SMOV Morning Meeting
2 Jul 09
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SMOV/Cycle 17 Focus Recommendation
All points from phase retrieval on ACS/HRC, ACS/WFC, WFPC2/PC, and STIS/CCD images.
Data has been model-corrected to reduce temperature associated effects. (reduces 1 sigma scatter from 2 to 1.7 mic)
Data plotted and fit from 2003. Two secondary mirror moves have been removed to present continuous curve.
Values expressed in microns at Sec. Mirror, relative to ACS/WFC best focus (Zero microns = ACS/WFC in focus)
1 micron at Secondary induces 6 nm of rms wavefront error ~1/90 wave (V-band) at focal plane.
Typical temperature-driven variations in OTA focus over orbits to days is usually ~5 microns +/–2
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Best fitting exponential:
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–6.16 + 201.64 e-T/1754.4
rms deviation 1.7 microns
predicts -2.5 microns on August 1
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Best fitting straight line:
• rms deviation 1.8 microns
• predicts –3.4 microns on August 1
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Double exponential fitting lifetime focus data:
• predicts -1.5 microns on August 1
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Primary goal is to have ACS/WFC & WFC3 confocal and to
maintain average focus at WFC3 within +/–2 microns:
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STScI will offset WFC3 & COS focus during fine alignment by
equiv. –2.5 microns to achieve confocality with ACS, and we
are requesting a follow-on Secondary Mirror move of 2.5+0.5 =
3.0 microns to leave the three SIs! focii ~ +0.5 micron. STIS slit
focus would remain within typical ranges seen in the past.
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From an OTA focus of +0.5 microns, we should pass through
best focus (0) in cycle 17, and not reach –2 microns until
mid-2012
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From the science and calibrations perspective, the best time for
this Secondary Mirror move is just prior to day 201 SMS. This is
sooner than originally estimated and will require GSFC/OTA
group availability.
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We are available to present further details at GSFC next week.
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