To guide the proposer through the FGS’s capabilities and help optimize the scientific use of the instrument, we have produced the FGS Instrument Handbook, the layout of which is as follows:

• Chapter 1: Introduction, describes the layout of the FGS Instrument Handbook and gives a brief overview of the instrument and its capabilities as a science instrument.
• Chapter 2: FGS Instrument Design, details the design of the FGS. Specific attention is given to the optical path and the effect of optical misalignments on FGS observations. The interferometric Transfer Function is described in detail, along with effects which degrade Transfer Function morphology. Descriptions of apertures and filters are also presented here.
• Chapter 3: FGS Science Guide, serves as a guide to the scientific programs which most effectively exploit FGS capabilities. The advantages offered by the FGS are described together with suitable strategies to achieve necessary scientific objectives. A representative list of publications utilizing the FGS for scientific observations is included for reference.
• Chapter 4: Observing with the FGS, describes the detailed characteristics of the two FGS observing modes - Position mode and Transfer mode - as well as the observational configurations and calibration requirements which maximize the science return for each mode.
• Chapter 5: FGS Calibration Program, describes sources of FGS errors, associated calibrations, and residual errors for Position and Transfer mode observations. A discussion of calibration plans for the upcoming Cycle is also included.
• Chapter 6: Writing a Phase II Proposal, serves as a practical guide to the preparation of Phase II proposals, and as such is relevant to those researchers who have been allocated HST observing time.
• Chapter 7: FGS Astrometry Data Processing, briefly describes the FGS astrometry data processing pipeline and analysis tools. The various corrections for both Position and Transfer mode observations are described along with the sequence in which they are applied.

In addition to the above chapters, we also provide two appendices:

• Appendix A: Target Acquisition and Tracking: Target Acquisition and Tracking, describes the acquisition of targets in both Position and Transfer modes. The target acquisition scenario may have implications for observations of moving targets or targets in crowded fields.
• Appendix B: FGS1r Performance Summary: FGS1r Performance Summary, describes the evolution of FGS1r during its first three years in orbit and the adjustment of the AMA to improve performance in this instrument.