Instrument Performance

Guiding Modes with the FOC

The FOC will default to fine lock (estimated RMS jitter 0.005 arcseconds out of day/night transitions) for all configurations. Users may not specify coarse track for their observations since it has been determined that guiding in this mode has a detrimental effect on the FGSs. Similarly, gyro hold may not be used because the large pointing and stability uncertainties are not conducive to optimization of the science.

Occasionally, it will be found that two guide stars are not available for an observation. In that case, STScI will notify the PI and offer the possibility of single guide star observing. For relatively short exposure times (< 2000 s), the impact on performance is very small. The observation will not be executed in this mode unless the PI gives approval.

Sometimes the FGSs fail to achieve fine lock for an observation. In this case, the Take Data Flag comes down, the shutter is not opened, and the users are notified of what happened.

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