PORTLAND, Ore. Astronomers at the University of Arizona are putting the final touches on a ground-based telescope that's said to offer three times better resolution than the Hubble Space Telescope. The last piece, a computer-controlled 2-foot flexible mirror, complements a 21-foot fixed primary mirror in a system that combines advanced algorithms with real-time DSP control to overcome distortions caused by Earth's atmosphere.
After eight years of work, astronomers buttressed their claims for the telescope with uncannily perfect infrared images of select "beacon stars," which are notoriously difficult to render since even the heat from the telescope distorts the image.
The 40-kHz closed-loop adaptive optics (AO) system adjusted the position of 336 points on its 640-mm (2.1-foot) deformable mirror 550 times per second to correct not only for atmospheric interference but also for ground-based interference from seismic vibrations to infrared glow in the telescope and observatory.
"The key was to build a mirror with a shape that deforms in real-time," said Michael Lloyd-Hart, project scientist for the closed-loop adaptive-optics system. "That's the final component we have been working on for the last two years." Astronomers Philip Hinz, Francois Wildi and Guido Brusa at the University of Arizona and Osservatorio Astrofisico di Arcetri (Florence, Italy) collaborated with with Lloyd-Hart. The U.S. Air Force, with an eye to having ground-based telescopes perform quality-control inspections on its low-orbit satellites, funded the closed-loop AO.
The mirror, an ultrathin (2-mm) reflective surface spun-cast from a borosilicate compound, floats in a magnetic field and changes shape in milliseconds, virtually in real-time. An array of 336 computer-controlled "actuators" electromagnetically grips the membrane, tweaking it into position a nanometer at a time. The adaptive secondary mirror, responding to the commands of 168 Analog Devices Inc. DSPs, focuses the starlight as steadily as if Earth had no atmosphere.
Feedback from sensors associated with each magnet read out the mirror's location, so that even if the wind buffets the telescope, the magnetic coils apply compensating forces to maintain the secondary mirror in perfect focus.
"Since our primary mirror is three times bigger than the Hubble, we have the potential to get images that are three times sharper than the Hubble. For wavelengths of 1.5 microns and longer, we can correct images to the diffraction limit of the telescope," said Lloyd-Hart.
The diffraction limit for the 6.5-meter closed-loop AO telescope is calculated by the astronomers to be good enough to enable it to image Jupiter-like planets in solar systems within 26 light-years, or more than 153 trillion miles. Future, larger telescopes with closed-loop AO, the astronomers say, could detect and study nearby Earth-like planets.
"This is the first time that anybody has done adaptive optics with a mirror that is an integral part of the telescope itself" said Lloyd-Hart.
While adaptive optics has fueled a revolution in telescope design for 10 years, progress has been slow because of the stringent requirements. First, a system must be able to detect and calibrate the optical effects of atmospheric turbulence. Next, that data must be translated into control information for altering an optical system to cancel the effects in real-time. Finally, an electromechanically driven optical system has to be devised to respond at those speeds.
"The key difference between our adaptive optics system and that of any other telescope in use today is that we are controlling the secondary mirror that is already a part of all telescopes, whereas everybody else adds on adaptive optics as an afterthought," said Lloyd-Hart.
The algorithms developed by Italy's Osservatorio Astrofisico di Arcetri have been able to calculate exactly the correct deformation coefficients for all 336 voice coils to cancel out the effects of Earth's atmosphere.
"We have a massively parallel computer with 168 processors DSPs one for every two actuators which sits hidden behind the secondary mirror, riding on the telescope," said Lloyd-Hart.
Once the main computer algorithm calculates the perfect deformation coefficients, at 550 times a second, it passes them to the massively parallel computer riding atop the secondary mirror. The heat generated by the 168 DSPs and associated circuitry is cooled by a half-water, half-methanol liquid cooling system that dissipates up to a kilowatt.
"The processors not only tell the actuators how to move but also get feedback from them revealing exactly where they are at a rate of 40,000 times per second," said Lloyd-Hart.
Each actuator has a capacitance that reveals how far it has moved, so that at 40 kHz the DSPs are constantly adjusting the actuators regardless of whether wind buffeting or other vibrations try to prevent the actuator from optimizing the deformation. In AO designs that run "open loop" without feedback any vibrations along the telescope will not be corrected. In tests of the closed-loop AO, winds of up to 30 mph had no effect on the final image.
"Closing the feedback loop is something that nobody else in the world has feedback enables us to make our adjustments very, very precisely, because of our constant stream of position feedback," said Lloyd-Hart.
For now, the system's only drawback is that a bright known star has to be within the field of view, limiting its usefulness in dark parts of the sky. To overcome that problem, astronomers are designing a "batman beacon" drawn in the sky with a laser. Lloyd-Hart said his team hopes to add the beacon to the telescope within the next year.
"In the future, we will create our own reference beacons in the sky by shining up a laser. Depending on the back-scattered light from molecules of air, we can point the telescope in any direction," said Lloyd-Hart.
Today, Lloyd-Hart muses mostly about his group's next big project the construction near the Mexican border of a large binocular telescope with two 27-foot mirrors on the same mount. Artist's renderings make it look almost comically like a gigantic pair of binoculars. Both side-by-side telescopes will be built from the beginning with separate closed-loop AO for twin 36-inch (91-centimeter) secondary mirrors each held by 672 actuators.
With stereoscopic vision enabled by binoculars, the astronomers hope to harness the telescope's unique capabilities to crack a number of scientific "hard cases." One of particular interest to Lloyd-Hart is looking for extra-solar planets. No one has ever taken a picture of a planet circling another star, though over 100 such objects have been inferred by their gravitational effects on their stars. One astronomer has photographed the "shadow" of a planet as it passes in front of its star, but no direct imaging of other planets has been done.
"If we can see the self-luminous light, and the reflected light from these objects, then we can begin to get crude spectra of them and find out what they are made of," said Lloyd-Hart.