NASA defines mirror technology needs for future space telescopes

Regardless of operating wavelength or mission application, all future space telescopes share common technical and programmatic challenges. One of the greatest technical challenges is finding a way to make large-aperture low-areal-density mirrors with the required surface figure precision, finish, and mechanical stiffness. Current observatories are mass- and volume-limited by the launch vehicle, and these limits constrain the aperture. Future observatories are being enabled by lower-areal-density (mass per square meter) mirrors with efficient launch packaging and deployment concepts. The greatest programmatic challenge is to rapidly manufacture affordable mirrors. The amount that can be spent on future space telescopes is limited. Reducing the areal cost (dollars per square meter) of mirrors makes larger-aperture systems cost effective. The ultimate goal is to mass produce either classes of telescopes or mirror segments for segmented telescopes, resulting in a cost/performance version of Moore's Law for space telescopes.

So, how do we achieve these goals? I don't know, but maybe you do. That's the philosophy behind the Mirror Technology Roadmap. The roadmap is not a plan for how to get from point A to point B but rather a statement of the current technological state of the art and where the technology needs to go to enable desired future space telescopes.

NASA chartered the Advanced Telescope and Observatory (ATO) Roadmap Committee (CRM) to assess technologies necessary to enable future space telescopes and observatories operating in electromagnetic bands ranging from x-rays to millimeter waves, and also gravity waves. Its 15 March 2005 report to the National Research Council covered six technology areas: optics; wavefront sensing and control, and interferometry; distributed and advanced spacecraft systems; cryogenic and thermal control systems; large precision structures for observatories; and the infrastructure essential to future space telescopes and observatories.

The ATO CRM defines an optics capability as a system of components—such as mirror substrates and facesheets, coatings, and actuators—along with the respective manufacture and test processes necessary to collect and concentrate electromagnetic radiation. The roadmap further defines four sub-capabilities based principally upon wavelength region: cryogenic optics (IR, far-IR, submillimeter, and microwave), precision optics (extreme UV [EUV], far UV [FUV], UV, visible, and lidar), grazing incidence optics (FUV and x-ray), and diffractive, refractive, and novel optics (gamma, x-ray, or other).

Roadmaps that lay out key technology readiness milestones supporting specific mission requirements were generated for each sub-capability. Capabilities are assumed to be required five years before a mission. Each technology has a cyclic development plan and is supported by four underlying technical metric requirements: mirror-surface figure error (or resolution for x-ray mirrors), areal density, size, and areal cost.

Future IR/far-IR/submillimeter wavelength missions require very large aperture but modest-quality mirrors operating at temperatures from 4–40K (see Figure 1). While current mirrors can satisfy most technical requirements for such missions, their cost is too great. Thus, for this waveband, the most important enabling capability is to reduce areal cost by an order of magnitude. Potential approaches include replication, nanolaminates, near-net shaping, and advanced polishing techniques. Another enabling technology for this spectral region is the application of coatings that preserve uniform polarization.

Future EUV, UV, and visible wavelength missions require large-aperture, extremely smooth, and highly stable ambient temperature mirrors (see Figure 2). The most challenging mission in the near term is the Terrestrial Planet Finder Coronagraph (TPF-C), which requires a primary mirror that has never been demonstrated on the ground, let alone in space. This is an extremely smooth (4nm rms surface), 4 × 8m, lightweight (∼40kg/m²) mirror with extremely uniform optical coating reflectivity and polarization properties. The cost-effective fabrication of this mirror requires the application of techniques previously only demonstrated on meter-class microlithography optics to an 8m-class mirror.

Because of launch vehicle constraints, some UV/optical missions may choose segmented mirror architectures. While it is easier to manufacture smaller mirror segments, to minimize scattered light and diffraction effects the segments must be accurately figured and polished completely to the mirror's physical edge. Additionally, for phasing, each segment's position must be mechanically controlled to extreme tolerances (0.1nm). Three specific enabling coating technologies are 80% reflectivity coatings from 90 to 120nm, and 0.1% uniform reflectivity and 0.1% uniform polarization coatings from 400 to 1000nm.

Future x-ray missions require large-aperture precision-quality grazing incidence mirrors (see Figure 3) that are truly revolutionary when compared to Chandra. The Constellation-X (ConX) mission plans a four-telescope architecture with 60 times the effective collecting aperture as Chandra (6m²). Each telescope is planned to be 1.6m in diameter and 1m long with 20 times lower areal density (<3 kg/m²) and 50 times lower areal cost (<0.1 M/m²) than Chandra. The technology needed to manufacture these mirrors requires new materials and fabrication processes. Obviously, mass production via some type of replication process would be ideal. The only aspect that is less demanding is that, at 15 arcsecond resolution, ConX has optical surface figure error requirements that are 30 times looser than Chandra's. However, because of the lower areal density, the mechanical support, alignment, and stability of such optics are a significant challenge. These challenges only increase for the Black Hole Imager (BHI) and Extreme Universe X-Ray Observatory (EUXO) missions.

The last, catch-all category—diffractive, refractive, and novel optics—includes coded apertures, occulting imaging, holographic optical elements (HOEs), Fresnel lenses, etc. These classes of novel optics are hard to roadmap, but they may enable enhanced (and more affordable) approaches to planned missions as well as unexpected missions through their clever use of novel concepts. This is a critical area to encourage, particularly as the technological challenges increase in difficulty for traditional optics approaches.

In conclusion, lightweight affordable optics are essential for future large-aperture space optical systems for Earth science, solar observations, and astronomy. The ATO CRM has developed roadmaps that lay out the necessary optics capabilities (systems of components and their manufacture and test processes) for future space telescopes for observing a range of electromagnetic bands.