Optical measurement of the pointing stability of the SOFIA Telescope using a fast EM-CCD camera

Abstract

The goal of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is to point its airborne telescope at astronomical targets stable within 0.2 arcseconds (rms). However, the pointing stability will be affected in flight by aircraft vibrations and movements and constantly changing aerodynamic conditions within the open telescope compartment. Model calculations indicate that initially the deviations from targets may be at the order of several arcseconds. The plan is to carefully analyse and characterize all disturbances and then gradually fine tune the telescope's attitude control system to improve the pointing stability. To optically measure how star images change their position in the focal plane, an Andor DU-888 electronmultiplying (EM) CCD camera will be mounted to the telescope instead of its standard tracking camera. The new camera, dubbed Fast Diagnostic Camera (FDC) has been extensively tested and characterized in the laboratory and on ground based telescopes. In ground tests on the SOFIA telescope system it proofed its capabilities by sampling star images with frame rates up to 400 frames per second. From this data the star's location (centroid) in the focal plane can be calculated every 1/400th second and by means of a Fourier transformation, the star's movement power spectrum can be derived for frequencies up to 200 Hz. Eigenfrequencies and the overall shape of the measured spectrum confirm the previous model calculations. With known disturbances introduced to the telescope's fine drive system, the FDC data can be used to determine the system's transfer function. These data, when measured in flight will be critical for the refinement of the attitude control system. Another subsystem of the telescope that was characterized using FDC data was the chopping secondary mirror. By monitoring a star centroid at high speed while chopping, the chopping mechanism and its properties could be analyzed. This paper will describe the EM-CCD camera and its characteristics and will report on the tests that lead up to its first use in a SOFIA flight.