HST Focus

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) has traditionally moved with respect to the primary (secondary mirror despace) as a result of overall shrinkage of the truss since launch. This shrinkage is presumably due to moisture desorption of the graphite epoxy structures of the OTA. Overall, the secondary mirror has position with respect to the primary mirror has changed about 150 microns since the launch of HST in 1990.

On shorter timescales, the focus will vary within acceptable ranges due to various heating loads and attitude histories ("breathing").

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 documentation concerning:

• The overall focus history of HST as monitored by the WFPC2 PC and ACS.
• Historical models of shorter term focus variations based on HST attitude and resulting temperatures.
• Dates, times, and amounts of all Secondary Mirror moves since the First Servicing Mission.

Conventions: In these documents, 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. One micron of secondary mirror despace is equivalent to 0.0061 microns rms wavefront error at the HST 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. Since the installation of the ACS during Servicing Mission 3B, the focus in the ACS High Resolution Camera (HRC) has also been monitored concurrently with WFPC2 PC focus. Variations between HST focus measured in ACS/HRC and WFPC2 are generally small, and suggest the focus variations seen in the SIs are indeed due to overall thermal variations in the OTA.

Note: Between SIs and even between detectors, there may be small offsets from best WFPC2 PC focus. ACS/HRC for example is measured to lie 0.44 microns less than PC. For additional discussion of instrument-specific optics please refer to the Instrument Pages.

For further details on HST Focus, please see the Performance Summaries section of the Document Archive.