NASA then contracted the work out to the Universities Space Research Association (USRA), and in 1996, NASA and the German Aerospace Center (DLR) signed a memorandum of understanding to build and operate SOFIA. The SOFIA telescope's primary mirror was 2.5 meters in diameter and was manufactured of Zerodur, a glass-ceramic composite produced by Schott AG that has almost zero thermal expansion. REOSC, the optical department of the SAGEM Group in France, reduced the weight by milling honeycomb-shaped pockets out of the back. They finished polishing the mirror on December 14, 1999, achieving an accuracy of 8.5 nanometres (nm) over the optical surface. The hyperbola-shaped secondary mirror was made of silicon carbide, with polishing completed by May 2000. During 2002, the main components of the telescope were assembled in Augsburg, Germany. These consisted of the primary mirror assembly, the main optical support and the suspension assembly. After successful integration tests were made to check the system, the components were shipped to Waco, Texas on board an Airbus Beluga aircraft. They arrived on September 4, 2002. SOFIA completed its first ground-based "on-sky" test on August 18–19, 2004 by taking an image of the star Polaris. The project was further delayed in 2001 when three subcontractors tasked with development of the telescope door went out of business in succession. United Airlines also entered bankruptcy protection and withdrew from the project as operator of the aircraft. The telescope was transported from Germany to the United States where it was installed in the airframe in 2004 and initial observations were made from the ground. In February 2006, after cost increases from $185 million to $330 million, NASA placed the project "under review" and suspended funding by removing the project from its budget. On June 15, 2006, SOFIA passed the review when NASA concluded that there were no insurmountable technical or programmatic challenges to the continued development of SOFIA. The maiden flight of SOFIA took place on April 26, 2007, at the L-3 Integrated Systems' (L-3 IS) Waco, Texas facility. After a brief test program in Waco to partially expand the flight envelope and perform post-maintenance checks, the aircraft was moved on May 31, 2007, to NASA's Armstrong Flight Research Center at Edwards Air Force Base. The first phase of loads and flight testing was used to check the aircraft characteristics with the external telescope cavity door closed. This phase was successfully completed by January, 2008