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Pointing



Chapter 4: Figures

Some examples of the observation log data and characteristics are provided in this chapter:

Jitter Data for a 13-minute Exposure -- An example of the motion at the position of the Science Instrument aperture. The data are SI_V2 and SI_V3 from the CMJ file. The jitter is derived from the combined motion of the dominant and roll guide stars.

Parallel Observations -- The top panel displays the jitter along V2 (SI_V2 from the CMJ table) as a function of time at the first primary instrument aperture. The middle panel is the jitter derived at the position of the secondary instrument aperture. The slight slope in the jitter in the middle panel is real and is due to differential velocity aberration effects. During parallels, the telescope (and so the observation log software) applies the correct differential velocity aberration correction to the primary instrument. Therefore the secondary instrument pointing experiences a small residual error. Also, notice the small region of missing telemetry between 300 and 400 seconds. In this case, the entire data stream during that interval was not available. The bottom panel is the jitter from another primary FOC exposure. (Note each exposure has its own set of Observation Logs).

HST Orbital Trace -- The latitude and longitude (from the CMI table) during an exposure are plotted on a mercator projection of the Earth. Below is the magnetic field (also from the CMI table) along the V2 direction as a function of longitude.

Recentering during an Exposure -- The central panel shows the Recentering Flag as a function of time. During the recentering event, pointing control is transferred to the gyros and from the FGSs. During recenterings and losses of lock, the jitter and pointing data in the Observation Logs are defaulted to INDEF (or 1.6e+38). These periods are left as blank intervals in the upper and lower panels of this figure.

On occasion the telemetry format stream changes during an observation. Usually, some of the engineering data is lost during these switches. From the lower panel, the latitude as a function of time, the telemetry switches and change in update rate are readily visible. The telemetry switches are labelled with a "T" in the upper panel. The upper panel displays the jitter (averaged over 3 seconds) as a function of time. The data drops during the switches are seen as smooth lines in this panel.

RA during GYRO Mode, FN Format -- The change in the position of the RA is a result of two effects: gyro drift which is, on average 1 milliarcsec/sec, and omitting the differential velocity aberration correction. The telemetry was transmitted in FN and so velocity information was not available. (The absolute velocity aberration was coarsely removed by fitting a sinusoid to the data).

Target Acquisition Peak-Up -- During a peak-up sequence, the telescope is moved to search for the target in a raster pattern and once found by the SI, automatically moves to center the target. This figure is a plot of the RA and Dec of an object centered at the aperture reference position. Note that the jitter statistics during a peak-up, spatial scan, or moving target observation will be incorrect because they will incorporate the motion during the observation.