We have recently demonstrated the ability to measure the absolute change in optical power (focus) of a 152 mm diameter flat mirror in vacuum between room and cryogenic temperatures (133K) with a peak-to-valley measurement error of only 22nm. Such a measurement would be crucial to the verification of the focus of a cryogenic instrument during ground testing.
The testing utilized a vibration-insensitive interferometer and a reference mirror maintained at room temperature located within the thermal vacuum chamber. Special considerations were taken to ensure that the reference mirror experienced low axial thermal gradients, since structural modeling indicated that axial thermal gradients and axial variation of substrate coefficient of thermal expansion are critical in maintaining flatness under cryogenic test conditions. This paper will discuss the testing equipment and methodology and the corresponding analysis and results.
The High Resolution Imaging Science Experiment (HiRISE) camera will be launched in August 2005 onboard NASA's Mars Reconnaissance Orbiter (MRO) spacecraft. HiRISE supports the MRO Mission objectives through targeted imaging of nadir and off-nadir sites with high resolution and high signal to noise ratio [a]. The camera employs a 50 cm, f/24 all-reflective optical system and a time delay and integration (TDI) detector assembly to map the surface of Mars from an orbital altitude of ~ 300 km. The ground resolution of HiRISE will be < 1 meter with a broadband red channel that can image a 6 x 12 km region of Mars into a 20K x 40K pixel image. HiRISE will image the surface of Mars at three different color bands from 0.4 to 1.0 micrometers. In this paper the HiRISE mission and its camera optical design will be presented. Alignment and assembly techniques and test results will show that the HiRISE telescope's on-orbit wave front requirement of < 0.071 wave RMS (@633nm) will be met . The HiRISE cross track field is 1.14 degrees with IFOV 1.0 μ-radians.
The Subscale Beryllium Mirror Demonstrator (SBMD) has been fabricated and tested, successfully demonstrating some of the necessary enabling technologies for the Next Generation Space Telescope (NGST) and other lightweight cryogenic space mirror applications. The SBMD is a 0.532-meter diameter concave spherical mirror with a 20-meter radius of curvature fabricated from a single billet of consolidated spherical powder beryllium. The mirror is lightweighted by 90% through the use of open back triangular cells and a thin facesheet. The mirror is mounted to a rigid backplane with titanium bipod flexures. Surface figure requirements at 35K of 1/4 wave p-v (full aperture) and 1/10 wave p-v (1-10 cm spatial frequency) required initial vacuum cryogenic characterization of the mirror. Cryogenic deformation and repeatability were characterized using the Optical Testing System (OTS) at the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The mirror underwent cryofiguring to optimize performance and was subsequently tested to verify final performance requirements of surface figure, radius of curvature, and microroughness. Presented here are the final results of the SBMD program, showing that all requirements have been met.
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.
Ball Aerospace & Technologies Corp. is currently under contract to design, build, and test a state-of-the-art lightweight beryllium mirror for cryogenic space applications. This Advanced Mirror System Demonstrator (AMSD) has been designed for lightweight, deployable, spaceborne mirror applications. The major components are currently being fabricated and will comprise a lightweighted mirror assembly including a composite reaction structure. The 1.4-m point-to-point hexagon, semi-rigid beryllium mirror will be integrated with the reaction structure, actuators, and flexures to achieve a mirror system capable of ambient and cryogenic (20 to 55K) operation. The mirror prescription is an off-axis asphere of a parent with a 10-m radius of curvature. Presented here is the current status and a summary of the planned optical fabrication and testing. This work is being performed under a contract to Marshall Space Flight Center (MSFC) in Huntsville, AL and is co-sponsored by the USAF and the NRO.
Ball Aerospace is currently under contract to Marshall Space Flight Center (MSFC) in Huntsville, AL to design, build, and test a state-of-the-art lightweight beryllium mirror for cryogenic space applications, the Next Generation Space Telescope Sub-scale Beryllium Mirror Demonstrator (SBMD). The mirror is manufactured from spherical powder beryllium and optimized for cryogenic use. This 0.53-meter diameter lightweight mirror (< 12 kg/m2) has been tested at MSFC at ambient and cryogenic temperatures down to 23 K, cryofigured for optimal performance at 35 K, and subsequently retested at cryogenic temperatures. In addition, Ball has a separate contract with MSFC for an Advanced Mirror system Demonstrator (AMSD) to fabricate and test an ultra-lightweight mirror system which extends the semi-rigid SBMD mirror design to a 1.4-meter point-to-point beryllium hexagon mirror, flexures, rigid body and radius of curvature actuators, and reaction structure. This paper will describe the SBMD mirror performance and its cryogenic testing and present an overview of the AMSD semi-rigid beryllium mirror.
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