The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for Observatory performance evaluation. These in-flight opportunities are viewed as a first comprehensive assessment of the Observatory's performance and are used to guide future development activities, as well as to identify additional Observatory upgrades. Pointing stability was evaluated, including the image motion due to rigid-body and flexible-body telescope modes as well as possible aero-optical image motion. We report on recent improvements in pointing stability by using an active mass damper system installed on the telescope. Measurements and characterization of the shear layer and cavity seeing, as well as image quality evaluation as a function of wavelength have also been performed. Additional tests targeted basic Observatory capabilities and requirements, including pointing accuracy, chopper evaluation and imager sensitivity. This paper reports on the data collected during these flights and presents current SOFIA Observatory performance and characterization.

The SOFIA telescope is a worldwide unique observatory that enables infrared astronomy aboard a Boeing 747SP at altitudes of up to 45[Formula: see text]kft. Contrary to any ground-based telescope, SOFIA is exposed not only to aerodynamic forces but also aircraft motion and excitation. Nevertheless, the ambitious scientific goals require a stable platform and very precise pointing. As of now, the telescope can be considered diffraction-limited in the far-infrared wavelengths beyond 50[Formula: see text][Formula: see text]m. A careful study of the different sources of blur revealed that image jitter is among the most influential. During the course of a flight, the telescope is exposed to various excitation levels, leading to deformation of its flexible structure and vibrations in a wide range of frequencies. Since SOFIA entered its operational phase, continuous efforts have been made to develop and implement upgrades in the pointing and control system. The original design was a robust and conservative structure aimed to ensure safe operations with several different science instruments and many unknown parameters. In recent years, more and more agile system components have followed to tackle the residual image motion. This paper introduces the telescope control system followed by a summary of recently installed upgrades and ends with an outlook on future developments on our way to diffraction-limited imaging beyond 25[Formula: see text][Formula: see text]m.

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