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5.1. Fine Guidance Sensors (FGS)

The three FGSs (1) on board the HST are primarily used to accurately point and control its stability. The third FGS can be used for astrometry while the other two are locked on target. These instruments are ideal to study close binary stars, to determine proper motions and parallaxes as well as to detect companions.

Relative positions between multiple components of a stellar system can be measured to around 3mas up to a limiting magnitude V = 17, using one of the operation modes (POS). For the TRANS mode, the values are 10mas and V = 16; while the difference in component magnitudes is 4.

The FGSs are essentially interferometers with a total FOV of 69 square arc minutes and an instantaneous FOV of 5" x 5". Each FGS sensor includes a filter wheel with 5 filters. In their optical path there is a pick-off mirror, an aspheric collimator, two star selectors, a polarizing beam splitter, corrective optics and a Koester's prism. Light is collected in four PMT included in each FGS.

Three observing modes are available:

To acquire a target, the instrument first searches the region in a spiral pattern. If the search was successful the coarse track phase begins and finally fine lock.

FGS data are the so called S-curves, the graphical representation of the count difference of the two photomultipliers (normalized by the sum of these counts) versus the tilt angle of the wavefront at the entrance of the Koester prism. An example of this transfer function is shown in Figure 1.

Figure 1

Figure 1. The tranfer function for a typical FGS observation.

1 A detailed description can be found in the latest edition of the intrument handbook available on-line at Back.

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