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HST Thermal Focus Modeling

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".

Year 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter
2003 03.001-0472      
2002 02.001-090 02.091-180 02.181-270 02.271-365
2001 01.001-090 01.091-1801 01.181-270 01.271-365
2000 00.001-090 00.091-180 00.181-270 00.271-366
1999 99.001-090 99.091-180 99.181-270 99.271-365
1998 98.001-090 98.091-180 98.181-270 98.271-365
1997 97.001-090 97.091-180 97.181-270 97.271-365
1996 96.001-090 96.091-180 96.181-270 96.271-366
1995 95.001-090 95.091-180 95.181-270 95.271-365
1994 94.001-090 94.091-180 94.181-270 94.271-365


  1. Four temperature model data not currently available for Days 2001.121 through 2001.151.
  2. Day 2003.047 is the last day for which temperatures used by the Hershey breathing model will be available through OMS.