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37.7 Spatial Uncertainty: Target Acquisition Problems

Most of this topic was discussed in the chapter on data assessment and in GHRS ISR 079, "Interpreting GHRS Target Positioning". An onboard target Acquisition performed a spiral search followed by returning to the dwell point with the most counts. The end phase of the onboard acquisition was the locate phase (coarse y and x centering with a fine x balance at the end of the locate phase).

GHRS target acquisitions of very bright stellar targets could fail during the locate phase. This resulted from low contrast between the central four diodes of the eight diodes used for acquisitions. The high count rate in the central four diodes flattened the point spread function (PSF) leading to wrong centering in the aperture. The SSA science observations in this case may contain just noise.

The header keyword FINCODE value indicates the success or failure of the GHRS observation. FINCODE=102 indicates the observation completed successfully as planned, but does not indicate whether the target was centered in the aperture. Any other value for FINCODE indicates a possible problem. During RSDP pipeline calibration, if a FINCODE value is encountered other than 102, a warning message is written to the trailer file (.trl).

From April 1989 through August 1994, OSS (now part of OPUS) created an Observation Comment file (extension .ocx) for every HST observation. After that time through January 1995, OSS created an .ocx file for degraded observations. If an .ocx file exists for an observation, it should be checked for anomalies. (See Chapter 38 for more information.)

If OMS observation logs were created for an observation, they should be checked to verify that no anomalies occurred during the target acquisition.

Low Count Rate

Observations with low count rates, containing mostly noise, may contain few detected or no counts from the target. Calibrated data may have a bell or U-shaped appearance, quantized data values, or extremely low flux values. These data indicate a missed target, a target too faint for the GHRS, or inappropriate use of the GHRS.

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