Division of Astronomy and Astrophysics, UCLA, 405 Hilgard Ave., Los Angeles, CA 90095-1562

The joint US and German SOFIA project to develop and operate a 2.5 meter infrared airborne telescope in a Boeing 747-SP began earlier this year. Universities Space Research Association (USRA), teamed with Raytheon E systems and United Airlines, was selected by NASA to develop and operate SOFIA. The 2.5 meter telescope will be designed and built by a consortium of German companies lead by MAN-GHH. Work on the aircraft and the primary mirror has started. First science flights will begin in 2001, and the observatory is expected to operate for over 20 years. The specifications, instruments and science potential of SOFIA are discussed.

The Stratospheric Observatory For Infrared Astronomy (SOFIA) is NASA's and DLR's premier observatory for infrared and submillimeter astronomy into the next century. A Boeing 747-SP aircraft will carry a 2.5-meter telescope designed to make sensitive infrared measurements of a wide range of astronomical objects. It will fly at and above 12.5 km, where the telescope collects radiation in the wavelength range from 0.3 micrometers to 1.6 millimeters region of the electromagnetic spectrum.

A team led by the Universities Space Research Association - USRA - has been selected by NASA to design, assemble, test, and operate SOFIA. USRA heads up a team that includes United Airlines, Raytheon E-Systems, Sterling Software, University of California, the SETI Institute, and the Astronomical Society of the Pacific. As part of NASA's privatization program to lower costs and improve efficiency, the SOFIA program has been contracted out to this team. Contract management will be performed by NASA Ames Research Center. The telescope and 20% of operations will be supplied by Germany.

The prime contractor, USRA, is a 26-year-old consortium of more than 80 universities set up to manage scientific research programs. USRA will have overall project responsibility and will manage the scientific operations of the observatory.

United Airlines, the world's largest air carrier, has provided the Boeing 747-SP to NASA, and will manage SOFIA's flight operations out of the Moffett Federal Airfield near San Francisco, California, traditional home of NASA's airborne astronomy program. Routine maintenance operations will be performed at United's facilities at San Francisco and Oakland.

Raytheon E-Systems of Waco, Texas will do the extensive aircraft modification required to build SOFIA. The five-year job of modifying the airplane for its new scientific role includes installing the telescope and the all-important mission control and communications systems for the observatory. The specialized computer systems needed to operate the telescope will be designed and integrated by Sterling Software.

The USRA/SOFIA Science Center will be located in hangar N211 at NASA Ames Research Center during both development and operation of the observatory. The University of California at Los Angeles and the University of California at Berkeley will be working closely with the USRA/SOFIA team to help provide scientific and technical support for the staff and facility. NASA Ames Research Center will also support the Science Center development through SOFIA work packages. Observatory instruments to be attached to the telescope will be designed and built at US universities and national centers through a peer review process. The German science community will also provide instruments.

DLR, Germany's space agency, is funding the design and construction of the telescope in cooperation with NASA. The telescope will be built by MAN GHH, MAN Technologies and Kaiser Threde of the Federal Republic of Germany. During operation, German scientists will utilize 20 percent of the flying observatory's telescope time. DLR will also provide 20 percent of the operations support through on site staffing, telescope maintenance and other provisions.

Presently, the SHOTT Zerodur mirror is being ground in Meinz, Germany. A picture of the SOFIA mirror is shown in Fig. 2. The Boeing 747-SP airplane was dedicated in San Francisco in March 1997 and is shown in Fig. 3. The SOFIA telescope will be delivered to Waco, TX in late 2000 for integration.

Nominal Operational Wavelength Range: 0.3 to 1600 microns; prime wavelengths 15-300 microns

Primary Mirror Diameter = 2.7 meters

System Clear Aperture Diameter = 2.5 meters

Nominal System f-ratio = 19.6

Primary Mirror f-ratio = 1.28

Telescope's Unvignetted Elevation Range:

20-60 degrees

Optical Configuration: Bent Cassegrain with oscillating secondary mirror and flat folding tertiary.

Unvignetted Field-of-View Diameter = 8 arcmin

Chopper Frequencies = 1 to 20 Hz for 2-point square wave chop

Pointing Stability

= 0."2 rms when using on-axis Focal Plane tracking

= 0."8 rms when using on-axis Fine-Field tracking

Pointing Accuracy

= 0."5 when using on-axis Focal Plane tracking

= 3" when using on-axis Fine-Field tracking

Sky Rotation Freeze Mode available for a sky rotation range of +3 degrees (i.e. 6 minutes in time for fast rotators)

Diffraction-Limited Wavelengths > 15 microns

Image Quality of Telescope Optics at 0.6 microns=1.5 arcsec on-axis (80% encircled energy)

=9."1 for 80% encircled energy diameter

=5."8 for 50% encircled energy diameter

(Chopped Image Quality for other chop throws scale linearly with throw, down to the diffraction-limit or optical quality of the optics.)

Total Emissivity of Telescope (Goal):

15% at 10 microns with dichroic tertiary

10% at 10 microns with aluminized tertiary

Recovery Air Temperature in Cavity (and Optics Temperature)=240K

960 successful flight hours per year between 12.5 km and 14 km in full operations

Will support both state of the art Facility Class and PI Class instruments which can be removed after each flight series

It is instructive to compare the time to make an observation on FIRST and SOFIA. This is done in Table 1 where the observing time on SOFIA is ratioed to a similar observing time on FIRST, for a point source and R<10⁴. The factors considered are the area of the telescopes, the thermal background, the effective emissivity and the transmission. The product of all effects shows that FIRST is 112 times faster than SOFIA. The signal to noise on a point source will be about 11 times greater on FIRST than SOFIA for the same integration time.

From the US side, SOFIA instruments will be selected for development by peer review of submitted proposals. In the USRA call for proposals, both Facility Class and PI class instruments were requested. It is anticipated that one or two Facility Class instruments will be delivered to the USRA Science Center during the first year of operation. The Facility Instruments are expected to be user friendly and will not require the builder to be present to operate. One member of the Science Center will work with the team developing a Facility Instrument (FI) and FIs will have PDRs and CDRs conducted. To cover all the science areas of SOFIA, a number of PI instruments will also be selected for development.

Table 2 shows the US SOFIA instruments selected by NASA for feasibility studies in 1996. It is anticipated that fewer than half of these instruments will be developed in the first call. The Germans are expected to develop P.I. class instruments. A list of possible German instruments is given in Table 3.

The primary wavelength regions where SOFIA will work are important for the following astrophysical reasons:

1. Most of the fundamental absorption and emission lines and bands of astrophysically and astrochemically significant molecules occur in this region;

2. Most of the luminosity of our galaxy and in other galaxies emerges in this wavelength region;

3. Low dust extinction at these wavelengths permits unbiased and potentially complete observations of statistically large samples of objects; and

4. Formation of galaxies in the early universe and the crucial stages of formation of stars and planets can be best studied in this range of wavelengths.