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HST's Optical Telescope Assembly (OTA) refers to the primary and secondary mirrors, and the truss structure supporting them. Although the mirrors spend virtually all of their time within scientifically acceptable focus limits, the secondary mirror (SM) does move in defocus with respect to the primary as a result of overall shrinkage of the truss presumably due to moisture desorption. On shorter timescales, the focus will vary within acceptable ranges due to various heating loads and attitude histories. The above effects are addressed here.

Users of this page should be aware of its scope. It is not intended as a thorough treatment of HST optics, or focus at any given science instrument (SI), the latter of which involves the optics and effects within the instruments. It does however present for the user data concerning:

Conventions: In this document, we express all focus changes in the SI-independent units of secondary mirror microns. A +1 micron change is equivalent to a physical movement of the secondary by that amount away from primary mirror. 1 micron of secondary mirror defocus translates to 110 microns at the focal plane. The focus zero point is defined as WFPC2 PC best focus as determined by PSF-fitting (phase retrieval) software for PSFs in the ~400-800 nm range

Note: Between SIs and even between detectors there may be an offset from best PC focus. For these relations and other focus information specific to the SIs:

Focus History

Since HST deployment, we have seen irregular shrinkage in the OTA truss presumably due to moisture desorption. Periodic secondary mirror moves are made to back up the SM and compensate for the resulting change in focus which since 1994 has been equivalent to over 30 microns. Until March '97 and the second Servicing Mission, values of +/- 5 microns away from optimal WFPC2 PC focus had been adopted as conservative focus guidelines for WFPC2, GHRS, and FOS, and compensating SM moves had been made at our then-best estimate of -3 microns in an attempt to keep the 1 sigma (1.9 micron) focus variations within these limits.

FOC, which better sampled the HST PSF required tighter focus control. This was accomplished by additional COSTAR Deployable Optical Bench (DOB) adjustments. A description of FOC-specific focus history and DOB-SM relations is given in the Instrument Science Report, FOC PSF Monitoring Program for Cycles 4-6, OSG-FOC-98-01

Focus requirements since Servicing Mission 2 (March '97) dictate keeping OTA focus within about +/- 3.5 microns. Desorption compensating Secondary Mirror moves are made when we determine a focus state of -1.5 microns based on fitting measured PC focus points, again ensuring 1 sigma variations stay within the adopted tolerance.

The data plotted and discussed below were measured from image analysis (phase retrieval) of a standard star in the WFPC2 PC chip, observed in suitable filters in the 400-800 nm range.

The two plots below are linked to high resolution pdf files which can be used to estimate focus for a given date. The latter of the two shows focus during the year 2000. We have not found a predictive function that can be fit to the focus data that describes the long term trend. Once a baseline focus estimate is made using the focus history plots, one can apply the breathing model at that time for the best estimate of the focus.

A more detailed discussion of HST focus monitoring, including techniques and error assessments, can be found in the memo OTA Focus Review...etc, a 500K pdf file.

Thermal Focus Variations

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, SESD-97-01 (400K pdf).

You may look up the modeled variation around the baseline focus value, for the exact time of your observation. The baseline focus can be obtained by inspecting the plots above.

Secondary Mirror Moves

29 Jun 1994 17:36 UT +5.0
15 Jan 1995 23:40 UT +5.0
28 Aug 1995 15:16 UT +6.5
14 Mar 1996 18:47 UT +6.0
30 Oct 1996 17:40 UT +5.0
18 Mar 1997 22:55 UT -2.4
12 Jan 1998 (NIC3 campaign 1) 01:15 UT +21.0
1 Feb 1998 16:40 UT -18.6
4 Jun 1998 (NIC3 campaign 2) 01:01 UT +16.6
28 Jun 1998 17:26 UT -15.2
15 Sep 1999 15:40 UT +3.0
09 Jan 2000 17:42 UT +4.2
15 Jun 2000 19:38 UT +3.6
02 Dec 2002 20:50 UT +3.6

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25 August 2003

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