Chapter Contents

• 1.1 Purpose
• 1.2 Instrument Handbook Layout
• 1.3 The FGS as a Science Instrument
• 1.4 Technical Overview
• 1.5 Planning and Analyzing FGS Observations
• 1.6 FGS Replacement in SM4

The precision pointing required of the Hubble Space Telescope (HST) motivated the design of the Fine Guidance Sensors (FGS). These large field of view (FOV) white light interferometers are able to track the positions of luminous point source objects with ~1 millisecond of arc (mas) precision. In addition, the FGS can scan an object to obtain its interferogram with sub-mas sampling. These capabilities enable the FGS to perform as a high-precision astrometer and a high angular resolution science instrument which can be applied to a variety of objectives, including:

• relative astrometry with an accuracy approaching 0.2 mas for targets with V < 16.8;
• detection of close binary systems down to ~8 mas, and characterization of visual orbits for systems with separations as small as 12 mas;
• measuring the angular size of extended objects
• 40 Hz relative photometry (e.g., flares, occultations, transits) with milli-magnitude accuracy.

The purpose of this Handbook is to provide information needed to propose for HST/FGS observations (Phase I), to design Phase II programs for accepted FGS proposals (in conjunction with the Phase II Proposal Instructions), and to describe the FGS in detail.