrument, CoarseLock, which is inferior to FineLock, is no longer allowed.

Requirement: Fine.

Reference: Chapter 5 and Chapter 7.
NULL
YES, NO (default)

Units: None
Determines whether the next FESTIME begins immediately after the previous one or not until the star selectors have been repositioned. This parameter may be applicable for very fast moving target. Observers should consult with STScI before specifying non-default values.

Recommendation: This use of this parameter is limited to a few specific observation scenarios (tracking solar system objects). Most standard astrometry programs benefit from NULL=NO.

Reference: STScI will alert the proposer if a non-default value is required.


Table 6.4: Trans Mode Optional Parameters  
Optional
Parameter
Allowed
Values
Comments and Recommendations
ACQ-DIST
DEF (default = 10),
0.0 - 90.0

Units: arcsec
Determines the size (in arcseconds) of the search region. For ACQ-MODE=SEARCH, ACQ-DIST is the maximum radius of the search spiral.

Recommendation: Although a search radius of up to 90 arcsec is allowed, larger values must be used with caution, i.e., the larger the search radius, the more potential to acquire a spurious nearby star (spoiler). If the target coordinates are expressed with respect to the Guide Star System, the default value of 10 arcsec (radius) should be sufficient to find the target. HST's pointing accuracy usually places the target within 1 arcsec of the expected location in the FGS. Note that large search radii incur a large overhead (time) cost.

Reference: Appendix 1: Target Acquisition
ACQ-MODE
SEARCH (default)

Units: arcsec
Determines the strategy for locating the target. The search phase is the initial outward spiral to find and identify the target. The sequence of Search, CoarseTrack, WalkDown, and FineLock is the standard astrometry target acquisition scenario. No other options are available. (Trans mode fringe scanning commences after the FineLock acquisition.)

Requirement: Default "SEARCH"

Reference: Appendix 1: Target Acquisition
SCANS
1 (default)-200

Units: None
Determines the number of separate scan lines. Successive scans are taken in opposite directions along a 45 degree diagonal path in the FGS detector space coordinates.

Recommendation: Determine the number of scans to achieve the required signal-to-noise.

Reference: Chapter 7
STEP-SIZE
1.0 (default)
0.3-10.0

Units: mas
The average angular distance (in mas) stepped by the IFOV along an interferometric axis in 0.025 seconds. To achieve the best results, step sizes of 0.3 or 1 mas should generally be selected.

Recommendation: Value depends on the science target and goals of the observation, required resolution and signal-to-noise.

Reference: Chapter 7


 6.1 Phase II Proposals: Introduction  6.3 Special Requirements
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