“We have a piezo stage; it's like an x/y control attached to the detector, which is where the focal plane of the telescope is,” Seager says. “And then we move that around and that gets us from that 60 arcseconds down to several arcseconds.”
The hardware is able to move the detector by microns in the two dimensions, but Seager notes that there is nothing new in the general approach of using starlight to guide a telescope.
“It's common,” she says. “That's basically how all telescopes are controlled. We're building upon things that we have done before. We're just trying to do it to a more precise level with smaller equipment.”
So far, the MIT team has tested the spacecraft's precision-pointing function with breadboard hardware on an air-bearing table. The camera and imaging-electronics board are also in hand, and have been tested both in the lab and outside against the night sky.
Although the work kicked off with a little astrobiology funding from NASA, the main financial support has come from Draper Laboratory, an MIT spinoff, and from MIT itself.
“That brought us about halfway in terms of the money spent, because we spent a lot of time on R&D,” says Seager. “We're still looking for more money to finish the project now.”
The MIT team has secured a launch, when the spacecraft is ready, via NASA's Educational Launch of Nanosatellites (ELaNa) program (see p. 44), and has a notional mission design and list of target stars. ELaNa payloads can't choose their orbits, but must follow the primary payload's route to space, so in general ExoplanetSat will go to an equatorial orbit in as low an inclination as possible, with an altitude that avoids the radiation belts to extend the lifetime of the detectors and other electronics.
“The field of exoplanets moves so quickly that by the time we launch, the list of targets will be different,” Seager says.
Those targets will be bright stars identified from the ground as having planetary systems. ExoplanetSat will determine if its target system includes a planet that transits the star, which could allow researchers to determine its size and fitness for study with larger and more expensive spacecraft.
“We ultimately want to do direct imaging from space, but that won't be done with cubesats unless you get them to self-assemble into something much bigger,” says Seager.