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FGS Instrument Handbook for Cycle 24 |
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Each of the corrections and calibrations are briefly discussed in Chapter 5, and are thoroughly reviewed in the FGS Data Handbook Many of the corrections specified on the list are determined by analysis of individual observations and removed later in the data reduction processing, (e.g., jitter data is retrieved from the guide star telemetry and removed from the target data). Calibrations, such as the plate scale, the OFAD, lateral color, and cross filter effect are derived from STScI calibration programs. A potentially dominant source of error, the FOV drift (time scale of several minutes), must be measured during the visit, and to that end, the sequence of exposures must be carefully arranged.For example, a visit could contain 10–35 exposures, provided the overheads are minimized and exposure times are less than 20 seconds. A sample geometry is given in Figure 4.2, where the science target is represented by the central object.Figure 4.2: A Sample Visit GeometryIn Position mode, the FGS can track a bright target (V) whose motion is less than ~ 0.1 arcsec per second. However, planning the observation requires extreme precision: the moving target must be accurately located, to within 10 arcsec, in the IFOV at the start of the exposure, implying a very accurate ephemeris. Transfer mode observations of moving targets are discussed in the next section.