Golden Age is indeed a fitting sobriquet for the state of astronomy today--thanks to the design, fabrication, optical, control, and sensing tools modern-day astronomers are using. These technologies are bringing scientists closer to a fundamental understanding of the origins of the universe and planets, knowledge that can help chart the course of Earth-bound programs to protect its environment and project climate trends.
Advanced optics is the key technology on the Advanced X-ray Astrophysics Facility (AXAF), to be launched by NASA next August. It's one of four great space-based observatories. The Hubble Space Telescope, launched in 1990, functions in the visible and near UV spectra. The Compton Gamma Ray Observatory lifted off in 1991 to study very-high-energy gamma rays at the upper end of the spectrum. AXAF will study high-energy X-ray sources with great sensitivity and resolution. And the Space Infrared Telescope Facility, launching at the turn of the century, will make detailed infrared observations.
Give 'em the AXAF. Features include:
- More than a ton of high-resolution mirrors consisting of two sets of four concentric, conical, nested mirrors--precisely polished, coated, and aligned.
- A High Resolution Camera (HRC) and the AXAF CCD Imaging Spectrometer (ACIS).
- A set of two gold transmission gratings that can be positioned behind the mirrors, ahead of the imaging instruments, for resolving spectra.
The Space Shuttle will place AFAX into a highly elliptical orbit, ranging from 6,200 miles to 87,000 miles, on a five- to ten-year mission. AXAF is not just for observation, but to test space-physics theories. Continuous observing time (up to 48 hr) above 37,000 miles is maximized by the orbital path, mitigating the effect of the Earth's radiation belts drowning out low-intensity deep-space X-rays. Solid-state recorders designed by prime contractor, TRW, can store up to 16 hours of data.
At the heart of AXAF are the concentric mirrors, one set curved parabolically along the common central axis and the other, hyperbolically curved, aligned behind it. X-rays enter the 47.2-inch-diameter aperture roughly parallel to the mirror surfaces. These photons graze at low angles from the paraboloid surfaces to the hyperboloids, ending up focused 10m (32.8 ft) away into the imaging detectors. Resolution is 0.5 arcsec, 10 times better than previous X-ray telescopes, like reading a newspaper at half a mile away.
Ground test images "are as good as, or better than, expected," says Martin Weisskopf, NASA Marshall chief scientist for AXAF. The high resolution, large collecting area, and sensitive instruments will enable AXAF to study faint sources, which are sometimes absorbed by intermediate matter or found in crowded star fields.
Grazing-incidence mirrors are necessary because short-wavelengths and high-energy X-rays are absorbed by mirrors when they strike the surface at angles greater than a few degrees. Hughes Danbury Optical Systems (HDOS, Danbury, CT) fabricated the mirrors from zerodur glass, from Schott (Mainz, Germany), polished within 3 angstroms root-mean-square micro roughness, according to Leon Van Speybroeck, telescope scientist at the Smithsonian Astrophysical Observatory (Cambridge, MA). That's the responsible agency for the AXAF science mission. "HDOS reduced polishing procedure to science and engineering rather than artistry," he adds. Optical Coating Laboratory Inc. (OCLI, Santa Rosa, CA) coated the mirrors, which were then assembled by Eastman Kodak (Rochester, NY).
Because it is more reflective of X-rays, engineers chose iridium instead of gold for the top coating on a binding layer of chromium. "The coatings are extremely thin," says Bob Hahn program chief engineer at OCLI. Vacuum ion deposition produced an iridium layer about 10-6 inches thick. The chromium is one-third of that. An ion beam removed atoms from a thin sheet of iridium placed in a vacuum chamber with the mirror. The iridium atoms "drifted" onto the glass, while "the glass was rotated very slowly to ensure a uniform coating," notes Jerry Johnston OCLI program manager. "Coating takes a little less than an hour."
Developed by the Smithsonian Observatory for imaging and fast-timing measurements, HRC is the highest resolution AXAF instrument. It uses microchannel plates (MCPs) for X-ray detection. MCPs for large field imaging are made by Galileo Electro-Optics (Sturbridge, MA), while the spectroscopy detector features ones from Philips Photonics (Brive la Gaillarde, France). Both firms developed radioisotope-free (low-noise) lead oxide glass for the detectors, which reduces background noise by a factor of 10. UV and low-energy ions are kept out of the detectors by an aluminized polyimide shield made by Luxel (Friday Harbor, WA).
A team from MIT and Penn State built the ACIS. It uses two CCD arrays, one for extended-object (wider-angle) imaging and one for simultaneous spectroscopy of each image component.
Reverse technology. Advances in astronomy are not confined to space. Developments are allowing unprecedented observations to be made from ground-based instruments--which, being larger, can gather much more light. The recently dedicated Hobby-Eberly Telescope (HET) near Ft. Davis, TX, operated by a five-university group headed by the University of Texas at Austin, is an example of how existing technologies being brought together. "It is not terribly innovative except in the way it is used to provide a large telescope at low cost," according to Facility Manager John Glaspey.
HET is the world's largest (11m diameter) primary-mirror telescope. But at $13.5 million, its cost is roughly one-sixth that for each of the comparable Keck telescopes in Hawaii. The secret is that HET does not move its primary mirror during observations, thus the main structure is very light. The mirror is made of 91 one-meter hexagonal segments. While the primary is locked at an elevation of 55 degrees, a lighter, secondary tracking module above the primary tracks a reflected image for up to 2.5 hr. The tracker moves the primary focus along X and Y orthogonal axes, and can rotate the image to keep an object in a fixed orientation. A six-linear-actuator hexapod also governs motion in tip, tilt, and Z directions. But with no more than 9.2m of the primary in use at any time, HET is effectively the third largest telescope after the Keck pair.
A rotation system developed by Comsat/RSI (Richardson, TX) turns the HET primary-mirror structure about the vertical on air bearings supplied by AeroGo (Seattle, WA). Once an azimuth is reached that permits observation, the structure settles onto the building foundation. Total sky coverage is about 70%. HET is designed mainly for spectroscopy.
To save cost, the HET primary mirror is spherical, so each segment can be an identically shaped hexagon. This allowed "assembly-line" production of the Schott zerodur segments, any of which replaces another. The disadvantage: spherical mirrors do not produce a sharp image, as do parabolic reflectors in other telescopes, due to spherical aberration. But parabolic segments would mean custom shaping each one. Light from the HET primary passes through a four-mirror "spherical aberration corrector" on the tracker before it is transmitted via fiberoptics to instruments below the telescope. The corrector provides sharp images over a small area, rather than typical wide-field astronomical images.
Each mirror segment has three actuators from TS Products (Post Falls, ID) to adjust in tilt, tip, and piston (in-and-out) directions. These units feature Arsape (La Chaux-De-Fonds, Switzerland) stepper motors driving a screw and levered to produce a 12X reduction. Thus, a basic 200-m step can be reduced by 12 times.
During an observation night, the truss structure changes shape due to temperature and loading and may need recalibration. The air bearings turn the telescopeto view a laser-interferometer/CCD-camera alignment system in a tower next to the dome. This system checks the position of each mirror segment relative to its neighbors. A single PC addresses 46 TS Products controllers, each driving the six actuators on two of the segments, effecting alignment in six minutes.
A dozen computers, each controlling a separate function, govern overall telescope control. Half are Sun workstations for telescope observations, while PC-based platforms handle facility management. According to Glaspey, to ease development each vendor system only had to integrate its functions into one computer. The development team then linked these to a master unit.
By producing and controlling precise machines--from polished mirrors, to actuators, and detectors--engineers give astronomers tools to discover the universe, from building blocks to overall structure--and the farflung sights in between.
Other applications for astronomy technology
- Adaptive optics profile laser beams, allow white-light metrology, such as semiconductor-wafer measurement, and can improve vision-inspection performance.
- Low-energy detectors facilitate image processing and data manipulation for better results and cycle times--applications range from medical (detailed images with less patient exposure) to entertainment (sharper realistic motion).
Increase performance without increasing costs by:
- Polishing, coating, and assembling large optical elements to tolerances measured in Angstroms.
- Designing low-noise detectors to optimize instrument sensitivity.
- Development of image/data capture software to maximize data usefulness.
- Controlling alignment of ground-based-telescope optical assemblies.