The Challenges for Next-Generation Space Telescopes
BY TED MOONEY, PHD
In the world of optical telescopes—the integral component in many Earth and space observation missions—the “big deal” is the diameter, or the aperture, of the primary mirror. This is because the telescope with the largest-aperture mirror can see the finest details. When the telescope is pointed into space, this can lead to exciting new revelations about the universe.
When the Hubble Space Telescope launched in 1990, NASA hailed it as the “most significant advance in astronomy since Galileo’s telescope.” Its primary mirror had an aperture of 2.4 meters. The aperture of the primary mirror in the James Webb Space Telescope*, expected to launch in 2019, is a massive 6.5 meters—enabling more than six times Hubble’s collection area and observation of infant galaxies born soon after the Big Bang. Future space-based telescopes will likely require mirrors that are even bigger for missions that will continue to uncover the secrets of the universe, including star, planet, and galaxy formation, the nature of dark matter, and potentially habitable planets.
So how do you get bigger mirrors into a telescope that can fit inside a launch vehicle? For the Webb telescope and others to come, the solution lies in segmentation. Eighteen hexagonal mirror segments made from lightweight beryllium comprise Webb’s primary mirror. These are mounted on a structure that folds up into compact payloads for launch and then deploys to full size in space. Having developed large optical systems for decades and as the integration and test lead for Webb’s primary mirror, we know firsthand the challenges precision segmented optical systems present for space applications. New manufacturing processes, materials, technologies, and test strategies will help address many of these.
RAPID, AFFORDABLE PRODUCTION
As telescope apertures increase in size, the requirements for precision and stability increase as well. Mirror segments must successfully come together in space to form a single reflective surface—and this means precise matching of each segment’s radius. Speeding up the process of making multiple identical segments can significantly reduce costs and improve the ability to get missions into space.
The desire to improve upon both the traditional cost and cycle time for producing large, space-based precision optics prompted Harris to engage in a series of multi-year research and development initiatives to advance mirror construction techniques. As a result of these efforts, we are applying new materials and processes that quickly replicate aspheric mirrors up to a certain performance point, or “capture range,” before final finishing processes are applied, thereby eliminating the high-cycle-time, high-cost steps associated with traditional early grinding and polishing steps. This same approach is advantageous when the mission requires multiple mirrors that have the same optical alignment, or “prescription,” such as segmented optical systems.
ADVANCED SYSTEM DEPLOYMENT AND CONTROL
Segmented mirrors add a whole new level of complexity to space telescope deployment and control, demanding improved opto-mechanical structures and mechanisms that enable successful precision deployment. This is because once deployed, the segments must be able to maintain their prescription in the changing environment of space to deliver a continual stream of high-resolution images.
Over the past 10 years, Harris has worked with government and industry partners to execute research and development programs to demonstrate key technologies that will accomplish this. Techniques like segment-to-segment sensing, advanced wavefront sensors, and precision mirror actuation are strategies that will provide the required ultra-stable system performance for tomorrow’s space telescope mirrors. More such demonstrations will be needed to test future concepts to push the envelope in mirror precision, size, and affordability. An integration infrastructure and disciplined testing process like those we have in-house will continue to play an important role in reducing mission risk.
GOING SMALL FOR BIG BENEFITS
Smallsat capabilities are maturing, presenting two important opportunities for those of us in the business of space-based optical systems. First, by providing rapid and affordable launch solutions, smallsats have the potential to encourage more missions—in number and type. These can include missions with optical payloads that would benefit from the use of constellations for broader coverage and resiliency. At Harris, we have adapted the telescopes on our world-class Earth imaging systems to fit a smallsat form factor. With the capability of capturing images with a 1-meter resolution, these systems open the door for new users and applications ranging from urban planning and investment analysis to disaster response and vegetation assessment.
Second, while smallsats, which typically travel in low Earth orbits, may not be able to provide the images desired for large universe exploration missions, they do offer a practical platform to demonstrate key pillars of segmented systems and accelerate new technology implementation. The development and rapid demonstration of key technologies would significantly increase the feasibility of large mission concepts and support early risk reduction for critical system components.
FURTHER EXPANDING THE CAPABILITIES OF SPACE OPTICS
Segmented mirrors are already in use in telescopes at some of the world’s most prominent ground-based observatories, and they will be part of the largest next-generation ground telescopes currently on the drawing board. As the first to adapt this technology for the challenging environment of space, the James Webb Space Telescope is leading the way forward for new and exciting applications. Through the use of new materials, processes, and approaches, we will further expand the capabilities of space optics to provide insights we cannot even guess at today.
Lightweighted primary mirror segment
*The James Webb Space Telescope is a NASA mission done in collaboration with the European and Canadian space agencies.
Ted Mooney, PhD, is chief technologist for Harris’ Civil and Commercial Imaging business area. Harris specializes in large precision optics and integration and testing services for deep-space observation programs, and designs and manufactures innovative imaging payloads for the world’s Earth-imaging satellites.