| HST Two-Gyro Handbook | |||
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1.2 Two-Gyro Pointing Jitter and Image Quality
Initial on-orbit tests of the HST two-gyro fine guiding mode and its impact on science instrument performance were carried out on 20-23 February 2005. Gyro #1 was removed from the pointing control loop, and Gyros #2 and #4 were used with the FGS to control the HST attitude during all science observations. The excellent pointing stability observed in fine lock during these tests was confirmed as part of the two-gyro transition orbital verification activities in August 2005 with a different pair of gyros (Gyros #1 and #2). The HST Project decided to turn off Gyro #4 on 28-August-2005 to extend its lifetime in case it is needed in the future.
HST fine-pointing performance in two-gyro mode is essentially indistinguishable from the fine-pointing performance observed previously in three-gyro mode.
Jitter Estimates
The HST Pointing and Control Systems group monitored the pointing jitter throughout the two-gyro on-orbit tests. For each science exposure, they calculated the jitter at 25 milli-second intervals as estimated by the attitude control law used to maintain the HST pointing. Summaries of the 10-second and 60-second jitter root-mean-square running averages and peak excursions are given in Table 1.1. This table lists the two-gyro 10-second and 60-second mean, median, and maximum jitter values for a sample of 454 exposures obtained in February 2005. Almost all of the exposures have a mean jitter less than 10 milli-arcseconds. In a few cases, transient pointing disturbances caused small enhancements in the jitter. These types of disturbances are also commonly seen in three-gyro mode.
Table 1.1: Jitter Measured During the February 2005 Two-Gyro On-Orbit TestNotes: Two-gyro values are based on a sample of 454 exposures taken during the two-gyro on-orbit test (20-23 February 2005). Three-gyro values are based on a sample of 24 exposures taken several days prior to the two-gyro on-orbit test.The mean two-gyro 60-second-averaged jitter in Table 1.1 is slightly higher than the mean 10-second-averaged jitter because the sample includes several series of short dithered exposures; the 60-second running averages span short periods of slightly increased jitter between exposures as the pointing was changed from one dither position to the next. Jitter values measured after the August 2005 transition to two-gyro mode are similar to the values listed in the table. The two-gyro jitter is comparable to the jitter observed in three-gyro mode.
Image Quality
The jitter observed in fine lock in two-gyro mode is small enough that it does not noticeably impact the quality of HST observations, even those obtained with the Advanced Camera for Surveys High Resolution Channel (ACS/HRC), which has a plate scale of ~25 milli-arcseconds per pixel. Figure 1.1 shows a histogram of point spread function (PSF) widths measured in 186 ACS/HRC exposures obtained during the two-gyro orbital verification in August 2005. The PSFs have full widths at half maximum intensity (FWHM) that range from 1.89 to 2.19 pixels (or 47-55 milli-arcseconds). This is indistinguishable from the resolution achievable in three-gyro mode. The tail of FHWM values above 2.07 pixels in this figure occurs for data taken at a larger Sun angle (~115°) than the Sun angle for the remaining data (<90°). Slight orbital variations in focus caused by telescope breathing and other effects are present in two-gyro data as well as previous three-gyro data.
Figure 1.1: ACS/HRC Point Spread Function Widths in Two-Gyro Mode
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