Optical testing of large mirrors for space telescopes can be challenging and complex. Demanding optical requirements
necessitate both precise mirror figure and accurate prediction of zero gravity shape. Mass and packaging constraints
require mirrors to be lightweighted and optically fast. Reliability and low mass imply simple mounting schemes, with
basic kinematic mounts preferable to active figure control or whiffle trees. Ground testing should introduce as little
uncertainty as possible, ideally employing flight mounts without offloaders. Testing mirrors with their optical axes
horizontal can result in less distortion than in the vertical orientation, though distortion will increase with mirror speed.
Finite element modeling and optimization tools help specify selective reinforcement of the mirror structure to minimize
wavefront errors in a one gravity test, while staying within mass budgets and meeting other requirements. While low
distortions are necessary, an important additional criterion is that designs are tolerant to imperfect positioning of the
mounts relative to the neutral surface of the mirror substrate. In this paper, we explore selective reinforcement of a 2-meter class, f/1.25 primary mirror for the proposed SNAP space telescope. We specify designs optimized for various
mount radial locations both with and without backup mount locations. Reinforced designs are predicted to have surface
distortions in the horizontal beam test low enough to perform optical testing on the ground, on flight mounts, and
without offloaders. Importantly, the required accuracy of mount locations is on the order of millimeters rather than
tenths of millimeters.
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.
A series of developmental as well as flight mirrors have been in process over the last few years for IR cryogenic telescope applications such as the Space Infrared Telescope Facility (SIRTF) and the Next Generation Space Telescope (NGST) and for visible ambient systems such as Space Based Laser (SBL). We will discuss the performance of the 0.85-m SIRTF primary mirror (26.6 kg/m2 areal density) and the 0.5-m Subscale Beryllium Mirror Demonstrator (SBMD) beryllium mirror (9.8 kg/m2 areal density) as well as the current status of the 1.4-m Ball semi-rigid, beryllium Advanced Mirror System Demonstrator (AMSD). The AMSD mirror itself has an areal density of 10.4 kg/m2 and is currently in polishing. The entire AMSD assembly including composite reaction structure, flexures, and actuators, has an areal density less than 15 kb/m2.
Cyrogenic test results of the SIRTF and SBMD mirrors will be presented along with test data on the AMSD actuators. The SBMD mirror wsa cryofigured based on ambient and cryo testing to achieve a wavefront quality of 19 nm rms at 35 K. In addition, the effects of optically coating SBMD with a protected gold multi-layer system will be shown - demonstrating that a lightweight mirror can be coated without adverse print-through due to coating stress at ambient or cryo operating temperatures.
KEYWORDS: Actuators, Cryogenics, Aerospace engineering, Nanotechnology, Interferometers, Control systems, Mirrors, Temperature metrology, Photography, Space telescopes
An effort has been in place at Ball Aerospace & Technologies Corp. (BATC) for over three years to develop a mechanism for precise positioning of optical elements for such applications as the Next Generation Space Telescope (NGST). It is desired for such a mechanism to be of low mass, to have nanometer-level positioning capability over a comparatively large range of travel, to be both ambient and cryogenically capable, and to have high strength and stiffness capabilities. The development effort has resulted in a simple 288-gram mechanism that meets these requirements, and does so with a single stepper motor and a simple control system. Performance has been verified at both ambient and cryogenic temperatures, and the mechanism design is currently being implemented on BATC's Advanced Mirror System Demonstrator program (AMSD). The current design achieves steps of less than 10 nanometers per step over more than 20mm of travel. We will present an overview of the capabilities of the mechanism, as well as a discussion of the test results achieved to date. Test results will include both ambient and cryogenic performance, hysteresis and stiffness measurement, as well as verification of single-stepping capability.
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.
Challenges in high-resolution space telescopes have led to the desire to create large primary mirror apertures. One such telescope is the Next Generation Space Telescope (NGST, 8-m primary). In order to accommodate launch vehicles, the optical systems using these large apertures are being designed to accommodate extremely lightweight, deployable, segmented primary mirrors. The requirements for these segments include: meter-class diameter, areal densities of the order of 15 kg/m2, aspheric surface figure, near infrared and visible spectrum operation, diffraction limited surface figure, high stiffness, tight radius of curvature matching, and excellent thermal stability. Operating temperatures for various systems include ambient as well as cryogenic ranges. A unique ceramic, carbon fiber reinforced silicon carbide, developed by the Industrieanlagen- Betriebsgesellschaft mbH, has shown potential for use as a mirror substrate. This paper presents the deign and predicted performance of this mirror system in various applications. Also included are issues related to the fabrication of the Advanced Mirror System Demonstrator.
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