The Hobby-Eberly Telescope (HET) is a one-of-a-kind telescope housed at the McDonald Observatory in west Texas. The Center for Electromechanics (CEM) has been collaborating with engineers and astronomers from the McDonald Observatory (MDO) for the past five years designing and testing an upgrade to the HET tracking system to increase the telescope field of vision. These upgrades are all in preparation for the Hobby Eberly Telescope Dark Energy Experiment (HETDEX) set to begin in 2014. Dark energy is thought to be a mysterious force that is causing the universe to expand faster as it ages. Designed to discover planets, explore distant galaxies, and to study exploding stars and black holes, the HET is capable of generating an incredible output of groundbreaking information. With the HET tracking system upgrade, scientists will have the opportunity to elicit undiscovered physics, identify new particles, and test the laws of gravity.
“A small positioning error here on earth results in a large error and losing the object in the sky.” – Ian Soukup
The new tracking system for the HET will increase the telescope field of vision to 25 times what it is today. To achieve this goal, upgrades had to be engineered to meticulous precision. Ian Soukup, an engineer of the HET project explains…“A small positioning error here on earth results in a large error and losing the object in the sky”. Using state-of-the-art controls and actuator technology, along with unique and innovative manufactured components, the precision and accuracy necessary were achieved.
“The optical package has to be positioned within a cylinder that’s 10 microns in diameter and about 10 microns high. A human hair is about 90 microns, so [the precision is down to] about a tenth of a human hair,” – Joe Beno
“Basically, the tracker is a big robotics system,” says program manager Dr. Joe Beno. “It’s got about 13 different major actuators. Each one of those has a series of control and subcontrol loops and safety stops. So it has about 150 things in the mechanical system to position the optical package where we want it.”
All of those pieces must work together to provide extremely precise control of the corrector. “The optical package has to be positioned within a cylinder that’s 10 microns in diameter and about 10 microns high. A human hair is about 90 microns, so [the precision is down to] about a tenth of a human hair,” Beno says.
The HET tracking system redesign set some extraordinary design goals that required several fascinating engineering developments. Here are a few examples:
Bridge Design and Installation
The tracking system bridge is essential for supporting the optical hardware and electronics system within the Primary Focus Instrument Package (PFIP). As the backbone of the telescope’s tracker system, scrupulous engineering was required. In addition to being the primary support structure, the bridge design had to avoid any obstruction or blocking of light throughout its range of motion. This required positioning the bridge support beams in unconventional directions, a process that demanded advanced structural engineering.
Installing the 9 ton bridge involved a tedious procedure and the collective brainpower of multiple engineers. To execute the difficult placement of the bridge on the telescope assembly in the CEM highbay, engineers worked with a local manufacturing facility to accurately position the large bridge gently onto the existing structure. Bridge positioning was verified throughout the installation with a laser measurement system. When the bridge was installed on the telescope structure, degrees of freedom were accurately addressed, in order to assure the bridge operated in a safe and repetitive manner.
Customized Solidworks Solutions
To optimize kinematic design of the entire tracker, CEM developed novel uses of constraints and drivers to interface with a commercially available CAD package (Solidworks). To insure critical hardware safety during various failure modes, CEM engineers developed Visual Basic drivers to interface with the CAD software and quickly tabulate distance measurements between critical pieces of optical hardware and adjacent components for thousands of possible hexapod configurations. These advanced techniques, applicable to any challenging robotic system design, are documented and describe new ways to use commercial software tools to clearly define hardware requirements and help insure safe operation.
Wide Field Corrector Installation
The wide field corrector (WFC) is a key component in the PFIP, where multiple mirrors of various shapes and sizes focus the light reflecting from the 9.2-meter primary mirror. The WFC was built at the University of Arizona and is a one-of-a-kind optical instrument that is very expensive and practically irreplaceable. The primary engineering challenge was to develop a reliable, safe installation for the WFC. The assembly procedure needed to be perfected on the ground to guarantee a successful final hardware installation 50 feet above the ground on the telescope.
The HET is currently one of the world’s largest and highly advanced optical telescopes. Though it is one-of-a-kind now, much of the work done here will serve as a basis for the Giant Magellan Telescope to be constructed in Chile and is scheduled for completion by the year of 2020.