A precision mirror positioning system for compact earth observation space telescopes
Coming from a background in mining and refineries, where one “could sometimes get away with just welding on another few centimetres of steel” to amend a piece of machinery, Sean Pepper has now won the Heinz Stoewer award for his thesis work on a nanometre-scale positioning system for mirrors used in a Deployable Space Telescope. Maintaining imaging resolution while drastically reducing the size of such telescopes may drive a step change in earth observation.
Disasters, such as the flooding catastrophe in Mozambique in March this year or the California wildfires end of last year, can’t always be prevented. But their impact can be minimized, for example by ensuring that evacuation plans and disaster relief efforts are based on the most accurate and up-to-date on-ground situation indicating, for example, which roads are still accessible. Google Maps shows that it is certainly feasible to make very high-resolution images from space. But it can take months, or even years, for these images to be updated. “High-resolution imaging from space requires a large mirror, resulting in telescopes the size of a delivery van,” Pepper explains. “These are expensive, bulky and heavy, meaning that we can’t launch very many. It therefore may take a while to refresh the image of earth, or certain parts thereof.” Whether it be for disaster relief, monitoring the impact of climate change or the prediction of cholera outbreaks, governments, industry and academia all want high revisit rates. It pushes the remote sensing market towards large constellations of more than one hundred small, milk-carton sized, satellites. “But this comes at the cost of image resolution,” Pepper says. “It is not yet economically feasible to have both a high resolution and a high revisit rate.”
Looking for a challenge to combine his skills in physics, mechanical engineering and systems engineering, Pepper joined the team working on designing a Deployable Space Telescope (DST#), headed by professor Hans Kuiper. “The DST is a proof-of-concept project,” Kuiper explains, “based on a vision I had to achieve a step change in earth observation imagery. It is a team effort aimed at achieving high-resolution, high-quality, images from a low earth orbit using deployable optics.” The large primary mirror of the DST consists of multiple smaller segments that are folded up during launch, and only deployed once in orbit. This way, a van-sized telescope can be reduced to the size of a washing machine. “It means that we can launch multiple high-resolution space telescopes in the same volume, dramatically reducing cost.” According to Pepper, “with the technology coming out of the DST project, perhaps in ten years, instead of getting updates in Google maps every few years, we’ll be getting it hourly and in much better resolution than we have today.”
Pepper decided to work on the system that allows high-precision positioning of its four primary mirror segments. It was a design choice in the earlier phases of the DST project to allow somewhat larger positioning tolerances for the secondary mirror, which is mounted at the ends of a number of deployable columns (see figure). As a consequence, to maintain image quality, tolerances for the primary mirror segments had to be especially tight. “Traditional telescopes use comparatively big, heavy, and very stiff structures to keep everything in position. You can’t do that easily with deployable optics,” Pepper says. “By definition, things have to move around.” The folding mechanism itself has an already very impressive accuracy on the order of micrometres. What was needed on top of that was a system allowing corrections more than a hundred times smaller, with a resolution of ten nanometres.
Stiffness and friction
Space is a very challenging environment in itself, but the first hurdle to overcome is to survive the launch into space. At rocket launch, the space telescope and all of its components are exposed to accelerations of up to 30 times that what we feel on earth. They also have to endure powerful vibrations originating from within the rocket structure. “Nothing is allowed to break off or bend irreversibly under those conditions,” Pepper explains. “Increasing stiffness by using thicker components or introducing clamping mechanisms was not an option as these would take up extra space and add unwanted weight.” Parts that rely on friction, contacts or sliding had to be avoided as well, as these would prevent reproducible positioning. “Our final design uses four actuators to hold each mirror segment in place, providing both stiffness for launch and high-precision position control,” Pepper says. “In space applications, these actuators are often based on the use of flexures – solid pieces of metal that are very thin in specific places allowing very small, but very predictable frictionless motions (see movie).”
Expanding and contracting
It was, however, heat that proved to be the biggest challenge. Being in a low earth orbit, each time the DST enters or leaves the shadow of the earth it can endure temperature swings of more than a hundred degrees. “The whole telescope system expands and contracts as it gets hotter and colder, necessitating re-alignment of the mirrors,” Pepper explains. It is difficult to underestimate this heat effect. Earth itself is warm and gives off infrared radiation. In an early design of the alignment mechanism, a student found out that pointing the telescope at earth would already heat the primary mirrors up enough to lose the image. “A lot of our modelling effort is going into limiting these thermal effects,” Pepper explains. “Doable, for sure, as various mechanical and thermal solutions have already been applied in space elsewhere.”
In March, Pepper received the 2019 Heinz Stoewer award, a prize for exceptional Master students of the faculty of Aerospace Engineering at the TU Delft. “I was happy to nominate Sean,” Kuiper says. “He tackled a very complex problem and showed that it is feasible to precision-align the segmented primary mirror using commercial off the shelf components.” Although certainly not a requirement, Pepper’s work is a good fit to that of last year’s prize winner, whose master’s thesis focussed on the commercial development of rockets specifically aimed at bringing small payloads into earth orbit.
A continuing mission
Although Pepper has left the group he is still in touch, contributing to a scientific paper and assisting a PhD student to continue his work. “And someone will have to develop a more thorough sensing and control approach to drive the mirror realignment process,” he adds. “That is yet another very complex problem to be solved.” The Deployable Space Telescope is a theoretical exercise and may never fly, but the idea(s) behind it most certainly will. Something to think about should you ever need a disaster relief team to come knocking on your door.
#The Digital Space Telescope is a university led project with no commercial funding, although Airbus Defence & Space Netherlands B.V. (ADSNL) have been providing coaching and periodic design reviews. TNO Space and Systems Engineering is also involved.
Here you can find a video demonstrating a compliant mechanism based on flexures.