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Chasing a Cosmic Phantom PUBLIC ACCESS

A New Telescope May Help Solve One of Astronomy’s Greatest Riddles-the Source of Gamma-Ray Bursts.

[+] Author Notes

David A. Rozansky is a freelance writer and amateur astronomer. He is also all aviation journalist and a public relations writer for the manufacturing and engineering technology industry.

Mechanical Engineering 120(09), 80-82 (Sep 01, 1998) (3 pages) doi:10.1115/1.1998-Sep-6

Engineers at Torus Precision Optics, in Iowa City, IA, have developed an innovative pair of telescopes for the University of Michigan, in Ann Arbor, which could pinpoint the sources of these mysterious bursts of energy. Searches show that gamma-ray bursts (GRB). Astrophysicists at the University of Michigan decided to take up the challenge of unlocking the mystery of GRBs. Their first course of action was to create a system that scans the sky quickly, deeply, and accurately. This meant designing a telescope with a large field of vision and a quick slewing rate. Torus Precision Optics completed and tested the first ROTSE II telescope in December 1997, within a year of the first proposal, and a second one is slated for completion this year. The ability of the three Mulherin brothers to accomplish so large a task in so little time is considered remarkable. Torus is working on projects that will search out near-Earth objects, those stray asteroids and comets that have received attention by smashing into Jupiter in 1996.

Gamma-Ray Bursts (GRBS) have been studied for more than 25 years, yet scientists are no closer to determining their sources than they were a quarter-century ago. These unpredictable bursts of gamma-ray photons, which can come from any direction, are one of the greatest mysteries of the cosmos. Now, however, engineers at Torus Precision Optics, in Iowa City, Iowa, have developed an innovative pair of telescopes for the University of Michigan, in Ann Arbor, which could pinpoint the sources of these mysterious bursts of energy.

Why is it so difficult to study GRBs? Gamma-ray photons interact with the upper atmosphere, so they cannot be detected on the ground; they can only be detected by orbiting observatories. Five satellites in orbit can detect GRBs, but they can calculate a GRB’s location only within 5 to 10 minutes of arc—an area about four times that of the moon. On average, the GRBs last about 10 seconds, then disappear as mysteriously as they appeared, further hindering all efforts to pinpoint their origins.

More than 2,000 GRBs have been detected since their discovery in 1973, but no one has ever identified a source object. Astronomers did make a valuable discovery in 1981—a comparison of GRB reports with old astrographic plates from observatory archives showed temporary, inexplicable light sources—called optical transients—in the “error boxes” of the more recent GRB reports. This spurred years of speculation that GRBs had brief optical counterparts visible through ground-based telescopes.

It would be 17 more years before the theory of GRB optical counterparts was proven. The optical transients in the archives were very dim—too dim, in fact, to be seen with any but the most powerful telescopes. However, such telescopes can see only a small piece of the sky (generally less than 0.1 degree of arc), and their slewing rates are so slow that it may take 10 minutes or more to begin searching the error box. A target GRB is long gone by that time, and such methods have not yielded results.

The only two ways to detect the optical counterparts, if at all, is either to point a telescope randomly at the sky and hope for a chance observation, or to point a telescope quickly in the general direction reported and scan the error box as fast as possible.

The first method requires a telescope with a large field of view, but the larger the field of view, the harder it is to discern dim optical events from the overall panoply of stars. The second method is more viable, but requires new telescope technology. The National Aeronautics and Space Administration (NASA), in Washington, D.C., had to expedite GRB reports; often 5 to 36 hours passed before the satellite telemetry could be delivered. In the past few years, the GRB Coordinates Network (GCN) was launched to transmit the GRB reports directly from satellites via the Internet. As a result, ground observatories are learning about any GRB within five seconds of its initial discovery.

Now astronomers have only the challenge of building a telescope that is able to hunt the sky quickly. Meanwhile, traditional methods have been used, and a lucky strike in mid-1997 found two optical counterparts to GRBs that lasted hours, perhaps days. Other searches show that many GRBs may have no optical counterpart at all. With so few data points, however, it is likely that many GRB optical counterparts have very short durations. It was discovered that the optical counterparts change wavelength as they age, moving from the X-ray to the ultraviolet, through the visual spectrum into the infrared.

Astrophysicists at the University of Michigan decided to take up the challenge of unlocking the mystery of GRBs. Their first course of action was to create a system that scans the sky quickly, deeply, and accurately. This meant designing a telescope with a large field of vision and a quick slewing rate. University staff believed the answer would he in a telescope control system they call the Robotic Optical Transient Search Experiment (ROTSE, pronounced rot-see), to be placed at Los Alamos National Laboratories in New Mexico.

The first phase—ROTSE I— used four telephoto lenses and a digital camera that can capture an image 2,048 by 2,048 pixels, with great sensitivity to dim light. The telephoto lenses were mounted to gimbals and driven by a sophisticated control system. There have been a few wrinkles to iron out, but now the ROTSE staff is confident that the system responds superbly and autonomously to the GCN reports. That left the better part of the challenge: build a pair of telescopes with a large field of vision and a fast slewing rate for the second phase. The university put out an open request for bids.

Torus Precision Optics, a small but growing company, won the bid in January 1997. The University of Michigan’s specifications were very high: a 2-degree field of view with a small focal length, and an astonishingly fast slew rate of 6 arc-degrees per second. These requirements drove up the cost, but Torus held its bid to $70,000 without sacrificing precision. In fact, Torus’s design exceeded the specified performance requirements.

The ROTSE II telescope's 2-degree field of view and 6-arcdegrees-per-second slewing rate enable it to scan the sky quickly, deeply, and accurately.

Grahic Jump LocationThe ROTSE II telescope's 2-degree field of view and 6-arcdegrees-per-second slewing rate enable it to scan the sky quickly, deeply, and accurately.

Torus Precision Optics began as James Mulherin’s personal business of producing and selling first-surface reflecting telescope optics to amateur astronomers. His high-quality workmanship became widely known, and he began receiving orders for larger optics from professional observatories. When customers began clamoring for complete telescopes, Mulherin invited his brother Tony, an expert machinist, to join the business and add a mechanical fabrication shop.

Orders were placed for increasingly larger projects, so James raised some capital in 1994 and stepped up the business’s capabilities of production and quality control. The fabrication shop’s inventory of machine tools grew, and now includes a Bridgeport computer numerically controlled milling machine.

James Mulherin worked with Opcon Associates of Cincinnati to design the optics to meet the two-degree field of view requirement, based on a larger, complex version of Torus’s smaller research telescopes. He started with a Classical Cassegrain design—one curved mirror collects the light and a smaller curved mirror magnifies the image—with a 45-centimeter aperture and a focal length of only 85 cm. A focal length this short will have off-axis aberrations; the collecting mirror is so large that images at the edge of the field of view are distorted. A series of six precision lenses corrects these aberrations.

Then the image is captured by an Apogee AP-10 CCD camera. The cameras will record a 4-million-pixel image, highly sensitive to a visual magnitude of 18 for exposures of only 15 seconds. Another Mulherin brother, John, joined Torus while studying mechanical engineering. The job of designing the telescope’s drive system and housing structure fell to him.

“Designing a telescope is unlike designing anything else,” John Mulherin said. “We are forced to approach this from so many aspects. Engineers are taught to design things like bridges and cars, where, the primary concern is whether or not the structures are going to fall apart. Instead, our concern here is whether or not the telescope is going to stop moving when you want it to.”

Getting a telescope to stop on a pinpoint isn’t easy, especially with a high slewing rate. John chose to use a 720:1 final drive ratio for the drive assembly, but accuracy had to be as high as possible. Traditional telescope designs depend on gears, but the ROTSE telescope needed zero backlash to handle wind loading. Precision gears are expensive in large sizes, and a periodic error becomes rapidly unacceptable as the gears wear. The Mulherin brothers chose to go with a friction drive system. The only gear is a zero-backlash planetary reducer from Harmonic Drives.

The wheels in the friction drive are stainless steel and hard-coat anodized aluminum. “These components can be easily manufactured to very high tolerances*” James Mulherin said.

In fact, the tolerances are so tight that the telescope has demonstrated an operational speed of 10 arc- degrees per second, almost twice what was required by the specifications.

The control system monitors a telescope’s position as it runs its drives. Positional information from toothless drive wheels is not enough for the accuracy of hunting GRBs, mostly due to hysteresis, so John had to bring in more instrumentation.

John decided upon industrial-grade optical encoders. Each encoder has two beams of light and a photo detector. The encoder measures the telescope’s position within 16 arc-seconds. The input from the encoder integrates with the input from the drive wheel for accurate, automatic positioning.

Torus Precision Optics completed and tested the first ROTSE II telescope in December 1997, within a year of the first proposal, and a second one is slated for completion this year. The ability of the three Mulherin brothers to accomplish so large a task in so little time is considered remarkable.

The ability to computerize most of Torus’s engineering work is the reason such a small company could master these innovations so rapidly. At Torus, information flows from mind to mind via computer-aided-design (CAD) and computer-aided-manufacturing (CAM) software. The selection of the company’s software has been critical to Torus’s success. John uses CAD- KEY, a mechanical CAD software program from Baystate Technologies (www.cadkey.com), in Marlborough, Mass., for the Pentium Pro-based CAD platform, while Tony drives the Bridgeport CNC milling machine with MasterCAM software from CNC Software in Tolland, Conn, (www.mastercam.com).

Based on a Classical Cassegrain design, the ROTSE II telescope has a 45-cm aperture and a focal length of only 85 cm.

Grahic Jump LocationBased on a Classical Cassegrain design, the ROTSE II telescope has a 45-cm aperture and a focal length of only 85 cm.

John Mulherin said it was important to have software that runs smoothly and integrates well in an overall system. “When I came on board, the software we were using had numerous bugs and was precise to only ten- thousandths of an inch,” he added. “When we switched to CADKEY, 10‘7 became our general working resolution. It is a solid, robust program.”

John was able to virtually prototype the entire telescope within CADKEY without having to create physical models or prototypes. All elements of the telescope were created within a single CADKEY part file. For manufacturing purposes, 3-D models were then patterned for use in separate part files used to generate custom fixturing. These files were sent to Tony’s CAM system for toolpath generation, which in turn would be used to mill the components with extreme tolerances. By creating a simple, inexpensive network where information flowed smoothly from one platform to another, Torus Precision Optics can quickly turn ideas into reality.

Torus’s success also comes from intensive research and development. James Mulherin reinvested nearly every penny of profit in research and development. He even invested in the research of others: By contributing to the development of Observatory Control and Astronomical Analysis System (OCAAS) software by Clear Sky Institute Inc. of Cedar Rapids, Iowa, Torus has secured license and exclusivity agreements, putting it in an enviable position in the world of astronomy research.

Torus forced so many innovations into a single telescope that it can be considered a new breed.

It was a firmly held belief that fast slewing meant less precision, that greater field of vision meant less discernibility, that automation meant restricted analysis and greater error. Torus Precision Optics turned those beliefs on their head, opening up new arenas for astronomers.

Torus is working on projects that will search out near-Earth objects, those stray asteroids and comets that have received attention by smashing into Jupiter in 1996 (and into theaters this summer). Currently, finding these objects has fallen primarily to amateur astronomers who are using CCD cameras and their personal telescopes to detect and discover asteroids.

Scientists have been anxious to develop a better system. A program known as the Taiwan-America Occultation Survey (TAOS) has been formed to measure the number of Kuiper Belt Objects (KBOs) and compile an accurate consensus of this newly discovered habitat of comets.

For the TAOS project, the Lawrence Livermore National Laboratory in Livermore, Calif., and two Taiwanese universities have ordered 50-cm aperture telescopes from Torus.

The TAOS program will place the entire system of three telescopes on a forest-covered mountain in Taiwan so remote that it can be reached only by helicopter. The telescope system will be completely autonomous, with its own independent source of electricity. Routine observation is slated for the year 2000. The three robotic telescopes will automatically monitor 3,000 stars every night for several years, consult among themselves to reject false events, and issue a warning if a collision course is detected. It could be the first warning humans have of impending extinction.

Meanwhile, short-lived gamma-ray bursts flicker across the universe. Some astronomers say they are the result of black holes gobbling up their companion stars. Others say GRBs originate from the centers of young galaxies clear across the universe. Some science-fiction writers have speculated that GRBs are the red-shifting engines of interstellar spaceships. No one really knows what GRBs are, but for the Mulherins, GRBs are an exciting path to the forefront of the now high-tech universe of optical telescopes.

Copyright © 1998 by ASME
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