Effective stray light control is a key requirement for wide dynamic range performance of scientific optical and infrared systems. SOFIA now has over 325 mission flights including extended southern hemisphere deployments; science campaigns using 7 different instrument configurations have been completed. The research observations accomplished on these missions indicate that the telescope and cavity designs are effective at suppressing stray light. Stray light performance impacts, such as optical surface contamination, from cavity environment conditions during mission flight cycles and while on-ground, have proved to be particularly benign. When compared with earlier estimates, far fewer large optics re-coatings are now anticipated, providing greater facility efficiency.
The NASA Stratospheric Observatory for Infrared Astronomy (SOFIA), is a 2.5 meter telescope in a modified Boeing 747SP aircraft that is flown at high altitude to do unique astronomy in the infrared. SOFIA is a singular integration of aircraft operations, telescope design, and science instrumentation that delivers observational opportunities outside the capability of any other facility. The science ground operations are the transition and integration point of the science, aircraft, and telescope. We present the ground operations themselves and the tools used to prepare for mission success. Specifically, we will discuss the concept of operations from science instrument delivery to aircraft operation and mission readiness. Included in that will be a description of the facilities and their development, an overview of the SOFIA telescope assembly simulator, as well as an outlook to the future of novel science instrument support for SOFIA
We present a performance report for FLITECAM, a 1-5 μm imager and spectrograph, upon its acceptance and delivery to SOFIA (Stratospheric Observatory for Infrared Astronomy). FLITECAM has two observing configurations: solo configuration and “FLIPO” configuration, which is the co-mounting of FLITECAM with the optical instrument HIPO (PI E. Dunham, Lowell Observatory). FLITECAM was commissioned in the FLIPO configuration in 2014 and flew in the solo configuration for the first time in Fall 2015, shortly after its official delivery to SOFIA. Here we quantify FLITECAM’s imaging and spectral performance in both configurations and discuss the science capabilities of each configuration, with examples from in-flight commissioning and early science data. The solo configuration (which comprises fewer warm optics) has better sensitivity at longer wavelengths. We also discuss the causes of excess background detected in the in-flight FLITECAM images at low elevations and describe the current plan to mitigate the largest contributor to this excess background.
A new stray light coating, called J-Black, has been developed for NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA). The coating is a layered composition of Nextel-Suede 3101 primers and top coats and silicon carbide grit. J-Black has been applied to large areas of the SOFIA airborne telescope and is currently operating within the open cavity environment of the Boeing 747. Over a series of discrete filter bands, from 0.4 to 21 microns, J-Black optical and infrared reflectivity performance is compared with other available coatings. Measured total reflectance values are less than 2% at the longest wavelengths, including at high incidence angles. Detailed surface structure characteristics are also compared via electron and ion microscopy. Environmental tests applicable for aerospace applications are presented, as well as the detailed steps required to apply the coating.
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 telescope pointing control of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is achieved during
science observations by an array of sensors including three imagers, gyroscopes and accelerometers. In addition,
throughout alignment and calibration of the telescope assembly, the High-speed Imaging Photometer for Occultation
(HIPO) is used as a reference instrument. A summary of the telescope pointing control concept is given and how HIPO
is used to calibrate the telescope reference systems on the sky. A method is introduced using simple maneuvers to
perform initial alignment of HIPO, the imagers and the gyroscopes by means of single star observations. During the first
on sky testing of the SOFIA telescope, these maneuvers were carried out and the alignment could be improved
iteratively. The corresponding alignment accuracies are identified considering repeated measurements, environmental
and sensor noise. Inertial and non-inertial observations, as well as measurements over the entire operational elevation
range provide additional alignment and sensor performance information. Finally, an overview is presented for future
improvements in alignment.
In the event of a cryostat vacuum failure, the subsequent liquid helium boil off can produce a significant pressure rise in the helium reservoir potentially causing damage to the cryostat. To preclude cryostat damage during such a failure, an analysis has been developed to predict the maximum internal pressure for the corresponding vent neck size and helium reservoir surface area at failure condition. To demonstrate that the analysis predicts correct pressure values, a series of experiments have been carried out at NASA Ames Research Center to measure the pressure profile during an induced failure. The maximum measured helium reservoir pressure is then used in the analysis to derive the heat load. The experiments have been conducted using a cylindrical helium and nitrogen cryostat with a variable size constriction insert in the helium neck to enable measuring the pressure rise across a range of effective neck areas. The experimentally derived heat flux on the uninsulated helium reservoir during an induced vacuum failure is 3.1 ± 0.2 W/cm<sup>2</sup>. Some preliminary test results are presented describing the effects of superinsulation, but this aspect of cryostat design is not extensively explored, as the design and materials selected may have highly variable results.
The SOFIA Airborne Observatory will operate a 2.5 m aperture telescope with the goal of obtaining over 960 successful science hours per year at a nominal altitude of 12.5 km and covering a wavelength range from 0.3 mm to 1.6 mm. The observatory platform is comprised of a Boeing 747SP with numerous significant modifications. The ground and flight mission operations architectures and plans are tailored to keep the telescope emissivity low and achieve high observing efficiency.
The Sloan Digital Sky Survey 2.5-meter telescope optical design is optimized for wide field (3 degree(s)), broadband (300 nm to 1060 nm) CCD imaging and multi-fiber spectroscopy. The system has very low distortion, required for time-delay-and- integrate imaging, and chromatic aberration control, demanded for both imaging and spectroscopy. The Cassegrain telescope optics include a transmissive corrector consisting of two aspheric fused quartz optical elements in each configuration. The details of the fabrication of these elements are discussed. Included is the design, development and performance of custom optical coatings applied to these optics.
The Cassini spacecraft will perform a detailed examination of the Saturnian system, including the release of a probe to study Saturn's largest satellite, Titan. The star tracker for the Cassini mission must provide accurate data during the entire flight including four years of measurement in a harsh radiation environment. The star tracker will provide autonomous star identification over the entire celestial sphere using a 4,000 entry on-board star catalog. Three axis attitude reference will be determined by measurements of two to five stars in the tracker field of view which will allow the gyroscopes to be powered off during the cruise phase of the flight. When the gyros are operational, attitude updates will be provided. The Cassini star tracker consists of a CCD based star camera, called the stellar reference unit (SRU), which is being designed and built by Officine Galileo. The operation of the SRU, including functional modes, exposure times, and areas of the CCD to digitize is under the control of the Cassini Attitude and articulation control subsystem (AACS) flight computer (AFC). The raw digital pixel data is transmitted from the SRU through a dedicated direct memory access (DMA) interface to the AFC memory for subsequent processing. All pixel processing and centroiding is performed within the AFC. Once the initial attitude has been determined, the AFC algorithms will choose which stars within the SRU field of view to track in order to maintain attitude knowledge. The SRU will have a 15 degree field of view and will provide 60 (mu) rad (3 (sigma) ) 2-axis position measurement accuracy for stars of approximately 6.05 visual magnitude and brighter. The required 1 mrad (3 (sigma) ) twist accuracy is provided by star separation.