HST Focus History
Since HST deployment, we have seen irregular shrinkage in the OTA
truss presumably due to desorption by the graphite epoxy structure.
Periodic secondary mirror moves have been required to back up the
secondary mirror (SM) and to compensate for the resulting change in
best focus. Since HST deployment in 1990 this motion has been the equivalent to over 150
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.
The Faint Object Camera (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.
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.
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 the 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.
The plots listed below can be used to estimate focus for a given
date in the past. We have not found a predictive function that can be fit to the focus data that describes any long-term trending in
focus.
A more detailed discussion of HST focus monitoring, including
techniques and error assessments, can be found in the memo OTA Focus
Review (PDF, 884KB).
Note: HST focus modeling information has not been updated since day
2003.047, since the models no longer accurately describe HST's less pronounced, though also less predictable overall focus trend.
|
 |
|
