SOFIA pointing and chopping: performance and prospect

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope built into a Boeing 747 SP. In 2014 SOFIA reached its Full Operational Capability milestone and nowadays takes off about three times a week to observe the infrared sky from altitudes above most of the atmosphere’s water vapor content. Despite reaching this major milestone the work to improve the observatory’s performance is continuing in many areas. This paper focuses on the telescope’s current pointing and chopping performance and gives an overview over the ongoing and foreseen work to further improve in those two areas. Pointing performance as measured with the fast focal plane camera in flight is presented and based on that data it is elaborated how and in which frequency bands a further reduction of image jitter might be achieved. One contributor to the remaining jitter as well as the major actuator to reduce jitter with frequencies greater than 5 Hz is SOFIA’s Secondary Mirror Assembly (SMA) or Chopper. As-is SMA jitter and chopping performance data as measured in flight is presented as well as recent improvements to the position sensor cabling and calibration and their effect on the SMA’s pointing accuracy. Furthermore a brief description of a laboratory mockup of the SMA is given and the intended use of this mockup to test major hardware changes for further performance improvement is explained.