Over 3000 exoplanets and hundreds of exoplanetary systems have been detected to date and we are now rapidly moving
toward an era where the focus is shifting from detection to direct imaging and spectroscopic characterization of these
new worlds and their atmospheres. NASA is currently studying several exoplanet characterization mission concepts for
the 2020 Decadal Survey ranging from probe class to flagships. Detailed and comprehensive exoplanet characterization,
particularly of exo-Earths, leading to assessment of habitability, or indeed detection of life, will require significant
advances beyond the current state-of-the-art in high contrast imaging and starlight suppression techniques which utilize
specially shaped precision optical elements to block the light from the parent star while controlling scattering and
diffraction thus revealing and enabling spectroscopic study of the orbiting exoplanets in reflected light. In this paper we
describe the two primary high contrast starlight suppression techniques currently being pursued by NASA: 1)
coronagraphs (including several design variations) and 2) free-flying starshades. These techniques are rapidly moving
from the technology development phase to the design and engineering phase and we discuss the prospects and projected
performance for future exoplanet characterization missions utilizing these techniques coupled with large aperture
telescopes in space.
NASA has decided to move forward with two complementary Terrestrial Planet Finder (TPF) missions, a visible coronagraph and an infrared formation flying interferometer in collaboration with ESA. These missions are major missions in the NASA Office of Space Science Origins Theme. The primary science objectives of the TPF missions are to search for, detect, and characterize planets and planetary systems beyond our own Solar System, including specifically Earth-like planets.
Mechanical cryocoolers represent a significant enabling technology for NASA's Earth and Space Science Enterprises. Over the years, NASA has developed new cryocooler technologies for a wide variety of space missions. Recent achievements include the NCS, AIRS, TES and HIRDLS cryocoolers, and miniature pulse tube coolers at TRW and Lockheed Martin. The largest technology push within NASA right now is in the temperature range of 4 to 10 K. Missions such as the Next Generation Space Telescope (NGST) and Terrestrial Planet Finder (TPF) plan to use infrared detectors operating between 6-8 K, typically arsenic-doped silicon arrays, with IR telescopes from 3 to 6 meters in diameter. Similarly, Constellation-X plans to use X-ray microcalorimeters operating at 50 mK and will require ~6 K cooling to precool its multistage 50 mK magnetic refrigerator. To address cryocooler development for these next-generation missions, NASA has initiated a program referred to as the Advanced Cryocooler Technology Development Program (ACTDP). This paper presents an overview of the ACTDP program including programmatic objectives and timelines, and conceptual details of the cooler concepts under development.
Four teams incorporating scientists and engineers from more than 50 universities and 20 engineering firms have assessed techniques for detecting and characterizing terrestrial planets orbiting nearby stars. The primary conclusion from the effort of the past two years is that with suitable technology investment starting now, a mission to detect terrestrial planets around 150 nearby stars could be launched within a decade. Missions of smaller scale could carry out more modest programs capable of detecting and characterizing gas giant planets around tens of stars and of detecting terrestrial planets around the nearest stars.
The Next Generation Space Telescope (NGST) is a major element of NASA's Origins Program. It is planned to be a deployable infrared telescope with an 8 m diameter aperture and a sensitivity approximately equals 1000 times greater than any currently existing infrared telescope. The scientific goals of NGST include imaging and spectroscopic characterization of the earliest galaxies and proto-galaxies, which formed following the `big bang'. Several years ago, NASA embarked on an aggressive technology development effort covering a number of technical areas including optics, detectors, deployable structures, wavefront control, passive cooling, operations, etc. This paper presents an overview of the status of the program NASA is pursuing to provide the necessary technologies, which will enable an exciting, affordable NGST mission.
NASA has embarked on the development of the NGST. The NGST is envisioned to be a large aperture deployable IR telescope with sensitivity 1000 times greater than any currently existing or planned IR telescope. The scientific goals of NGST include imaging the earliest galaxies and proto- galaxies which formed following the 'big bang'. Several studies have concluded that the mission is feasible within the proposed cost if a well-planned, aggressive technology development effort is implemented early in the development phase. This paper present an overview of the technology program NASA is pursuing to provide the necessary technology to enable an exciting, affordable NGST mission to launch early in the next century.
As part of the technology validation strategy of the next generation space telescope (NGST), a system testbed is being developed at GSFC, in partnership with JPL and Marshall Space Flight Center, which will include al of the component functions envisioned in an NGST active optical system. The system will include an actively controlled, segmented primary mirror, actively controlled secondary, deformable, and fast steering mirrors, wavefront sensing optics, wavefront control algorithms, a telescope simulator module, and an interferometric wavefront sensor for use in comparing final obtained wavefronts from different tests. The developmental cryogenic active telescope testbed will be implemented in three phase. Phase 1 will focus on operating the testbed at ambient temperature. During Phase 2, a cryocapable segmented telescope will be developed and cooled to cryogenic temperature to investigate the impact on the ability to correct the wavefront and stabilize the image. In Phase 3, it is planned to incorporate industry developed flight-like components, such as figure controlled mirror segments, cryogenic, low hold power actuators, or different wavefront sensing and control hardware or software. A very important element of the program is the development and subsequent validation of the integrated multidisciplinary models. The phase 1 testbed objectives, plans, configuration, and design will be discussed.
The Infrared Technology Testbed Telescope (1T1T) is a demonstration telescope meeting the needs of the SIRTF mission. It is a Ritchey-Cretien form designed for diffraction limited performance at 6.5 pm, at 5.5 K with an 85 cm. clear aperture. The mirror and system focal ratios are f/1.2 and f/12 respectively. This paper describes the design and fabrication of the efficient, ultra-lightweight, all-beryllium telescope. The design incorporates a central metering tower and single arch primary mirror to achieve a total telescope mass of less than 30 kg. Cryogenic testing of the primary mirror demonstrates the stability of the I-70-H (special) Be and the fabrication process. No thermal hysteresis was observed after repeated cycling to 5 K, and cryo-null figuring was utilized to overcome the small thermal instability observed at that temperature.
The SIRTF Telescope Test Facility (STTF) consists of an optical dewar for testing mirrors of up to 1m diameter and f < 6 at temperatures from 300K to 5K and a phase shift interferometer for optical characterization. The STTF was brought on-line in early 1995. The STTF was initially used to cool a 50cm diameter beryllium mirror that had been previously tested at NASA Ames Research Center. The initial tests validated the performance of the STTF by proving that the STTF could cool a mirror to 5K and achieve high quality optical data on the mirror, consistent with the previous results achieved at NASA Ames. The STTF has also been used to provide cryogenic optical testing of the ultra- lightweight 85cm diameter beryllium primary mirror assembly for the Infrared Telescope Technology Testbed (ITTT). Currently the facility is preparing for testing the complete ITTT. Also, the long wavelength photon background in the facility will be measured and characterized in 1996.
Stability of beryllium mirrors is said to be unpredictable. Three recent mirrors demonstrate excellent stability. JPL produced a plano-concave, 0.5 m solid test mirror that was machined from a HIP'ed billet of special I-70 Be powder, polished bare and tested to 4K. It was thermally stable and had no hysteresis. The JPL ITTT 0.85 m primary mirror used similar material and processes, but with more stress relied treatments. Tests of this bare-polished, very lightweight single arch hyperboloidal mirror to 5K showed similar excellent results. The ESO VLT chopping secondary is a 1.12 m, machined lightweight, nickel plated, convex paraboloid. Similar fabrication processes are being used but with higher strength I-220 Be. In-processes testing indicates a stable mirror. The results show beryllium to be a stable and predictable mirror material.
In this paper we describe the key features of the SIRTF Telescope Test Facility developed at the Jet Propulsion Laboratory. Information on the cryogenic performance including details of the test cycle time and cryogen hold time are included. Emphasis is on the operation of the facility. Data are presented on the cryogenic optical testing of the ultra-lightweight 85 cm diameter beryllium primary mirror assembly for the infrared telescope technology testbed.
Many future space optical systems are dependent on large apertures to achieve the collecting power or resolution necessary to meet mission goals. Traditional mirror materials such as glasses and metals result in optics which are heavy and costly to both fabricate and deploy. In recent years, an approach for fabricating large, lightweight, precision optics from fiber reinforced organic matrix composite materials has been developed and demonstrated. These mirror panels consist of composite facesheets bonded to an open cell core. A key element of this technology is the durability of the composite construction materials. Extensive testing has been performed on a number of composite materials based on carbon fiber and organic resins. Ultimately, a high modulus graphite fiber/cyanate ester composite system was chosen for the panel facesheets due to its superior mechanical properties, processability, thermal stability, radiation resistance, very low water absorption, and temporal stability. This program has successfully demonstrated the feasibility of fabricating space durable mirrors using composite materials technology.
Ion beam deposition processes were used to influence the microstructure and scattering characteristics of gold and platinum coatings. These metallic coatings were deposited onto super-polished fused silica and Zerodur substrates. They were deposited using thermal evaporation, ion assisted deposition, sputtering, and ion assisted sputtering. The microstructure of these films was characterized using an optical scatterometer, an optical profilometer, a stylus profilometer, scanning tunneling microscopy, and atomic force microscopy. We found that these metallic coatings deposited using ion beam deposition produced lower optical scattering and related surface microroughness.
The Multi-angle Imaging SpectroRadiometer (MISR) plans to use deployable diffuse reflectance panels to provide periodic radiometric calibrations of its nine cameras while in-flight. Near-Lambertian reflectance characteristics are desirable to facilitate flat-field camera intercomparisons. Also required is panel spatial and spectral uniformity, and stability with time. Spectralon, a commercially available polytetrafluoroethylene (PTFE) compound, has been baselined in the MISR design. To assess the suitability of this material, a series of degradation tests were planned and implemented. These included UV vacuum exposure and proton bombardment tests which simulated the exposure levels to be encountered during the mission life. Proton levels are now considered too low to be of concern, but UV vacuum tests demonstrate sensitivity to material contamination. Material investigations have concluded that hydrocarbons are present in the bulk of the material, and that plastic packaging materials can introduce additional surface-layer contamination. It is found however, that these unwanted elements can be eliminated through vacuum pumping at elevated temperatures. Exposure to a UV source, while in vacuum, is again planned for a set of targets which have been vacuum baked. This will assess the stability of the pure PTFE form.
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