Effect of jet engine exhaust on SOFIA stray-light performance

摘要

Abstract: The Stratosphere Observatory For Infrared Astronomy, or SOFIA, is being designed at NASA's Ames Research Center as a replacement for the Kuiper Airborne Observatory (KAO). A 2.5 meter Nasmyth telescope will be mounted in a Boeing 747 SP and flown at 41,000 feet, above most of the H$-2$/O in the Earth's atmosphere. In the original SOFIA design, the telescope is located in front of the wings, as it is in the KAO. An alternative design with the telescope placed behind the wings is being studied as part of an effort to reduce cost and weight. In this location, the emission from the engines and the hot H$-2$/O molecules in the exhaust become significant straylight sources. The engines and exhaust radiate into the telescope cavity, and illuminate the primary and tertiary mirrors at low telescope elevation angles. The APART/PADE program was used to analyze the straylight at the SOFIA focal plane as a function of wavelength and telescope elevation angle. The emission from the engines and exhaust gas is compared to that from the Earth and the telescope itself. The secondary mirror will be chopped during most observations, allowing subtraction of a constant background signal. The importance of fluctuations in the exhaust emission is that they limit the amount of straylight that can be reduced by chopping. For the aft position of the SOFIA telescope to be acceptable from a scientific point of view, the fluctuations in the exhaust straylight must be lower than the shot noise in the telescope mirror emission. Based on the results of this analysis, the SOFIA telescope has been moved behind the wings. The degradation in performance at low elevation angles reported here has been deemed acceptable.!3