In this Stratospheric Observatory for Infrared Astronomy (SOFIA) news note, I would like to describe how the 2.5 meter telescope will point and track to under one arcsec in the open port airborne environment. This is one of the remarkable challenges of the telescope and aircraft design, engineering, and construction. The German MAN corporation is providing the pointing system as part of the telescope development.

A Close-up of a Gyro Image courtesy of Man Technologie

Gross pointing of SOFIA to within 3 degrees of the desired target is determined by the flight path of the aircraft and the coarse elevation gear drive of the telescope, which operates from 20 to 60 degrees elevation angles. Finer pointing and tracking to within a few arcsecs is determined by Fiber Optic Gyros. The zero points of the Gyros are determined from stars in the tracker cameras and the focal plane camera.

These SOFIA gyros have two sets of 3.0 Km optical fiber wound on a spool. Angular motion of the gyros is determined by timing the arrival of light pulses traveling through the two sets of fiber in opposite directions. There are three gyros that are mounted orthogonally, allowing three axis stabilization. The gyros work in concert with three sets of specially designed magnetic linear motors, in a closed loop servo system. Position sensors mounted on the telescope accurately monitor the movement of the motors and the motion of the telescope relative to the airplane. Stabilization is enabled by a nearly frictionless spherical hydrostatic bearing supporting the telescope. Testing and tuning of the closed loop servo system with the hydrostatic bearing is now underway in the MAN facility in Augsburg, Germany.

Because of aircraft turbulence and wind buffeting on the telescope structure and primary mirror, additional image motion control is necessary. Much of the aircraft turbulence will be isolated from the telescope by a MAN designed vibration isolation system; this system is similar to the isolation system used on MAN trucks that travel across Europe. Residual motion not sensed by the gyros will be detected by accelerometers on the telescope structure and primary mirror. Compensation of this motion will be done by either the linear drive motors, if it is less than ~10 Hz, or with the two axis chopping secondary mirror.

The gyros have a drift rate of 3.6 arcsec/hour. They also have a random walk error of a few arcsec after one hour. These will enlarge the 100 micron diffraction-limited beam of ~10 arcsec by about 15% for daytime observations. To provide a reference for pointing, the gyros must have their zero point calibrated. This will be done with bright stars in the two tracker cameras during climb out of the aircraft to the observing altitude. For night time observations or for bright objects such as planets, updates of the gyros will be made as needed by observing objects in the focal plane CCD imaging camera or the tracker cameras to achieve higher pointing and tracking accuracy.

The above described systems should produce pointing accuracy and tracking stability of about 1 arcsec rms. Some SOFIA science requires better tracking and pointing than this value. During the first few years of operations we will replace the focal plane guide camera with a fast low-noise CCD camera. The output of this camera will be fed to the secondary mirror drive in a closed servo loop. Such a control system is similar to the tip-tilt correction used in many adaptive optics (AO) systems on ground based telescopes. This will allow better than 0."5 rms pointing and tracking in most of the night time sky.

If you have any questions about the SOFIA tracking and control system please contact Eric Becklin, Chief Scientist at SOFIA@usra.edu

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