On sky testing and preliminary sensor alignment for the SOFIA Telescope

The telescope pointing control of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is achieved during science observations by an array of sensors including three imagers, gyroscopes and accelerometers. In addition, throughout alignment and calibration of the telescope assembly, the High-speed Imaging Photometer for Occultation (HIPO) is used as a reference instrument. A summary of the telescope pointing control concept is given and how HIPO is used to calibrate the telescope reference systems on the sky. A method is introduced using simple maneuvers to perform initial alignment of HIPO, the imagers and the gyroscopes by means of single star observations. During the first on sky testing of the SOFIA telescope, these maneuvers were carried out and the alignment could be improved iteratively. The corresponding alignment accuracies are identified considering repeated measurements, environmental and sensor noise. Inertial and non-inertial observations, as well as measurements over the entire operational elevation range provide additional alignment and sensor performance information. Finally, an overview is presented for future improvements in alignment.