1.2 Pointing and Jitter

The HST Pointing and Control Systems group monitored the pointing jitter throughout the two-gyro on-orbit test. 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. A summary of the 10-second and 60-second jitter root-mean-square running averages and peak excursions is given in Table 1.1. The table lists the two-gyro mean, median, and maximum jitter values for the 10 and 60 second quantities in the sample of 454 exposures. 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 (see Chapter 5 of the HST Two-Gyro Handbook for more details).

Table 1.1: Jitter Summary

Jitter (milli-arcseconds, RMS)

10-sec Avg

Peak 10-sec

Avg 60-sec

Peak 60-sec

Two-Gyro Mean
5.6

6.5

6.0

6.2

Two-Gyro Median
5.5

6.2

5.7

6.0

Two-Gyro Maximum
9.5

22.2

10.7

18.0

Percentage of Two-Gyro Exposures with Jitter < 10 mas

100%

97.8%

99.1%

98.7%

Three-Gyro Mean
4.1

5.2

4.2

4.3

	Jitter (milli-arcseconds, RMS)
10-sec Avg	Peak 10-sec	Avg 60-sec	Peak 60-sec
Two-Gyro Mean	5.6	6.5	6.0	6.2
Two-Gyro Median	5.5	6.2	5.7	6.0
Two-Gyro Maximum	9.5	22.2	10.7	18.0
Percentage of Two-Gyro Exposures with Jitter < 10 mas	100%	97.8%	99.1%	98.7%
Three-Gyro Mean	4.1	5.2	4.2	4.3

Notes: 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. The jitter values measured during the two-gyro on-orbit test are only slightly larger than those observed in three-gyro mode. The two-gyro values are similar to those predicted by high fidelity simulations conducted in late 2004 and are significantly better than the conservative "worst case" jitter ellipse of 30 x 10 milli-arcseconds adopted for Cycle 14 Phase I preparations.

There was no loss of fine lock resulting from large pointing disturbances during any of the science observations obtained in two-gyro mode. Loss of lock did occur for 5 of the 36 acquisitions, but these failures have been traced to either bad guide stars or to a minor problem with roll adjustments during the acquisitions at the beginning of the second orbit of several visits. This latter problem is correctable with a minor change to the flight software. The acquisition success rate in two-gyro mode is expected to be >98% once the software patch is in place.

HST Two-Gyro Handbook Update for Cycle 14 Phase II Proposals

1.2 Pointing and Jitter