Autonomous Assembly of a Reconfigurable Space Telescope (AAReST)
In recent years, there has been a desire to develop space-based optical telescopes with large primary apertures. Currently the largest primary aperture under development is that of the James Webb Space Telescope with a diameter of 6.5 m. It represents a major shift in telescope design due to the use of a deployable primary mirror. However, its size is still limited by the diameter of the launch vehicle; a limitation for all current space-borne telescopes. One method to overcome this obstacle is to autonomously assemble small independent spacecraft, each with their own mirror, while in orbit. In doing so, a telescope with a large, segmented primary mirror can be constructed. Furthermore, if each of these mirrors is manufactured to have an identical initial shape and then adjusted upon assembly, a substantial reduction in manufacturing costs can be realized. In order to prove the feasibility of such a concept, a collaborative effort between the California Institute of Technology and the University of Surrey has been formed.
In order to demonstrate this concept, the "Autonomous Assembly of a Reconfigurable Space Telescope" (AAReST) mission has been developed. The AAReST telescope is shown in Figure 1. It is a prime focus design (1.2 m focal length, 0.3 deg field-of-view) with the primary mirror divided into a sparse aperture consisting of an arrangement of 10cm diameter circular mirrors. The primary mirror segments are attached to a cluster of Cubesats, two of which are able to undock from the cluster and navigate independently. The telescope is to launch as a small secondary payload in a stowed state as seen in Figure 2. The stowed volume of the telescope is 0.5m by 0.5m by 0.6m. After separation from the primary payload, the telescope deploys its sensor package to the focus of the mirror array using a deployable boom.
Using wavefront sensors, the mirrors can be adjusted and calibrated in order to minimize the size of the mirrors' individual point spread function (PSF). The mirrors are not co-phased down to sub-wavelength levels (as would be required for an actual science mission), as this would require an additional metrology system that is prohibitively expensive for a small mission such as this. Instead, images from each mirror are to be captured in order to demonstrate the ability of the mirrors to self-correct their shape.
Once the initial calibration and imaging requirements have been met two of the mirror segments, carried by independent Cubesats equipped with propulsion systems, are to detach from the mirror cluster, perform an orbital maneuver to reposition themselves at a new location in the array, and then redock to the cluster as shown in Figure 2. This will demonstrate on-orbit assembly of the mirror segments. Once the cluster is reassembled, the mirror calibration and imaging are to be performed again in order to show the capability of calibration in various configurations.