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FGS Instrument Handbook for Cycle 24 |
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7.3.1 Position ModeThe correction of the astrometry centroids for vehicle motion (as determined by changes in guide star positions) is referred to as pos-mode dejittering. Transient corrections can be as large as 3 to 5 mas, such as when HST enters orbital day, but the adjustments are typically small—less than 1 mas. This underscores the excellent performance of HST’s pointing control system under the guidance of the FGSs.“Drift”, as discussed in Chapter 5, is defined as the apparent motion of the astrometer’ s FOV on the sky during the course of the visit as detected by the astrometry targets that are observed more than once during the visit (the check stars). Drift must be removed from the measured position of all astrometry targets. This is accomplished by using check star data to construct a model to determine the corrections to be applied. If at least two check stars are available and were observed with sufficient frequency (i.e., at least every seven minutes), a quadratic drift model (in time) can be used to correct for both translation and rotation of the FOV. The availability of only one check star will limit the model to translation corrections only. If the check stars were observed too infrequently, then a linear model will be applied.The visit level corrections to Position mode observations, i.e., pos-mode de-jittering and the drift correction are performed by calfgsb (Figure 7.3).7.3.2 Transfer Mode