How to Use this Handbook

This handbook is designed to help you manipulate, process and analyze data from the three Faint Guiding Sensors (FGSs) on-board the Hubble Space Telescope (HST). The book is presented as an independent and self-contained document, providing all the required information for understanding the FGS observations and obtaining high precision astrometric data on milli-arcsec scales. This can also be used as a cookbook, guiding users through various steps of FGS data reduction.

The HST Data Handbook has now been subdivided into separate volumes for each instrument. Users are referred to a companion volume, Introduction to the HST Data Handbooks, for more general information about the details of acquiring data from the HST archive, HST file formats, and general purpose software for displaying and processing HST data.

The current edition of the FGS Data Handbook is being written early in 2002, as we approach Servicing Mission 3b (SM3b). The behavior of this instrument is not likely to change after the servicing mission. Therefore, the material presented in this handbook is expected to remain up-to-date during the next cycles.

The present revision incorporates significant changes into the data handbook. These include details on data calibration and description of CALFGSA and CALFGSB IRAF/STSDAS tasks, required to process FGS Position and Transfer Mode observations for data analysis. Also, The Data Analysis chapter has been extensively revised to include details on how to run a set of stand-alone tasks to analyze both Position Mode and Transfer Mode data.

While the present version of the Data Handbook contains the latest information for accurate reduction and analysis of the FGS data, readers are advised to consult the FGS Website ( for the most recent up-dates regarding the FGS operation.