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Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

Real-Time Control for the World's Largest Telescope

Using commercial off-the-shelf solutions for high-performance computing in extremely large telescopes
Figure 1
© European Southern Observatory
Figure 1: For a size comparison, two humans and a car stand next to the E-ELT. The M1 primary mirror, which is 42 meters in diameter, features segmented mirror construction. European Southern Observatory
Figure 2
© European Southern Observatory
Figure 2: The E-ELT features a total of five mirrors.
Figure 3
© European Southern Observatory
Figure 3: LabVIEW software controls the M1 system comprised of 984 segments at 1.5 meters each with six sensors and three actuator legs that provide three degrees of freedom for movement deviation.
Figure 4
© European Southern Observatory
Figure 4: A thin, flexible mirror spread across 8,000 actuators, the M4 can be deformed every few milliseconds to compensate for atmospheric interference.
Figure 5
© European Southern Observatory
Figure 5: NI engineers validated the mirror control system (right) with the M1 mirror Hardware-in-the-loop simulation (left). The E-ELT features a total of five mirrors. The E-ELT features a total of five mirrors. Click to view larger image.
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By Jason Spyromilio, Ph.D., European Southern Observatory

The European Southern Observatory (ESO) is an astronomical research organization supported by 13 European countries. We have experience developing and deploying some of the world's most advanced telescopes. Our organization currently operates at three sites in the Chilean Andes—the La Silla, Paranal, and Chajnantor observatories. We have always commanded highly innovative technology, from the first common-user adaptive optics systems at the 3.6-meter telescope on La Silla to the deployment of active optics at La Silla's 3.5-meter New Technology Telescope (NTT) to the integrated operation of the Very Large Telescope (VLT) and the associated interferometer at Paranal. In addition, we are collaborating with our North American and East Asian partners in constructing the Atacama Large Millimeter Array (ALMA), a $1 billion (U.S.) 66-antenna submillimeter telescope scheduled for completion at the Llano de Chajnantor in 2012.

The next project on our design board is the European Extremely Large Telescope (E-ELT). The design for this 42-meter diameter primary mirror telescope is in phase B and received $100 million in funding for preliminary design and prototyping. After phase B, construction is expected to start in late 2010.

Active and Adaptive Optics

The 42-meter telescope draws on the ESO and astronomical community experience with active and adaptive optics and segmented mirrors. Active optics incorporates a combination of sensors, actuators, and a control system so that the telescope can maintain the correct mirror shape, or collimation. We actively maintain the correct configuration for the telescope to reduce any residual aberrations in the optical design and increase efficiency and fault tolerance. These telescopes require active optics corrections every minute of the night, so the images are limited only by atmospheric effects.

Adaptive optics uses a similar methodology to monitor the atmospheric effects at frequencies of hundreds of hertz and corrects them using a deformed, suitably configured thin mirror. Turbulence scale length determines the number of actuators on these deformable mirrors. The wave front sensors run fast to sample the atmosphere and transform any aberrations to mirror commands. This requires very fast hardware and software.

Controlling the complex system requires an extreme amount of processing capability. To control systems deployed in the past, we developed proprietary control systems based on virtual machine environment (VME) real-time control, which can be expensive and time-consuming. We are working with National Instruments (Austin, Texas) engineers to benchmark the control system for the E-ELT primary segmented mirror, called M1, using COTS software and hardware. Together we also are exploring possible COTS-based solutions to the telescope's adaptive mirror real-time control, called M4.

M1 is a segmented mirror that consists of 984 hexagonal mirrors (Figure 1), each weighing nearly 330 pounds with diameters between 1.5 and 2 meters, for a total 42-meter diameter. In comparison, the primary mirror of the Hubble Space Telescope has a 2.4-meter diameter. The single primary mirror of the E-ELT alone will measure four times the size of any optical telescope on the earth and incorporate five mirrors (Figure 2).

Defining the control system

In the M1 operation, adjacent mirror segments may tilt with resect to the other segments. We monitor this deviation using edge sensors and actuator legs that can move the segment three degrees in any direction when needed. The 984 mirror segments comprise 3,000 actuators and 6,000 sensors (Figure 3).

The system, controlled by LabVIEW software from National Instruments, must read the sensors to determine the mirror segment locations and, if the segments move, use the actuators to realign them. LabVIEW computes a 3,000-by-6,000-matrix-by-6,000-vector product and must complete this computation 500 to 1,000 times per second to produce effective mirror adjustments.

Sensors and actuators also control the M4 adaptive mirror. However, M4 is a thin deformable mirror—2.5 meters in diameter and spread over 8,000 actuators (Figure 4). This problem is similar to the M1 active control, but instead of retaining the shape, we must adapt the shape based on measured wave front image data. The wave front data maps to a 14,000 value vector, and we must update the 8,000 actuators every few milliseconds, creating a matrix-vector multiply of an 8 by 14 k control matrix by a 14 k vector. Rounding up the computational challenge to 9 by 15 k, this requires about 15 times the large segmented M1 control computation.

We were working with NI on a high-channel-count data acquisition and synchronization system when they began working on the math and control problem. NI engineers are simulating the layout and designing the control matrix and control loop. At the heart of all these operations is a very large LabVIEW matrix-vector function that executes the bulk of the computation. M1 and M4 control requires enormous computational ability, which we approached with multiple multicore systems. Because M4 control represents 15 3-by-3 k submatrix problems, we require 15 machines that must contain as many cores as possible. Therefore, the control system must command multicore processing. This is a capability that LabVIEW offers using COTS solutions.

Because we required the control system engineering before the actual E-ELT construction, the system configuration could affect some of the construction characteristics of the telescope. It was critical that we thoroughly test the solution as if it were running the actual telescope. To meet this challenge, NI engineers not only implemented the control system, but also a system that runs a real-time simulation of the M1 mirror to perform a hardware-in-the-loop (HIL) control system test. HIL is a testing method commonly used in automotive and aerospace control design to validate a controller using an accurate, real-time system simulator. NI engineers created an M1 mirror simulator that responds to the control system outputs and validates its performance. The NI team developed the control system and mirror simulation using LabVIEW and deployed it to a multicore PC running the LabVIEW Real-Time Module for deterministic execution.

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