HST Call for Proposals and HST Primer for Cycle 24

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4.3	Fine Guidance Sensor (FGS)

There are three Fine Guidance Sensors (FGSs) onboard HST. Two FGSs are used to point the telescope at a target and hold that target in the primary scientific instrument's field of view. This task can be performed with a 2 to 5 milliarcsecond pointing stability. The third FGS (FGS1R in Figure 2.2) is used as a sub-milliarcsecond astrometer and a high angular resolution interferometer. This instrument has two operating modes:

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In POS (Position) mode, the FGS measures the relative positions of objects in its 69 square arcminute field of view to a precision of ~1 milliarcsecond for targets with magnitudes 3.0 < V < 16.8. Position mode observing is used to determine relative parallax, proper motion, and reflex motion of single stars and binary systems. Multi-epoch programs have resulted in parallax measurements accurate to 0.2 milliarcsecond or less.

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In the TRANS (Transfer) mode, the 5" x 5" instantaneous field of view of the FGS is scanned across an object to obtain an interferogram with high spatial resolution. This is conceptually equivalent to an imaging device that samples an object’s PSF with 1 milliarcsecond pixels. The scientific goal of the TRANS mode is to study binary star systems (measure the separation, position angle, and relative brightness of the components), as well as to determine the angular size of extended objects such as the disks of resolvable giant stars or asteroids down to ~8 milliarcseconds.

By using a “combined mode” observing strategy, employing both POS mode (for parallax, proper motion, and reflex motion) and TRANS mode (for determination of visual and relative brightnesses of components in a binary), it is possible to derive the total and fractional masses of binary systems, indicating the mass-luminosity relationship for the components. Additional information can be found in the FGS Instrument Handbook.