SOFIA telescope predicted aero-optics performance at flight conditions

Abstract

The results of detailed predictions are presented for the blur circle of the proposed Stratospheric Observatory for Infrared Astronomy (SOFIA) which will have a 2.5 meter diameter telescope in a open cavity aft of the wing on a Boeing 747SP aircraft. The blur circle wavelength dependence from visible to the long wave infrared wavelengths is presented. The analysis is based on theory 1, and uses detailed wind tunnel measurements of the turbulent shear layer^, CFD calculations of the telescope cavity shear layer organized air density fluctuations^ and extrapolation of the cavity optical effects of the Kuiper Airborne Observatory (KAO)4 to the SOFIA cavity noise2. The results indicate a visible wavelength blur circle diameter of several arc seconds which raises the issue of the use of such guide stars to obtain long term pointing accuracies of 0.14 (arc-seconds). Further studies on star brightness and pointing accuracy, optical tests in a quiet transonic wind tunnel and independent CFD calculations of the air density fluctuations in the telescope cavity are indicated. The methodology demonstrated is applicable to other airborne large optics, such as the Air Force's Airborne Laser weapon system.