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Aperture is the generic term used for any targetable area in an SI; a small slit, an occulting finger, different points on a CCD, a virtual aperture that encompasses a number of chips (e.g. WFALL), anything that can be specified in the APERTURE field of the proposal file, and a few more that can't. The scales, orientations, locations in the HST FOV, and a number of other properties of these targetable areas are calibrated and updated periodically. Though these positional calibrations largely fall under the calibration plans of the SI Groups, some special calibrations are made by the Observatory Science Group, which also maintains the aperture data and relations. This information, in the form of the Science Instrument Aperture File (SIAF.DAT), is installed into the HST Project Data Base whenever updates are calculated and needed.


In 1991, the Observatory Support Group developed code for accurately determining telescope pointing by extracting FGS guidestar measurements. 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. More recently however, the same algorithm has been used to produce `jitter files', part of the observation log files. 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 StarView.

Observers who employ this information scientifically should of course 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 the observation log documentation for a discussion of pointing error budgets.

A report on the program findtarget, which predated the routine jitter files, is available. Though the document is obsolete in that the jitter files now provide all the information that findtarget would, there is some pertinent description and background of general interest for pointing calculations.

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21 May 1998

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