|FGS Instrument Handbook for Cycle 24|
This section is designed to complement the section on FGS instrument parameters contained in the Phase II Proposal Instructions. Table 6.1, FGS Instrument Parameters summarizes the FGS instrument parameters.
Table 6.1: FGS Instrument Parameters 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” 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 Chapter 5 and 7. Table 6.2, Summary of Calibrated Mode and Filter Combination presents a summary of the calibrated modes and filter configurations.
Availability of standard stars with appropriate spectral energy distributions, i.e., colors of reference star and science target should differ by no more than δ(B-V) ~ 0.3 for a given filter (F583W or F5ND).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.A program is customized by specifying optional parameters for each exposure. Table 6.3, Pos Mode Optional Parameters and Table 6.4, Trans Mode Optional Parameters 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 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. 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” 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. 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. 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. 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 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. 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” 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. 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.