We present results of a study of a deployable version of the Advanced Technology Large-Aperture Space Telescope
(ATLAST), designed to operate in a Sun-Earth L2 orbit. The primary mirror of the segmented 9.2-meter aperture has 36
hexagonal 1.315 m (flat-to-flat) glass mirrors. The architecture and folding of the telescope is similar to JWST, allowing
it to fit into the 6.5 m fairing of a modest upgrade to the Delta-IV Heavy version of the Evolved Expendable Launch
Vehicle (EELV). We discuss the overall observatory design, optical design, instruments, stray light, wavefront sensing
and control, pointing and thermal control, and in-space servicing options.
The James Webb Space Telescope (JWST) consists of an infrared-optimized Optical Telescope Element (OTE)
that is cooled down to 40 degrees Kelvin. A second adjacent component to the OTE is the Integrated Science
Instrument Module, or ISIM. This module includes the electronic compartment, which provides the mounting
surfaces and ambient thermally controlled environment for the instrument control electronics. Dissipating the 200
watts generated from the ISIM structure away from the OTE is of paramount importance so that the spacecraft's
own heat does not interfere with the infrared light detected from distant cosmic sources. This technical challenge
is overcome by a thermal subsystem unit that provides passive cooling to the ISIM control electronics. The
proposed design of this thermal radiator consists of a lightweight structure made out of composite materials
and low-emittance metal coatings. In this paper, we will present characterizations of the coating emittance,
bidirectional reflectance, and mechanical structure design that will affect the performance of this passive cooling
Future large aperture infrared space telescopes such as the Next Generation Space Telescope will require lightweight, deployable sunshields to enable passive radiative cooling to cryogenic operating temperatures. In addition to the requirement for a high performance thermal design, mechanical and structural requirements are also demanding due to constraints on mass and volume. NASA has supported several technology development efforts to reduce risks in the area of sunshield structures, including: system packaging and deployment, film management, materials characterization, modeling tools for thin-film membranes, and ground test capabilities for characterizing structural performance. This paper discusses recent progress in sunshield structures technology development relating to post-deployment structural performance. First, improved approaches for analyzing partially wrinkled, thin-film membrane structures will be discussed. Next, new techniques for static and dynamic testing of ultra-lightweight structures will be described. Finally, analytical and experimental results from two recent studies will be described: (a) dynamic characterization of a 1/10th scale sunshield model and (b) static shape characterization of a 1/20th scale sunshield membrane layer. Results from these studies will provide valuable resources for use in design of sunshields for future space telescopes.
An Integrated Product Team was formed to develop a detailed concept for optical test methodology for testing of the NGST individual primary, secondary and tertiary mirrors and the full telescope system on the ground. The large, lightweight, deployable primary mirror, and the cryogenic operating environment make optical testing of NGST OTA (Optical Testing Assembly) extremely challenging. A telescope of the complexity of NGST has never been built and tested on the ground in 1-g environment. A brief summary of the preliminary metrology test plan at the mirror component and telescope system level is presented.
The detection of exo-Zodiacal discs is an important step in the NASA program for the detection of exo-solar planets, in particular, the detection of earth-like planets. We show that by incorporating a nulling interferometer NGST would be very well suited to study exo-zodiacal disks in nearby stars. Over 400 stars could be surveyed and at least 40 could be resolved such that structural parameters on the 2-3 AU scale could be measured. This can be done within the existing optical NGST design but demands a wavefront RMS error less than (lambda) /100 at 10 micron in order to assure a wavefront cancellation of 1000. In addition, a new concept for an off-axis NGST design is discussed.
Current concepts for the NGST call for an 'open' telescope design in order to passively cool the telescope and instrument to cryogenic temperatures. We show that, in spite of the lack of an external baffle, straylight due to off- axis sources is negligible compared to the zodiac light. We also show that the instrumental thermal emission is in general dominated by scatter from the sunshield and not by emission from the optics. The back surface of the sunshield needs to be at about 180 K or less to reduce the self emission of the observatory to a negligible level in the near-IR, and at about 90 K or less in the case of the mid- IR.
Preliminary studies of passively cooling the NGST utilizing a lightweight deployable subshield are described. The NGST mission concept of a passively-cooled large-aperture optical telescope is unique from any other mission flown to date. We show that achieving operational temperatures of less than 50 K appears feasible by passive cooling alone through a combination of (i) operating the observatory far from the Earth so that the Sun becomes the only significant source of environmental heating, (ii) selecting an observatory configuration that isolates all significant heat dissipation from the cold telescope, and (iii) employing a high performance sunshield to attenuate the incident solar radiation. The observatory configuration consists of the sunshield with cold telescope and instrument elements on the anti-sun side, and warm spacecraft avionics and propulsion elements on the sun-side of the sunshield. A sunshield thermal configuration trade study, preliminary telescope thermal analyses, and a mechanical concept for a lightweight deployable sunshield are presented. Also discussed are the remaining issues to be addressed.