The 'ultra' in ultralight will be redefined this year as several of the world's leading optics companies strive to meet some of the most stringent mirror requirements ever issued for the Next-Generation Space Telescope (NGST) program.
Figure 1. The COI mirror (left) is a 1.6-m hybrid and the SBMD (right) is a 0.5-m mirror currently undergoing cryogenic testing.
The roadmap for NGST mirror technology is broken down into several large demonstration mirrors: the NGST Mirror Systems Demonstrator (NMSD), which consists of 2- and 1.6-m mirrors to be delivered this year; the 0.5-m Subscale Beryllium Mirror Demonstrator (SBMD); and the Advanced Mirror Systems Demonstrator (AMSD), which is just beginning the fabrication phase and focuses on new mirror designs. The AMSD program is a multi-agency procurement involving the Air Force Research Lab, NASA, and various other organizations.
According to David Jacobson, project lead of the AMSD at Marshall Space Flight Center (MSFC; Huntsville, AL), all the agencies involved in AMSD are interested in ultralight mirrors that weigh less than NASA's 15 kg/m2. Developing a 3-ft.2 mirror that weighs about seven pounds would itself be an impressive achievement; however, that weight specification also includes first-stage actuators capable of deforming the mirror surface 10 to 20 µm and the structure the actuators react against. In addition to surface deformation, the mirrors will also require tip, tilt, and piston actuation in the range of 6 mm in stroke.
Figure 2. Because the NGST will make the majority of its observations in the infrared part of the spectrum it is important that the telescope be very cold. Any excess heat from the telescope creates an artificial background signal that would swamp the faint light from distant astronomical sources. This requirement is one of the reasons that an orbit that takes the NGST far from Earth (and the reflected sunlight it produces) is desirable. There are several orbits that would be satisfactory from a thermal point of view, but only two remain under consideration due to other constraints (for example, ease of communication and launch-weight considerations). These two orbits are called "L2" and "1 X 3 AU" orbits. Both the TRW/Ball Aerospace design (left) and the Lockheed-Martin design (right) will use the L2 position rather than the 1 X 3 AU, which refers to an orbit that varies between one and three AU.
Jacobson said phase 1 of the AMSD project, which details various concepts and materials for mirror construction and actuation, has been completed. So far, eight designs involving five industry partners -- including the Univ. of Arizona (Tuscon, AZ), Composite Optics Inc. (COI; San Diego, CA), Ball Aerospace (Boulder, CO), Raytheon Optical Systems (Danbury, CT), and Kodak (Rochester, NY) -- are being built as prototype ultralight mirrors. The Univ. of Arizona's 2-m "bed of nails" mirror, which uses a 2-mm facesheet of glass over a bed of more than 160 actuators, is called a high-authority mirror because it enables minute control of the deformable surface. COI and IABG (Germany) developed a carbon-fused silica-carbon semirigid mirror that uses sparse actuation to control radius of curvature and tip, tilt, and piston of the mirror.
The NMSD and SBMD mirrors are in various stages of construction and cryogenic testing. For the AMSD program a multi-agency team must now reduce the initial eight designs down to approximately four technologies for final manufacture and testing. System specs
Lockheed/Raytheon and TRW/Ball Aerospace are competing for the contract to build the 8-m-dia. spaceborne telescope. According to Jacobson, the two groups have widely different plans; one uses 36 petals (mirror segments) of approximately 1.2 m2 each, and the other uses seven petals measuring approximately 3 m2.
The final system configuration will depend on the results of the AMSD program and architecture assessments over the next year. Other considerations for a final design include the type of mirror, methods of actuation and folding, and many other mechanical factors. For example, the 8-m-dia. specification for NGST could change based on the estimated cost of the overall system and the requirements of the scientific package. Jacobson said the decision about the final mirror will be made by the contractor chosen by NASA officials.
The mirrors themselves are required to weigh less than 15 kg/m2, perform at ambient temperatures down to 35 K, and have a base requirement for reflectance from 0.6 to 10 m. NASA hopes to extend that from the visible to 30 µm. Scott Smith, director of the Space Optics Manufacturing Technology Center at MSFC, said the final mirror must provide a diffraction-limited spot at 2 µm. This mirror could scale up to 20 µm or beyond for imaging distant red-shift galaxies and other celestial bodies, but only if detector elements can handle the spot size.
"The nice thing about AMSD is that we've excited the optical industry -- asked them to deliver and we really got what we wanted," Jacobson said. "Our job is to just bring these different mirror technologies to the table and see how they perform."
The final phase of the NGST prime architecture contract, which will cover the overall design of NGST, is expected to be awarded by the first quarter of 2001.
R. Winn Hardin
R. Winn Hardin is a science and technology writer based in Jacksonville, FL.