They call them “sky islands,” these mountainous outcroppings that rise suddenly from the Arizona high desert. As if separated in an ocean, each has its own ecosystem and unique character. The highest of these, Mount Graham, rises to an altitude of 10,700 feet above sea level. It is here that the University of Arizona’s Steward Observatory in partnership with the Max Plank Institute and other participants, is building what will for a time be the world’s most powerful optical telescope, the Large Binocular Telescope (LBT).

When it is completed, the LBT will have two 8.4 meter mirrors, each of which is over half again the diameter of Palomar. When used together, the mirrors will provide a diffraction-limited image sharpness of a 22.8 meter aperture. Astronomers will be able to use the mirrors in tandem or individually. For example, different instruments on each mirror could capture optical and infrared images of the same object. The binocular design enables a number of unique instrumentation and research possibilities.

The LBT weighs approximately 580 metric tons and has a combination of focal stations for different instruments and observation tasks (Figure 1). Accurately pointing, focusing and controlling a precision instrument of this size is an enormous challenge, especially in the light of external factors such as temperature variations and the buffeting of high winds at such an altitude. It is a challenge that could only be met and overcome through the application of powerful embedded and real-time computing technology consisting of a combination of COTS and custom hardware and software. The open-loop, full-sky pointing accuracy is currently 0.3 arcseconds with a tracking performance of 0.01 arcseconds.

Like the end users of any other embedded system, astronomers do not want to have to worry about the details of pointing and controlling the telescope. They want it pointed and focused accurately and reliably so that they can effectively use their instruments and obtain the data needed for their science projects. So the engineering issues in question here are with those tasks rather than with the scientific instrumentation.

Basically, the LBT is what is called an “altazimuth” mount. That is, one axis turns in relationship to the surface of the Earth, the azimuth, while the other moves up and down in elevation. This is different from more traditional telescope equatorial mounts in which one axis is aligned with the axis of Earth’s rotation. Equatorial mounts become prohibitive in size for telescopes beyond a certain aperture. To point an altazimuth telescope, a pointing control system must translate the standard location of an object in the sky, which is cataloged in terms of right ascension with respect to 0 degrees longitude, and declination with respect to the equator into altitude and azimuth coordinates for the telescope. These are based on its geographical coordinates and the local time. But that’s the easy part. Once pointed, the instrument has to track the object, incrementally changing its altitude and azimuth settings, as the Earth moves on its axis.

In addition to the pointing problem, there is also the issue of optical accuracy. Corrections must be done through a system of adaptive optics to cancel the distortion of the Earth’s atmosphere and the stresses from tilting, and temperature on the 8.4 meter primary mirrors must be sensed and corrected to keep a perfect shape. In the case of the primary mirrors these are small increments, but we are dealing here with the wavelengths of light, and each mirror weighs 41,500 pounds.

Accurately moving such a massive piece of equipment is done by essentially “floating” it on a film of oil supplied by hydrostatic bearings and using four powerful motors on each axis with position information gathered from a set of inductive strip encoders mounted on the outer radius of each axis. It also involves a dynamic balance system, which operates by shifting water in and out of ballast tanks to keep the telescope in balance. Three major control systems include the Mount Control System (MCS), the Adaptive Optic System (AOS) and the Primary Mirror Controller (PMC). These operate under the main Telescope Control System (TCS), which includes, among others, the MCS GUI and the Pointing Control System (PCS).