Technology
Nugget—Controlling all those Segments In designing TMT, a major challenge is how to make the 738 hexagonal segments of the primary mirror work together to imitate a single, nearly perfect monolithic mirror. In detail, this is a very complex question that requires much more space to answer than available for this one article. But we can start the discussion by looking at a small slice of the big picture. This month, we take a close look at the actuators that control the primary mirror. In Newscast Issues 4 and 5, TMT Project Scientist Jerry Nelson described the process used to fabricate individual segments and how each of the 738 segments can be thought of as a small piece of a large 30-meter diameter monolithic mirror (see Figure 1.). One might think that once the fabrication of all 738 segments is successfully completed, all that remains is to mount them in their correct positions on the telescope and "PRESTO!" we have a 30-meter primary mirror. Unfortunately, things aren’t nearly that simple; it turns out that the "correct position" is not easily defined nor is it "correct" all of the time.
As the telescope tracks a star, its orientation changes, resulting in changes to the gravity-driven deformations of the telescope structure. In addition, the telescope structure deforms in a different manner as the local temperature changes. The combination of these effects can result in many millimeters of deformations for a structure as large as the TMT. If nothing is done about these deformations, our nearly perfect mirror will become jagged due to the growth of inter-segment height differences, and its overall shape will wander from the desired shape. To be useful as a science instrument, the telescope must keep the segments aligned to within tens of nanometers—an improvement of nearly 100,000 over that which occurs naturally without control. We solve this problem by mounting each segment on three actuators. If used properly, the actuators can remove the negative effects of gravity and temperature. We can also use the actuators to partially mitigate the effects of wind-induced segment motion. [In Newscast Issue 3, George Angeli explained the complex simulations and models that are under development to better understand the characteristics of the wind in the telescope enclosure, its effects on the telescope, and how best to utilize the actuators to mitigate the wind driven deformations.]
The three actuators are positioned to allow control of each segment in piston, tip, and tilt (See Figure 2.). Motions within the plane of the segment are controlled passively with a complex structure called a Segment Support Assembly (SSA). The most important performance characteristics of the actuators are their abilities to position each segment to better than 5 nanometers over a nearly 5 millimeter range, and to support the weight of a segment and SSA (~ 130 kilograms), along with secondary characteristics such as low power dissipation, low weight, and high reliability. Oh… and by the way, the actuators also must be inexpensive, since well over 2,000 actuators are required to control the 738 segments of the TMT primary mirror. The TMT project office has contracted with Marjan Research to design and build a prototype actuator. After an intensive 14-month design, fabrication, and test program, Marjan successfully demonstrated the first TMT prototype actuator.
The key components of the actuator are a voice coil, a novel state-of-the-art optical sensor, an off-load mechanism, and the control computer (See Figure 3.). The voice coil is a type of linear motor that is commonly found in stereo speakers. The optical sensor provides position feedback to the computer that controls the actuator. The off-load mechanism is used to minimize the power required to support a segment. The control computer commands the actuator based on inputs for the desired positions for each actuator and real-time feedback from the actuators internal optical sensor. Without the off-load mechanism, the power dissipated to support a segment would be so large that it would degrade the local thermal environment, thus compromising the scientific performance of the telescope. The off-load mechanism works via a complex—but elegant—system of levers and springs. One saving grace about gravity and temperature effects is that they produce deformations that vary slowly over time. This means that the actuators aren’t required to respond quickly to a changing environment. Astronomers call relatively slow control of telescope optics "active control." This is not to be confused with "adaptive optics," which is used to remove image degradation caused by Earth’s atmosphere. Actuators that are used for adaptive optic systems typically must run at much higher speeds than those required for active optics. So far, we've described how the segments are moved; the remaining pieces of the puzzle include the sensors that are used to measure the shape of the segmented array in real time, the control algorithm (which converts the sensor measurements and desired sensor readings into actuator commands), and the system that determines the desired sensor readings that will achieve the required shape of the overall array. We’ll address these questions in a future issue of the Newscast. |
