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FGS Data Handbook
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Fine Guidance SensorData Handbook > Chapter 5: Astrometric Error Sources > 5.1 Levels of Position Mode Errors

5.1
An FGS in Position Mode measures the relative angular separations of several objects. The objects generally will be separated by angles on the order of arc minutes, ranging over several magnitudes in brightness (possibly from V = 3 to 16.7), and will have different B-V colors. Because their parallaxes and proper motions need to be determined, HST will visit a given astrometric field several times, spanning perhaps two to four years or longer, over the course of an observing program. During each visit, the FGS will track each target in FineLock, one at a time, according to the instructions and sequence specified in the proposal.
Sources of error in these Position Mode measurements can be categorized into three distinct levels:
Observation level: errors associated with each individual FineLock acquisition and tracking sequence.
Visit level: errors involved in constructing a virtual plate for a given FGS astrometric visit.
Field level: errors that arise when comparing virtual plates of the same field taken during different visits.
Errors from lower levels will percolate through to the top.

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