A starshade occulter is a large space structure whose shape is specially designed to produce a diffraction pattern in starlight that can aid a telescope in direct imaging of exoplanets. The diffraction pattern produces extremely high-contrast dark regions in the starshade’s shadow on the order of 10-9 or 10-10. To do so, the edge shape of the structure must be held to extremely tight tolerances. In addition, potentially obscuring glint light from the Sun must be minimized to prevent loss of contrast.
This project sought to find suitable materials for the space environment that could be used to manufacture flat edge segments up to 1 meter in length, with tight in-plane shape tolerances on the order of tens of micrometers, along with sharpened edges with sub-micrometer, out-of-plane radii at the edge tips to reduce sunlight glint and loss of contrast.
In addition to the above requirements, the parts should be flexible so as to be stowed within a launch shroud prior to deployment in space. They must be able to survive thermal cycling on the order of ±50 to 100 ºC, and not unduly distort the shape of the low-thermal-expansion host structure.
The solution is the use of thin, highly elastic, amorphous metal foils carefully shaped by a photo-chemical etching process. The combination of the manufacturing process and amorphous material inherently produces sharp edges that are sub-micron in radius. Two similarly shaped foils are bonded on either side of a thicker carbon fiber-reinforced polymer laminate for stiffening. The overall laminate is thermally balanced. One of the foils is slightly oversized with respect to the others, and is the “optical edge” layer. This layer’s sharp edge is the only edge encountered by the target star’s light on its way to the telescope. The undersized foil is referred to as the “dummy,” “shadowing,” or “baffle” layer.
The amorphous metal alloys are a combination of various metallic elements, and are quenched rapidly from the melt to prevent the growth of crystalline grain structures, and freeze the atoms in a random, glassy state. This increases the materials’ elastic yield limits, and minimizes the materials’ plastic deformations experienced during handling and application use, i.e. permanent wrinkling, creasing, etc. typically experienced with other thin metal foils.
The carbon fiber-reinforced polymer laminate used for mechanical stiffening is constructed from a stack of unidirectional plies. The fiber laminate plies are optimally oriented to obtain good thermal expansion performance with respect to the system requirements in both longitudinal and transverse directions. The dummy foil layer allows for the stack-up to be neutral in bending under temperature changes, since the entire stack-up is symmetric about the midplane. It also provides extra shading for the actual optical edge for off-axis stray light sources such as the Sun. Sunlight coming towards the starshade off-axis would need to scatter around both foils’ edges to enter the telescope aperture. Note that this added double scattering benefit only applies to the half of the starshade oriented opposite the stray light source — the socalled “trailing edges.”
This work combines the use of etched amorphous metal materials and carbon fiber-reinforcement to create 1-meterlong parts with precision-controlled edge geometries and sharp edge profiles that are robust. These precision, sharpedge parts are one of the key “high tentpole” enabling technologies that would allow successful construction and operation of a high-contrast-producing starshade occulter. Without these edge parts, the carefully contrived dark shadow behind the shade would be partially filled by bright sunlight glint.