In addition to a long term shrinkage of the OTA, HST undergoes focus variations having a period of an orbit, and also lower frequencies as combinations of sun angle, off-nominal roll, earth heating, shadowing by parts of the vehicle, and day/night produce longer term temperature trends.
The Engineering Team at STScI has invested effort in modeling this scatter. The most effective model succeeds in reducing the RMS scatter in our routinely collected focus data set by approximately 25-50%. A large number of temperature telemetry points from stations throughout HST were combined to produce a resulting function which tracks the OTA focus. This model does not provide a complete description of thermal focus variations. As well it does not describe the long term, non-thermal effects. A more detailed discussion can be found in the document, Modeling HST Focal-Length Variations (PDF, 467KB).
The baseline focus can be obtained by inspecting the plots:
You may look up the modeled variation around the baseline focus value, for the exact time of your observation. Tabulated below are links to quarterly files containing focus model values. The values are given in microns of secondary mirror and represent offsets or excursions from some overall focus state which can be estimated by inspection of the focus history plots. Add the looked up model value to your estimate of the baseline focus state for the best determination of the HST focus at a given time. Zero microns is defined as best focus for the PC in the visible.
The column labeled "full-temp" in each file is the appropriate model to use. The others are obsolete. Ignore the last column labeled "observed".
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