PSF-G1 » History » Version 4

Stephen Kent, 07/31/2012 01:06 PM

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h1. PSF-G1
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Telescope alignment errors / aberrations inferred from field dependence of moments of PSF on science array agree with those inferred from donut data, to within ??? waves of wavefront error.  NOTE: the PSF is bandpass-dependent.  This test should be done for r, i filters.  g and z may be more problematic (g band PSF is not very good towards the edge of the focal plane; z band affected by CCD transparency).
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h2. Prerequisites
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Static LUT for alignment has been installed and is being applied.  F/A donuts are being analyzed, but alignment tweaks need not yet be implemented.  Results of F/A analysis are stored in database or equivalent.  Focus loop is implemented and working.  (One MIGHT be able to do this test with open-loop focus adjustments, but not sure.)
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h2. Procedure
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This test can be performed using science data alone.  Science data in r and/or i bands must be taken in good seeing (FWHM < 1.2" ???).  One computes moments of stars on all science CCDs and uses the patterns to derive a misalignment (including defocus).  The numbers are compared with those derived from F/A donuts.
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As a cross-check, we also want to obtain in-focus and out-of-focus images in the science array twice per SV night during evening and morning twilight to monitor the correspondence between the F/A data, PSF moments, and science CCD donuts.
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h2. Verdict
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The numbers agree at the same level that BCAM and donut data should agree (translation within 200 microns; tilts and defocus to some equivalent values).
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h2. Consequences
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This test is intended to determine what is possible using the pattern of PSF shapes.  It is possible that one cannot derive a non-degenerate solution.