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In 1991, the Space Telescope Science Institute developed code to accurately determine telescope pointing by extracting FGS guidestar measurements from engineering telemetry data. The output mapped any point in the telescope's focal plane, such as an aperture, onto RA and DEC. The determination was accurate to the level of the guidestar positional uncertainty and any errors in our knowledge of focal plane geometry, distortions, etc.

The same algorithm was used to produce 'jitter files', part of the Observation Log files produced by the Observation Monitoring System (OMS) from 20 Oct 1994 through February 2003. OMS has since been replaced by the Engineering Data Processing System (EDPS), and with that change came a revision of the jitter file format definition. More information about this change and on the new jitter files can be obtained from the Jitter File Format Definition.

Jitter files are routinely available tables and images that trace an aperture or reference pixel's RA and DEC during an observation. These files are archived for all observations after 20 Oct 1994, and are available, along with the science data, through the HST Archive.

Historical information on the OMS Observation Logs is available from the link above. This link also contains descriptions of pointing calculations, sources of pointing error, and cautions on the use of jitter files for pointing information.

Observers who employ the information contained within the jitter files for scientific anlaysis should be aware that accurate absolute astrometry depends on many calibrations, and that such a pointing determination is only as accurate as guidestar positions, positional knowledge of the focal plane, and internal instrument calibrations. HST pointing generally has extremely good relative precision. See either the Instrument Handbooks for each particular instrument, or the discussion in the OMS Observation Log Documentation for a discussion of pointing error budgets.

For further detailed information please see the HST Pointing Documentation page.