Space Telescope Science Institute  Chapter 2:  Special Considerations for Cycle 15 Chapter 3:  Introduction to ACS

2.1 Two-gyro Guiding


HST 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, the HST Project has decided to turn one of the functioning gyros off and to upload a new attitude control system that functions with only two gyros in the fine pointing control loop. This new two-gyro control mode was tested extensively during the past year and will be used during Cycle 15. Tests of the HST pointing capabilities while guiding with the new attitude control algorithms in place indicate that the pointing performance in two-gyro mode is essentially indistinguishable from previous performance in three-gyro mode.

The differences in the ACS point spread function (PSF) properties in two-gyro mode and three-gyro mode are so small that the information in this Instrument Handbook is applicable to either two-gyro or three-gyro operations. The formal changeover to 2-gyro operations occurred on August 28, 2005. To keep up to date with the status and performance of this new guiding mode, please check the Two-Gyro Science Mode Web site:

http://www.stsci.edu/hst/HST_overview/TwoGyroMode,
 

for up to date information about HST pointing in this mode. Details about the visibility and scheduling of observations in two-gyro mode can be found in the HST Two-Gyro Handbook at:

http://www.stsci.edu/hst/HST_overview/TwoGyroMode/handbook/
cover.html. 
 

Additional information and restrictions on observations can be found in the Cycle 15 Call for Proposals at:

http://www.stsci.edu/hst/hst/proposing/documents/cp/.
 

Chapter 2:  Special Considerations for Cycle 15 Chapter 3:  Introduction to ACS
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