Determination of the SIRTF Focus from IRAC Images without a Focus Slew

Details Determination of the SIRTF Focus from IRAC Images without a Focus Slew Determination of the SIRTF Focus from IRAC Images without a Focus Slew

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003aas...203.2215h&link_type=abstract

American Astronomical Society Meeting 203, #22.15; Bulletin of the American Astronomical Society, Vol. 35, p.1244

Astronomy and Astrophysics

Astronomy

Scientific paper

Prior to launch, the SIRTF secondary focus mechanism was set to a predicted desired in-orbit focus value. This predicted setting, determined from double-pass cold chamber measurements and calculated ground-to-orbit corrections, had an uncertainty greater than the required on-orbit focus accuracy. Because of concern about the potential for failure in a cryogenic mechanism affecting all SIRTF instruments, it was required that any focus correction be made in a set of moves directly from the initial to the desired setting. The task of determining the required focus moves fell to IRAC, the instrument most affected by and sensitive to defocus.
To determine the focus directly from examining images at a fixed focus, we developed two methods, "Simfit" and "Focus Diversity" (Hoffmann W. F., et. al., 2003, SPIE, 4850, 428). Simfit finds the focus by obtaining the best match between observed images and families of simulated images at a range of focus settings. Focus Diversity utilizes the focal plane curvature to find the best fit of the varied image blur over the focal plane to a model defocus curve. Observations of a single star at many field locations in each of the four IRAC channels were analyzed before and during the refocus activity. The resulting refocus moves brought the focus to well within the specified requirement of 0.3 mm from the desired IRAC optimum focus. This is substantially less than a "Diffraction Focus Unit" (wavelength times focal ratio squared) of 0.48 mm at the shortest IRAC band (3.6 microns). The improvement in focus is apparent in both the appearance and the calculated noise-pixels of star images.

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