This section contains information about the performance of the Fine Guidance Sensors when HST is guiding in two-gyro mode. Observers should use the information in this chapter to assess and justify the feasibility of their proposed FGS observations.
Several FGS science observations were performed as part of the two-gyro on-orbit test. These observations were used to check the pointing stability as measured by FGS tracking of guide stars. The FGS POS-mode data analyzed to date indicates that RMS pointing errors within an orbit are typically 3.0-3.5 milli-arcseconds for a range of guide star magnitudes. This performance is similar to that observed in three-gyro mode and is in good agreement with the jitter estimates from the gyro data. Observers filling out their Cycle 14 Phase II proposals should consult Chapter 11 of the HST Two-Gyro Handbook
for restrictions on astrometric observations in two-gyro mode. For information about FGS performance in three-gyro mode, see the FGS Instrument Handbook
.
As a science instrument the FGS is used for high angular resolution observing in transfer (TRANS) mode and astrometry in positional (POS) mode. Spacecraft jitter and drift in three-gyro mode introduces a source of positional error that is typically much larger than the scientific measurement being pursued. For example, parallaxes can be measured to an accuracy of 0.2 milli-arcseconds (mas), while the drift of the FGS1r field of view during the course of an orbit can be as large as 20 mas. For both POS and TRANS mode data reduction, the FGS calibration pipeline uses the 40 Hz data from the guiding FGSs to model and remove jitter and drift over the time scales of a single POS mode exposure (typically 20 seconds) or single TRANS mode scan. Drift over times scales up to the duration of an orbit can be monitored, modeled, and eliminated during data analysis provided the proposer employs the appropriate "check star" strategy for POS mode observations (as described in this Handbook). For TRANS mode, the cross correlation of individual scans compensates for the drift.
In two-gyro mode, the pointing jitter displays a ~0.8 Hz periodicity with an amplitide of about 6 mas (peak-to-peak). Application of routine jitter and drift removal tools in the FGS calibration pipeline effectively eliminates this pointing error in the science data just as effectively as is in three-gyro mode. Therefore, the operation of HST in two-gyro mode is transparent to observers using the FGS as a science instrument.
The greatest impact to astrometry parallax programs in two-gyro mode, as shown in Figure 1.1: for an object at (,
) = (53°, -28°), will be the inability to schedule observations over an appreciable part of the object's parallactic ellipse, an effect which is more pronounced for low declination fields (see the general discussion of scheduling issues in Chapter 6). The inability to observe an object at both epochs of parallactic extremes in two-gyro mode will directly reduce the accuracy of the parallax measurement.
The above plot shows the schedulability (blue triangles) of observations in two-gyro mode for a target at (,
) = (53°, -28°) with observing windows of at least 30 minutes in length. Additional constraints may apply that would reduce the range of available roll angles, which in turn may preclude an observer from using an optimal set of reference field stars for positional astrometry.
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