Figure 1. AAReST spacecraft components
Figure 1 depicts the individual components of AAReST. It is comprised of a number of subsystems: the central "Coresat", 2 separate "MirrorCraft", a camera package and a deployable boom. The CoreSat is the central hub of the telescope containing the bulk of the spacecraft's control hardware. The camera package, located at the focus of the telescope contains various elements such as corrective optics, wavefront sensors and an imaging detector. A foldable composite boom separates the camera package from the primary aperture of the telescope.
The MirrorCrafts are small independent spacecraft used to house the deformable mirrors. The deformable mirrors sit atop these spacecraft with the control electronics directly underneath. Each MirrorCraft is equipped with its own propulsion system in order to perform the autonomous reconfiguration and docking maneuvers. The docking procedure is aided through the use of electromagnets and a computer vision system.
Figure 2. The AAReST spacecraft in the (a) "Launch" configuration, (b) "Compact" imaging mode, and (c) "Wide imaging mode.
Figure 2 shows the 3 configurations of the AAReST spacecraft. The first is the "Launch" configuration where the deployable boom is folded tightly around the CoreSat. This is done in order to minimize the launch volume. Once in orbit, the boom will deploy and position the camera package at the focus of the telescope. The telescope is now in the "Compact" imaging configuration. At this point in the mission, the initial mirror calibration will take place and the first set of images will be captured. Once all objectives have been completed in this configuration, two of the MirrorCraft will perform an autonomous reconfiguration maneuver and position themselves at the outer edges of the spacecraft. The telescope is now in the "Wide" configuration where the mirror calibration and imaging procedure will be repeated, thus demonstrating the unique capabilities of the deformable mirrors.
The deformable mirrors are made using thin glass wafers and a layer of piezoelectric polymer. A custom electrode pattern is incorporated onto the backside of the piezoelectric polymer allowing for shape control to be performed. The mirrors have a total stroke of ~40um allowing for the correction of large shape errors. More information on the deformable mirrors can be found here.
Figure 3. (Left) Exploded view of the various layers within the deformable mirror. (Right) CAD model of the deformable mirror package used for AAReST.
The focal length of the telescope is provided by a thin-walled composite boom. The boom, fabricated in collaboration with our partners at AFRL, is constructed using a combination of unidirectional carbon fiber and plain-weave fiber glass. Material is removed at select locations along the length of boom creating self-deploying tape-spring hinges. Four hinges are implemented in the design of AAReST, allowing the boom to be tightly packaged around the spacecraft for launch. Deployment is performed using a two-stage process as shown in Figure 4. More information on the design and characterization of the boom can be found here.
Figure 4. Overview of the two-stage deployment process for the AAReST spacecraft.
The camera package houses the optical and electronic hardware required for imaging. Figure 5 is a ray-trace of the optical path within the camera. Light originating from the prime focus of the telescope enters the camera and is passed through a series of collimating lenses. A pair of beam splitters redirect a portion of this light onto two Shack Hartmann wavefront sensors providing knowledge of the shape of the deformable mirrors. The remaining portion travels through a pupil mask and through another set of lenses to reimage the light onto the imaging detector. Images are captured using this detector once the telescope has gone through its calibration process.
Figure 5. Geometric ray-trace of the optical path within the Camera Package.