The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for Observatory performance evaluation. These in-flight opportunities have been viewed as a first comprehensive assessment of the Observatory's performance and will be used to address the development activity that
is planned for 2012, as well as to identify additional Observatory upgrades. A series of 8 SOFIA Characterization
flights have been conducted from June to December 2011. The HIPO science instrument in
conjunction with the DSI Super Fast Diagnostic Camera (SFDC) have been used to evaluate pointing stability,
including the image motion due to rigid-body and
flexible-body telescope modes as well as possible aero-optical
image motion. We report on recent improvements in pointing stability by using an Active Mass Damper system
installed on Telescope Assembly. Measurements and characterization of the shear layer and cavity seeing, as
well as image quality evaluation as a function of wavelength have been performed using the HIPO+FLITECAM
Science Instrument conguration (FLIPO). A number of additional tests and measurements have targeted basic
Observatory capabilities and requirements including, but not limited to, pointing accuracy, chopper evaluation
and imager sensitivity. This paper reports on the data collected during these
flights and presents current SOFIA
Observatory performance and characterization.
The SOFIA airborne observatory flies in the lower stratosphere above more than 99.9% of the Earth's water vapor. As
low as this residual water vapor is, it will still affect SOFIA's infrared and sub-millimeter astronomical observations. As
a result, a heterodyne instrument operating at 183 GHz will be used to measure the integrated water vapor overburden in
flight. The accuracy of the measured precipitable water vapor must be 2 microns or better, 3 sigma, and measured at
least once a minute. This presentation will cover the design and the measured laboratory performance of this instrument,
and will discuss other options for determining the water vapor overburden during the SOFIA Early Science shared-risk
SOFIA, the Stratospheric Observatory for Infrared Astronomy, is an airborne observatory that will study the universe in
the infrared spectrum. A Boeing 747-SP aircraft will carry a 2.5 m telescope designed to make sensitive infrared
measurements of a wide range of astronomical objects. In 2008, SOFIA's primary mirror was demounted and coated for
the first time. After reintegration into the telescope assembly in the aircraft, the alignment of the telescope optics was
repeated and successive functional and performance testing of the fully integrated telescope assembly was completed on
the ground. The High-speed Imaging Photometer for Occultations (HIPO) was used as a test instrument for aligning the
optics and calibrating and tuning the telescope's pointing and control system in preparation for the first science
observations in flight. In this paper, we describe the mirror coating process, the subsequent telescope testing campaigns
and present the results.
The SOFIA Water Vapor Monitor (WVM) is a heterodyne radiometer designed to determine the integrated amount of water vapor along the telescope line of sight and directly to the zenith. The basic technique that was chosen for the WVM uses radiometric measurements of the center and wings of the 183.3 GHz rotational line of water to measure the water vapor. The WVM reports its measured water vapor levels to the aircraft Mission Controls and Communication System (MCCS) while the SOFIA observatory is in normal operation at flight altitude. The water vapor measurements are also available to other scientific instruments aboard the observatory. The electrical, mechanical and software design of the WVM are discussed.
SOFIA will permit observations from the visible to mm wavelengths, and offer higher spectral and spatial resolution than any other facility at some wavelengths. Nine focal-plane instruments are being developed to exploit this capability during the first several years of SOFIA operation. These instruments are being built at universities, at research institutes in Germany, and at NASA's Goddard Space Flight Center. The broad wavelength span of SOFIA implies a wide variety of Science Instrument characteristics, including detector technologies, spectral definition techniques, and science objectives. Here we summarize the performance of the nine instruments in relatively uniform format to facilitate evaluation of feasibility of desired observations. For each instrument, three basic aspects are described: (1) spectral resolution or passbands (2) sensitivity for emission lines and/or continuum (3) angular resolution. Spectral resolution ranges from several hundred km/s down to 0.01 km/s; some of the instruments have several modes spanning several orders of magnitude within this range. Sensitivities for continuum and for emission line integrated fluxes are given in Janskies and W/m2 respectively, for specified integration time and S/N. For reference some Pogson magnitudes are also given at short (visible, near-IR) wavelengths, and some antenna temperature values are also given at sub-mm wavelengths. Angular resolution is expressed as the FWHM beam size in seconds of arc, as a function of wavelength. With this compilation of basic performance, any researcher may estimate the feasibility of potential observations with any of the first generation instruments. The performance summaries are available online at the SOFIA web site: http://SOFIA.arc.nasa.gov.
The far-infrared reflectance and scattering properties of telescope surfaces, surrounding cavity walls, and surfaces within focal-plane instruments can be significant contributors to background noise. Radiation from sources well off-axis, such as the earth, moon or aircraft engines may be multiply scattered by the cavity walls and/or surface facets of a complex telescope structure. The Non-Specular Reflectometer at NASA Ames Research Center was reactivated and upgraded, and used to measure reflectance and Bi- directional Reflectance Distribution Functions for samples of planned telescope system structural materials and associated surface treatments.