1. STScI
2. HST
3. Two Gyro Mode Handbook

	HST Two-Gyro Handbook

Chapter 1:
Introduction to Two Gyro-Mode

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1.1 Introduction

This Handbook is a shortened edition of versions that were released previously. It contains only the basic two-gyro information needed to submit your Phase I proposal. Observers interested in the HST gyroscopes, slewing and guide star acquisition processes in two-gyro mode, and the impact of observatory disturbances on HST pointing should consult earlier versions of the Handbook.

The Hubble Space Telescope was originally designed to use three rate-sensing gyroscopes to provide fine pointing control of the observatory during science observations. In order to conserve the lifetime of the HST gyros, one of the functioning gyros was turned off on 28-August-2005 and a new attitude control system that functions with only two gyros was activated. In this mode, two gyros used in combination with the Fine Guidance Sensors provide fine-pointing information during science observations.

The HST Two-Gyro Handbook contains information about target visibility and scheduling of HST observations conducted with an attitude control system relying upon just two gyroscopes. Use the Handbook in conjunction with the appropriate Instrument Handbooks and the Hubble Space Telescope Call for Proposals and Hubble Space Telescope Primer when assembling your Cycle 16 Phase I proposal. The individual Instrument Handbooks contain detailed technical information about the science instruments and their on-orbit performance. The Call for Proposals contains policies and instructions for proposing.