SOFIA Telescope In-flight Activation – What it Involved, Why it's Important
The main objective of the SOFIA telescope assembly activation flight Jan. 15 was to verify that the telescope assembly could achieve inertial stabilization mode in flight and document how stable it is. In principle, this was the second part of a test flight conducted about two years ago during which inertial stabilization was not achieved because of a gyroscope fault.

The telescope activation test flight Jan. 15 gave us the opportunity to evaluate the updated gyroscope, demonstrate improved vibration isolation system performance and operate the telescope assembly with its coarse and fine drives coupled together and active in flight. Activating the two drives while they were coupled was important, since this is the configuration we will use while making science observations.

Although no images of the sky were taken due to the telescope cavity door remaining closed, gyro output signals were used to evaluate the level of stabilization provided by the gyro feedback in the telescope's pointing control system.

One objective of the mission was to test the vibration isolation system to determine if it kept the un-caged telescope assembly centered in its mount during the altitude and heading changes that occurred during flight. We also wanted to see how subtly aircraft headings as small as a half-degree could be changed upon demand.

The temperature at high altitude is much colder than on the ground. Even with the telescope cavity door closed, the cavity is not sealed from the outside atmosphere. The temperature of the telescope components inside the cavity gradually drops while flying at high altitude. Since there is not as much air flow with the door closed, it isn't quite as cold as with the door open, but still provided a chance to see how the temperature drop affected operation and performance of those telescope assembly systems located in the cavity, including the Wide Field Imager, Fine Field Imager and Secondary Mirror Mechanism. (The temperature dropped to –15 degrees Celsius (+5 degrees Fahrenheit) as the aircraft flew at 35,000 feet.)

The Secondary Mirror Mechanism has the capability to move side to side, up and down, in and out, and tilt in all directions. In order to chop the instrument's field of view, a necessity in infrared astronomy, the mirror must tilt back and forth roughly 10 times per second. We exercised all functions of this mount after we reached the coldest temperature, and tuned its controller for optimum performance at low temperature.

We also looked at the dark field images of the two imagers on board during the period when we were experiencing the -15 degrees Celsius temperature. We also checked noise levels, as they are usually lower when flying in the low temperature of high altitude.

Among other tests, we operated the balancer drives of SOFIA's telescope assembly, the filter fan unit, the Nasmyth tube fans and opened the gate valve for the first time in flight. This valve allows light from the telescope in the cavity to reach the science instruments that will be installed in the cabin when astronomical observations are under way. No science instruments were mounted for this flight.

Phil Watts, SOFIA Chief Engineer
Universities Space Research Association