The Fine Guidance Sensors (FGS), originally designed and built by the Perkin-Elmer Corporation in Danbury, CT (now Goodrich Corporation’s Optical and Space Systems), comprise a set of three radial-bay instruments on board the Hubble Space Telescope (HST). The main purpose of the FGS is to maintain the pointing stability of the telescope at the milliarcsecond level, often over exposure times as long as tens of minutes. The HST pointing requirements necessitated a design with a large observable field of view (FOV) with a high dynamic range in order to take advantage of the variety of observing scenarios HST was expected to encounter.

The resultant instruments, the FGS, are dual-axis white light shearing interferometers, each with a ~69 square arcminute FOV. Under nominal operating conditions, the FGS are routinely able point the spacecraft with a precision of ~2 mas or less. Unfortunately, the original design of the FGS did not compensate for the spherical aberration of the mis-formed HST primary mirror, and as a result the original FGSs suffered from degraded performance. In response to this, Goodrich re-engineered the spare FGS to include a commandable mechanism to mitigate the deleterious effects of the spherical aberration. This revised instrument replaced FGS1 during the second Hubble Servicing Mission in 1997. This device, now designated FGS 1R, was joined by FGS2R during the HST Servicing Mission 3a. Goodrich is currently upgrading the old FGS2 with this commandable mechanism for an expected return to the telescope (and replacement of FGS3) during the fourth HST servicing mission.

The high precision pointing capabilities of the FGS coupled with a fourteen magnitude dynamic range enable the FGS to perform as a high-precision astrometer and a high angular resolution science instrument. The 40 Hz readout time and detector stability allow for milli-magnitude relative photometry over orbital timescales and 1-2% relative photometry over long baselines (i.e., months). As a science instrument, the FGS have been used to determine the parallaxes of cataclysmic variable and dwarf novae stars (McArthur et al. 2001, Harrison et al. 2000), to provide dynamical mass constraints for Pre-Main Sequence evolutionary tracks (see Steffen et al. 2001), and to calibrate the optical mass-luminosity relationship at the end of the Main Sequence (Henry et al. 1999). The FGS have been used to search for the astrometric signatures of planets orbiting nearby stars (Benedict et al. 1999), to constrain the angular diameters of Mira variables (Lattanzi et al. 1997) and to place limits on the spatial extent of active galactic nuclei. Within the solar system, FGS observations have yielded information on the structure of the atmosphere of Triton (Elliot et al. 2000)

This chapter briefly describes how the Fine Guidance Sensors operate, summarizing its capabilities, its design, and its modes of operation. A more complete discussion of the design and of the scientific capabilities of the FGS can be found in the FGS Instrument Handbook. In addition, the FGS Web pages at STScI, http://www.stsci.edu/hst/instrumentation/fgs, are also valuable sources of information for the FGS user, particularly concerning technical details of the instrument, as well as a history of its performance throughout its lifetime.

1.1.1 The FGS as a Science Instrument

The FGS has two modes of operation: Position Mode and Transfer Mode. In Position Mode the FGS locks onto and tracks a star’s interferometric fringes to precisely determine its location in the FGS FOV. By sequentially observing other stars in a similar fashion, the relative angular positions of luminous objects are measured with a per-observation precision of about 1 mas over a magnitude range of 3.0 < V < 16.8. This mode is used for relative astrometry, i.e., for measuring parallax, proper motion, and reflex motion. Multi-epoch programs achieve accuracies approaching 0.2 mas.

In Transfer Mode an object is scanned to obtain its interferogram with sub-mas sampling. Using the fringes of a point source as a reference, the composite fringe pattern of a non-point source is deconvolved to determine the angular separation, position angle, and relative brightness of the components of multiple-star systems or the angular diameters of resolved targets (Mira variables, asteroids, etc.).
As a science instrument, the FGS is a sub-milliarcsecond astrometer and a high angular resolution interferometer. Some of the investigations well suited for the FGS are listed here and discussed in detail in Chapter 3:

• Relative astrometry (position, parallax, proper motion, reflex motion) with ­single-measurement accuracies of about 1 milliarcsecond (mas). Multi-epoch observing programs can determine parallaxes with accuracies approaching 0.2 mas.
• High-angular resolution observing:

-detect duplicity or structure down to 8 mas
-derive visual orbits for binaries as close as 12 mas.

• Absolute masses and luminosities:

-The absolute masses and luminosities of the components of a multiple-star system can be determined by measuring the system’s parallax while deriving visual orbits and the brightnesses of the stars.

• Measurement of the angular diameters of non-point source objects down to about 8 mas.
• 40Hz 1–2% long-term relative photometry:

-Long-term studies or detection of variable stars.

• 40Hz milli-magnitude relative photometry over orbital timescales.

-Light curves for stellar occultations, flare stars, etc.