Jeffrey R. Kegley

Cryogenic performance of lightweight SiC and C/SiC mirrors

The technology associated with the use of silicon carbide (SiC) for high-performance mirrors has matured significantly over the past 10-20 years. More recently, the material has been considered for cryogenic applications such as space-based infrared telescopes. In light of this, NASA has funded several technology development efforts involving SiC mirrors. As part of these efforts, three lightweight SiC mirrors have been optically tested at cryogenic temperatures within the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The three mirrors consisted of a 0.50 m diameter carbon fiber-reinforced SiC, or C/SiC, mirror from IABG in Germany, a 0.51 m diameter SiC mirror from Xinetics, Inc., and a 0.25 m diameter SiC mirror from POCO Graphite, Inc. The surface figure error was measured interferometrically from room temperature (~290 K) to ~30 K for each mirror. The radius-of-curvature (RoC) was also measured over this range for the IABG C/SiC & Xinetics SiC mirrors. This paper will describe the test goals, the test instrumentation, and the test results for these cryogenic tests.

Lessons we learned designing and building the Chandra telescope

2014 marks the crystal (15th) anniversary of the launch of the Chandra X-ray Observatory, which began its existence as the Advanced X-ray Astrophysics Facility (AXAF). This paper offers some of the major lessons learned by some of the key members of the Chandra Telescope team. We offer some of the lessons gleaned from our experiences developing, designing, building and testing the telescope and its subsystems, with 15 years of hindsight. Among the topics to be discussed are the early developmental tests, known as VETA-I and VETA-II, requirements derivation, the impact of late requirements and reflection on the conservatism in the design process.

Cryogenic optical performance of a lightweighted mirror assembly for future space astronomical telescopes: correlating optical test results and thermal optical model

A 43cm diameter stacked core mirror demonstrator was interferometrically tested at room temperature down to 250 degrees Kelvin for thermal deformation. The 2.5m radius of curvature spherical mirror assembly was constructed by low temperature fusing three abrasive waterjet core sections between two CNC pocket milled face sheets. The 93% lightweighted Corning ULE® mirror assembly represents the current state of the art for future UV, optical, near IR space telescopes. During the multiple thermal test cycles, test results of interferometric test, thermal IR images of the front face were recorded in order to validate thermal optical model.

Helium cryo testing of an SLMS (silicon lightweight mirrors) athermal optical assembly

SLMS athermal technology has been demonstrated in the small 4-foot helium cryogenic test chamber located at the NASA/MSFC X-Ray Calibration Facility (XRCF). A SLMS Ultraviolet Demonstrator Mirror (UVDM) produced by Schafer under a NASA/MSFC Phase I SBIR was helium cryo tested both free standing and bonded to a Schafer designed prototype carbon fiber reinforced silicon carbide (Cesic) mount. Surface figure data was obtained with a test measurement system that featured an Instantaneous Phase Interferometer (IPI) by ADE Phase Shift. The test measurement systems minimum resolvable differential figure deformation and possible contributions from test chamber ambient to cryo window deformation are under investigation. The free standing results showed differential figure deformation of 10.4 nm rms from 295K to 27K and 3.9 nm rms after one cryo cycle. The surface figure of the UVDM degraded by lambda/70 rms HeNe once it was bonded to the prototype Cesic mount. The change was due to a small astigmatic aberration in the prototype Cesic mount due to lack of finish machining and not the bonding technique. This effect was seen in SLMS optical assembly results, which showed differential figure deformation of 46.5 nm rms from 294K to 27K, 42.9 nm rms from 294K to 77K, 28.0 nm rms from 294K to 193K and 6.2 nm rms after one cryo cycle.

Cryogenic optical testing results for the Subscale Beryllium Mirror Demonstrator (SBMD)

An Optical Testing System (OTS) has been developed to measure the figure and radius of curvature of Next Generation Space Telescope (NGST) developmental mirrors in a vacuum, cryogenic environment using the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The OTS consists of a WaveScope Shack-Hartmann sensor from Adaptive Optics Associates as the main instrument and a Leica Disto Pro distance measurement instrument. Testing is done at the center of curvature of the test mirror and at a wavelength of 632.8 nm. The error in the figure measurement is <EQ(lambda) /13 peak-to-valley (PV). The error in radius of curvature is less than 5 mm. The OTS has been used to test the Subscale Beryllium Mirror Demonstrator (SBMD), a 0.532-m diameter spherical mirror with a radius of curvature of 20 m. SBMD characterization consisted of three separate cryogenic tests at or near 35 K. The first two determined the cryogenic changes in the mirror surface and their repeatability. The last followed cryo-figuring of the mirror. This paper will describe the results of these tests. Figure results will include full aperture results as well as an analysis of the mid-spatial frequency error results. The results indicate that the SBMD performed well in these tests with respect to the requirements of (lambda) /4 PV (full aperture), (lambda) /10 PV (mid-spatial, 1-10 cm), and +/- 0.1 m for radius of curvature after cryo-figuring.

Newly modified cryogenic optical test facility at Marshall Space Flight Center

NASA Marshall Space Flight Center has maintained and operated a world-class x-ray optics and detector testing facility known as the X-ray Calibration Facility (XRCF) since the mid 1970s. The ground testing and calibration of the Chandra X-ray Observatory optics and detectors were successfully completed at the XRCF in 1997. In 1999, the facility was upgraded in preparation for cryogenic testing of lightweight telescope mirrors without compromising the existing x-ray testing capability. A gaseous Helium cooled enclosure or shroud capable of 20 degrees Kelvin and vibration isolated instrumentation mount were added to the existing facility. A precision remote-control five-axis motion mirror support was modified to operate under cryogenic conditions. Mirrors with diameters as large as two meters, and radii of curvature up to twenty meters can be accommodated in the He shroud.

Cryogenic Performance of a Lightweight Silicon Carbide Mirror

Low cost, high performance lightweight Silicon Carbide (SiC) mirrors provide an alternative to Beryllium mirrors. A Trex Enterprises 0.25m diameter low areal density SiC mirror using its patented Chemical Vapor Composites (CVC) technology was evaluated for its optical performance at cryogenic temperature. CVC SiC is chemically pure, thermally stable, and mechanically stiff. CVC technology yields higher growth rate than that of CVD SiC. NASA has funded lightweight optical materials technology development efforts for future space based telescope programs. As part of these efforts, a Trex SiC mirror was measured interferometrically from room temperature to 30 degrees Kelvin. This paper will discuss the test goals, the cryogenic optical testing infrastructure and instrumentation at MSFC, test results, and lessons learned.

AMSD test error budget sensitivity analysis

The successful augmentation of NASAs X-Ray Cryogenic Facility (XRCF) at the Marshall Space Flight Center (MSFC) to an optical metrology testing facility for the Sub-scale Beryllium Mirror Development (SBMD) and NGST Mirror Sub-scale Development (NMSD) programs required significant modifications and enhancements to achieve useful and meaningful data. In addition to building and integrating both a helium shroud and a rugged and stable platform to support a custom sensor suite, the sensor suite was assembled and integrated to meet the performance requirements for the program. The subsequent evolution from NMSD and SBMD testing to the Advanced Mirror System Demonstrator (AMSD) program is less dramatic in some ways, such as the reutilization of the existing helium shroud and sensor support structure. However, significant modifications were required to meet the AMSD programs more stringent test requirements and conditions resulting in a substantial overhaul of the sensor suite and test plan. This overview paper will discuss the instrumentation changes made for AMSD, including the interferometer selection, null optics, and radius of curvature measurement method. The error budgeting process will be presented, and the overall test plan developed to successfully carry out the tests will be discussed.

Actively cooled SLMS technology for HEL applications

Schafer has demonstrated two different methods for actively cooling our Silicon Lightweight Mirror System (SLMSTM) technology. Direct internal cooling was accomplished by flowing liquid nitrogen through the continuous open cell core of the SLMSTM mirror. Indirect external cooling was accomplished by flowing liquid nitrogen through a CTE matched Cesic® square-tube manifold that was bonded to the back of the mirror in the center. Testing was done in the small 4- foot thermal/vacuum chamber located at the NASA/MSFC X-Ray Calibration Facility. Seven thermal diodes were located over the front side of the 5 inch diameter mirror and one was placed on the outlet side of the Cesic® manifold. Results indicate that the mirror reaches steady state at 82K in less than four minutes for both cooling methods. The maximum temperature difference of the eight diodes was less than 200 mK when the mirror was internally cooled and covered with MLI to insulate it from the large 300 K aluminum plate that was used to mount it.

GoldHelox solar x-ray telescope testing progress report

The GoldHelox Solar X-ray Telescope underwent several tests during the years of 1997 - 1999, and continues through the testing phase of the project. The instrument itself, a solar telescope to ride on board the Space Shuttle, is designed to photograph the sun in soft x-ray wavelengths between 171 angstroms to 181 angstroms. Critical to its success, many tests are required to insure safety, robustness, and overall accuracy of the telescope during its mission. Among these are shake table tests, optical tests, vacuum integrity, and thermal analysis. This paper describes the GoldHelox project including its current status as a mission, the tests performed on the instrument to date, and the tests pending.