The results of detailed predictions are presented for the wavelength dependence of the blur circle of the proposed Stratospheric Observatory for Infrared Astronomy (SOFIA), which will have a 2.5 m diameter telescope in an open cavity aft of the wing on a Boeing 747SP aircraft. The analysis uses wind tunnel measurements of the turbulent shear layer (Rose Engineering and Research, 1996), computer calculations of the telescope cavity shear layer organized air density fluctuations (Atwood, 1993) and extrapolation of the cavity optical effects of the Kuiper Airborne Observatory (KAO) (Sutton, 1992) based on the expected SOFIA cavity noise (Rose Engineering and Research, 1996). The results indicate a visible wavelength blur circle diameter of many arc seconds, which then raised the issue of the use of the visible spectrum for natural guide stars to obtain the long term pointing accuracy of 0.14 arcsec. Detailed calculations of SNR indicate there are a sufficient number of stars having adequate visible brightness to achieve this objective. Optical tests in a quiet transonic wind tunnel and independent calculations of the air density fluctuations in the telescope cavity would confirm this result. # 1998 Society of Photo-Optical Instrumentation Engineers, S0091-3286(98)02211-9 Subject terms: astronomy; infrared imaging; point spread function; pointing; propagation; turbulence. Paper 980045 received Feb. 9, 1998; revised manuscript received July 20, 1998; accepted for publication Aug. 3, 1998.
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