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FGS Data Handbook
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Fine Guidance SensorData Handbook > Preface > Handbook Structure

Handbook Structure
The FGS Data Handbook is organized in five chapters, which discuss the following topics:
Chapter 1:Instrument Overview presents a brief overview of the scientific capabilities of the Fine Guidance Sensors, their readout modes, and data products. The material presented here is excerpted from the information presented in the FGS Instrument Handbook, and we refer the reader to that document for more complete information about the properties of the FGS as a science instrument.
Chapter 2:FGS Data Products describes how to identify and interpret the contents of FGS data files. An FGS data set received from the Archive contains information from all three FGSs, plus supporting data on the spacecraft itself. These files arrive in FITS format and need to be converted to GEIS format before processing.
Chapter 3:Calibration describes the routine reduction and calibration of raw FGS data. Unlike data from other HST instruments, data from the FGS are not calibrated by an automated pipeline process. This task is left to the user. To facilitate FGS data calibration, STScI provides two IRAF/STSDAS tasks - calfgsa and calfgsb - which enable observers to calibrate their data using the current best set of reference files and algorithms. This chapter describes these tasks in detail, and should serve as a guide for when to use calfgsa or calfgsb on FGS data.
Chapter 4:Data Analysis provides insight and instructions for the analysis of FGS astrometry data. This includes the reduction of multi-epoch Position Mode observations (used, for example, in the determination of an object’s parallax), multi-epoch Transfer Mode observations (used to derive a binary system’s orbital parameters), as well as the detailed inspection of the data from a single exposure or observation.
Chapter 5:Astrometric Error Sources discusses those uncertainties, both statistical and systematic, which remain after the pipeline calibrations of raw Position Mode and Transfer Mode observations. Each step in the calibration procedure leaves its own residuals which contribute to the overall error budget.
There are some important pieces of general information about HST data, the HST Archive, and the IRAF and STSDAS analysis software that are not specific to the FGS, and which are therefore not discussed here in the FGS Data Handbook. We refer the reader to the most recent version of the companion Introduction to the HST Data Handbooks for this information. Additional help with HST data is always available via email to the STScI Help Desk at help@stsci.edu.

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