A. B. Meinel

Ultralightweight and hyperthin rollable primary mirror for space telescopes

The aperture of monolithic space telescope primary mirrors placed on orbit is limited to payload faring diameters, the largest being about 4-meters. This requires a novel stowage approach for monoliths larger than 4-meters. Very large aperture telescopes, 50 to 100-meter diameters, planned for deployment in the next 10 to 20 years will also require very large mirror segments in an effort to manage the phasing of the entire surface. The larger the mirror panels the fewer that will be required for such apertures. If the mirrors can be made thin enough to be deformed into a cylinder or undeformed but closely nested, enough surface area can be placed on orbit to facilitate large aperture telescope mirrors. 8-meter monolithic mirrors can be rolled into a 2.5-meter diameter cylinder with the secondary support structure stowed in the cylinder to maximize the payload faring volume. Hyper-thin mirrors can be closely nested in order to maximize volume as well. Presented is a design and engineering model of a 0.9-meter diameter hyper-thin, ultra- lightweight spherical composite mirror and methods, which led to the fabrication of the mirror.

Two-stage optics: high-acuity performance from low-acuity optical systems

We developed the concept of two-stage optics in 1984 under the Jet Propulsion Laboratorys 20-m Large Deployable Telescope (LDR) program to enhance the performance, lower the cost, and increase the reliability of LDR. It permitted the large primary mirror to remain as deployed or as space-assembled, with phasing and subsequent control of the system done by a small, fully assembled optical active element placed at an exit pupil. The performance and tolerances of such a system were explored. A two-stage optics testbed was constructed that successfully demonstrated the concept. Extension of the concept for upgrading the perlormance of other large, low-cost, wide-field, space optical systems was explored. An opportunity to utilize this concept arose when the problems with the Hubble Space Telescope (HST) were discovered. Its advantages for use in future very large space telescopes are addressed.

Inflation-deployed camera and hyper-thin mirrors

Inflatable optics have the potential for large reduction in launch mass and volume, but they involve significant challenges to achieve the wavefront accuracy required for diffraction-limited operation in the visible and near IR. Current studies identify two major subsets of this topic: 1) inflation-deployed structures with a monolithic, but rolled, hyper-thin primary mirror and 2) an inflatable structure and inflatable membrane primary mirror. We address the current state of the art, the challenges involved, and a program development plan.

FAST: A Folded Astronomical Space Telescope

A Folded Astronomical Space Telescope is a 2.4-m Hubble Space Telescope class of telescope that can be packaged in a 1.5-m diameter cylinder through use of a single ring of eight deployable segments. Because it has less mass and uses a much smaller booster to inject it into orbit, the cost is greatly reduced. The enabling rationale, general configuration, and optical technologies for such a telescope are presented.

Precision segmented reflectors for space applications

The Precision Segmented Reflectors (PSR) project represents a first step toward developing the technology base needed to support future advanced astrophysics missions, especially those that operate at submillimeter wavelengths. The focus of the effort is to develop a lightweight, low cost option for building large aperture, segmented reflecting space-based telescopes. The principal driver for PSR technology is the Submillimeter Telescope program including the proposed 20 meter Large Deployable Reflector (LDR) telescope, and its precursor-survey mission which is currently in early development and definition. Four major technical areas, reflector panels and materials, structures, and figure control are under development by PSR. These technical areas are, however, generic in nature and can be applied to other future missions such as optical communications, optical interferometers, and missions requiring large diameter, segmented reflectors. In this paper, specific project objectives, approaches and significant challenges will be described. Technology development issues encountered will also be discussed. Finally, project status will be reported.

Wavefront control of large optical systems

Several levels of wavefront control are necessary for the optimum performance of very large telescopes, especially segmented ones like the Large Deployable Reflector. In general, the major contributors to wavefront error are the segments of the large primary mirror. Wavefront control at the largest optical surface may not be the optimum choice because of the mass and inaccessibility of the elements of this surface that require upgrading. The concept of two-stage optics was developed to permit a poor wavefront from the large optics to be upgraded by means of a wavefront corrector at a small exit pupil of the system.

Deployable space telescopes for planetary and astronomical missions

A telescope folded into minimum volume for launching can be a powerful technique for maximizing the aperture size that can be accommodated in a given launch vehicle shroud. As an example we show our concept for a Folded Astronomical Space Telescope where a 2.4-m Hubble Space Telescope class of telescope can be packaged in a 1.5-m diameter cylinder. The enabling rationale, general configuration, and optical system technologies for such a telescope will be presented.

Infrared and the search for extrasolar planets

Search for evidence concerning the existence of extrasolar planets will involve both indirect detection as well as direct (imaging). Indirect detection may be possible using ground based instrumentation on the Keck telescope, Imaging probably will require an orbiting system. Characterizing other planets for complex molecules will require a large orbiting or lunar-based telescope or inteferometer. Cryogenic infrared techniques appear to be necessary. Planning for a NASA ground and space-based program, Toward Other Planet Systems (TOPS), is proceeding.