Two guiding modes are available: Gyro Hold, and Fine Lock. Fine Lock (PCS MODE FINE) is used by default, since use of Coarse Track may be harmful to the Fine Guidance Sensors. Use of Gyro Hold (PCS MODE GYRO) is not generally recommended, even for snapshot (SNAP) observations, since the pointing accuracy is only 14". Also the drift rate is 0.0014" s-1 so exposures >100s can result in smeared images. However, if the reduced pointing accuracy can be tolerated, and the exposures are only a few seconds or less, Gyro Hold can give a significant savings in the target acquisition overhead time.
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