IQ-R2 » History » Version 3

Aaron Roodman, 07/30/2012 02:48 AM

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h1. IQ-R2
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Optical prescription, fabrication errors, alignment errors, flexure, and CCD non-planarity, etc. contribute no more than 0.36" FHWM to images taken at best-focus position [DECam TO-2 (see Table 2 for budget), TO-4] To be tested over full HA/Dec range of Survey.
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As a wavefront analysis, the donuts measure contributions to the PSF from the average wavefront.  These include the optical prescription from both the primary and DECam, and any deviations from the ideal optics, but not seeing, tracking or CCD(diffusion) contributions.
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The wavefront analysis returns results as aberrations across the focal plane, and the sources of contributions may be explored by comparing the results with those from the optical prescription.  Beyond that it will be difficult to separate out much of these contributions.  One exception is the contribution from focal plane height, which can be explicitly measured.
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h2. Prerequisites
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* Same as for IQ-R1
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h3. Procedure
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* Use the same science CCD images as for IQ-R1.
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* The images may also be reanalyzed incorporating a few higher-order Zernike terms in the fit. At most spherical and quadrefoil may be included.
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* Convert the wavefront from the measured values to PSF, by numerical calculation of simulated stars.
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h3. Verdict
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* Compare inferred PSF contributions from the average wavefront with 0.36 arcsec FWHM. 
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* Perform this comparison at the full HA/Dec range of the survey.
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h4. Consequences
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* If we fail this test, see if we at least pass IQ-R4.  Assign excess WFE to contingency portion of PSF budget.
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* Alternatively, if some of the reason for failure is attributable to residual misalignments or primary mirror Astigmatism, work to improve
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either of these contributions.