Baade then brought William Baum, an expert in photometry, up to Palomar to measure the light-gathering and resolving power of the telescope by setting up photometric sequences. Baum’s tests agreed with the earlier calibrations. The telescope could detect magnitude 22.4 stars in a thirty-minute exposure. Separately, from a study by one of his doctoral students, Baade confirmed the absolute magnitude of RR Lyrae stars. That Baade couldn’t detect these stars in the central region of Andromeda, after a careful survey, confirmed his division of the variables into two distinct populations. In Rome at the 1952 meeting of the International Astronomical Union, Baade announced his findings. Shapley’s light curves for variable stars were too simple. By identifying two distinct populations of variable stars, Baade had corrected Hubble’s scale, doubling the size of the universe. The first major discovery of the two-hundred-inch telescope was from what it couldn’t see. Imagine, astronomers said to themselves, what the telescope will discover from what it can see.
Astronomy is an incremental science. Although reporters puff each report from a meeting of the AAS or the IAU into what sounds like a definitive proof of the big bang, black holes, or dark matter—a night, or even an entire observing run on a telescope, rarely produces a revolutionary discovery. Each night adds data, fragmentary glimpses and measurements of the reaches of universe that astrophysicists and cosmologists can use to build, modify, or undermine features of a constantly evolving model.
Yet amidst that steady accumulation of knowledge, the achievements of the two-hundred-inch telescope stand out as a history of twentieth-century astronomy. It was on the two-hundred that Baade’s student Allan Sandage pursued his long search to refine the Hubble constant, the magic number that would define the age and size of the universe. It was on the two-hundred that Baade and others went beyond the geometry of space that Hubble had explored to identify distinct populations of stars and to explain the evolution of stars, the processes at work as stars were born and died. As the geometry and astrophysics came together, mostly from research done on the two-hundred-inch telescope, it became possible to age-date stars, to begin to understand the mysterious processes at work in the galaxies, and to discover large-scale structures in the universe.
With new instruments, tiny Schmidt cameras that Don Hendrix built with sapphire or diamond lenses, more sensitive photographic emulsions, phototubes and photomultipliers that could record light too faint for a photographic emulsion, corrective lenses with even broader fields than the Ross lens, finer spectrographic gratings, and tricky observational techniques that pushed the equipment to the limits—the reach of the telescope extended further than even the wildest optimists had dared to predict.
After Baade retired back to Germany, in 1958, Rudolph Minkowski took over some of the research Baade had pursued. Minkowski, a big hulk of an astronomer, was famed for ineptness around machines that seemed the perfect inverse of Baade’s skill. Night assistants who drifted over to the two-hundred knew two sure ways to tell when Minkowski was on the telescope: the aroma of the smokey Lapsang souchong tea he brought to the dome with him, and the sounds—many not suitable for polite company—that came over the intercom from the prime-focus cage as Minkowski’s body protested at the cramped quarters. Baade claimed that the hayrake seat in the prime focus, too big for his diminutive frame, had been shaped from a plaster cast of Minkowski’s derriere.
Minkowski was persistent, and he got results. On his last run on the two-hundred-inch telescope, in the spring of 1960, he was determined to get a spectrum from an elusive and suspicious object, identified from the Cambridge compilation of radio sources as 3C 295. The object was so faint he couldn’t see it to center it on the slit of the spectrograph. He would have to guide the telescope, for a whole night of exposure, on a dark area of sky where previous direct photographs had identified the object. His final run on the telescope was scheduled for four nights. The first two nights were too cloudy to observe. Minkowski moped around the Monastery, watching his chances slip away. The next night the weather cleared, and he held a faint guide star in the crosshairs of his guiding eyepiece for an entire night. He and Allan Sandage had previously taken “trial plates” to determine that the tiny area of blackness that hid 3C 295 would be on the slit of the spectrograph if the guide star was in the crosshairs. Minkowski gambled, and on the fourth night continued the exposure—trusting Sinclair Smith’s wonderful control system to keep the telescope pointing to exactly the same spot. At midnight on the last night he rushed the slide down to the darkroom and developed it. The red shift he measured for 3C 295 was .46, meaning that the strange object was receding at 46 percent of the speed of light. It was the most distant and fastest-moving object yet discovered. Minkowski bounded into the Palomar library with a bottle of bourbon and three glasses, for himself, Sandage, and night assistant Robert Seares. Despite Byron Hill’s rules, he was going to celebrate. The “look back time” to 3C 295 was somewhere between one-third and one-half the age of the universe.
Not every experiment was a success. There were some legendary failures. Fritz Zwicky was allocated little time on the two-hundred-inch telescope. In one experiment he used the two-hundred-inch telescope and the “little” Schmidt telescope, a few hundred yards away, to study the effect of artificial meteors on the atmosphere. The experiment required Ben Traxler, Zwicky’s favorite night assistant, to stand in the open shutter of the telescope, firing his .30-.30 carbine into the air while both the two-hundred-inch and Schmidt telescopes tried to record the path of the bullet. When the report from the rifle echoed over the mountaintop, astronomers accused Zwicky of trying to punch a hole in the atmosphere to improve the seeing. Experiments like that one weren’t repeated.
Hubble suffered a heart attack in 1949 and was ill by the time the telescope was ready. In a photograph taken in the prime focus, he looks wan and pale. A bell was rigged near his bed in the Monastery, and the night assistants went out of their way to accommodate him on his few observing runs. When he could no longer observe, he picked a surrogate observer to carry on his work, Allan Sandage. As an undergraduate at the University of Illinois, Sandage had read about the telescope in David Woodbury’s book and decided that he would go to California to study with Hubble, the most famous astronomer in the world, and somehow work on the most famous telescope in the world. As one of the first Ph.D. students in astrophysics at Caltech, Sandage learned from Walter Baade the skill of identifying a Cepheid star among the thousands of stars in an image of a distant galaxy and then using the remarkable abilities of a trained human eye to compare the brightness of the Cepheid with other stars so its light curve could be calibrated. Allan Sandage probably spent more hours of dark time on the two-hundred than any other observer during its early years, as he relentlessly pursued the goal of refining the Hubble constant that relates the red shifts of distant galaxies to their distance, and thus gives the size and age of the universe.
Sandage used the two-hundred to show that the most distant “stars” Baade had observed were actually H II (ionized hydrogen) clouds. That finding again doubled the scale of the universe. Later, on plates taken with the two-hundred-inch telescope, Sandage discovered a new class of stellar object. He called them “quasi-stellar radio sources,” a name that was later shortened to “quasars.” For a long time no one could explain these strange objects, until Maarten Schmidt, using spectra taken on the two-hundred, concluded that the light from the quasars was shifted so far toward the red end of the spectrum that they had to be moving at inconceivable speeds. Quasars were the most distant and most luminous objects ever recorded. The telescope, it seemed, was reaching to the very edge of the universe.
The race to find objects with ever greater red shifts was on. Reporters clamored for more exciting reports, and the Caltech publicity office was ever ready to satisfy the requests with a new breakthrough. “Those are the days when you’d come down from Palomar,” Sandage recalled, “and everyone would expect you to come down with a pot of gold. Sometimes, almost always,
it worked: new quasars, the biggest red shift, variability, are they galaxies or are they nearby?” The two-hundred had gone beyond even the most optimistic of hopes.
Even when larger telescopes came along, and when research on radio telescopes, neutrino detectors, and satellite-borne instruments compete for space in the journals and headlines in the science sections, those heady days of reaching for the edge anointed the two-hundred-inch telescope as the machine that opened the universe.
Astronomers usually show up at the dome around dusk. When the telescope first went into service, observers came down early to sensitize photographic plates in special hypering solutions. For some observation programs the astronomer would have to work in absolute darkness to cut the glass emulsions into small squares and bake them in an atmosphere of dry nitrogen to increase their sensitivity. An invisible speck of dust in the wrong place on the emulsion would ruin a night’s work. Before the run began, the observer would give the night assistant a handwritten or perhaps typed list of objects with their coordinates. The observing positions in the prime-focus cage and the swinging seat on the Cassegrain focus were connected by intercom to the night assistant’s console, but the astronomer, once loaded into his seat, was alone for hours. For the prime focus the observer would ride a small open elevator up the inside of the shutter opening. The elevator would stop at the level of the prime focus, and the observer would step out, across a ten-inch gap, into the cage.
Once, when the prime-focus elevator broke before a scheduled observing run, the night assistant loaded a young, willing Allan Sandage into the cage by lowering the tube to its lower limit and letting Sandage climb up on a rickety ladder, like a carnival stunt man loading himself into the muzzle of a cannon. As the telescope slewed up, he was a prisoner in the cage. For cold winter nights the observers wore war-surplus electrically heated flight suits. For hours they would sit, cramped in a small tube, in subzero temperatures, without light, food, drink, or access to the toilet. As the suits wore out, sometimes the heating wires would chafe and be exposed. Bladder control in an electrical “hot” suit was at a new premium.
Yet for all the rigors of a cold night in a cramped cage, observing at the prime focus is a magical experience. The observer can glance over the edge of the cage, or down from the prime-focus elevator, and see directly into the mirror. The achievement of seventeen years of work—the masterpiece that McCauley, Brown, Anderson, Hendrix, Bowen, and uncounted other men created—is revealed in all its glory. Stars, millions of stars, seem to float above the disk. The focus of the disk creates an illusion: It seems as though there is nothing between the observer and the heavens. The great mirror has reached out and grabbed a chunk of the universe.
Few observers today use the prime focus. The last regular exposure of a photographic plate at Palomar was on September 29, 1989, almost forty years after Hubble exposed the “official” first plate at the prime focus on November 12, 1949. Photographic plates are still occasionally used on the two-hundred for late-epoch proper-motion measurements, but in the search for more sensitive detection devices, astronomers shifted to phototubes, photomultipliers, and solid-state light-sensitive devices for most imaging and spectra. The black-box devices of the newest imaging technology are phenomenally sensitive, but they have stolen the mystique of developing a plate in the darkrooms around the perimeter of the observatory and wondering what results would emerge from the developing and fixing baths. The move from the prime focus to the Cassegrain focus, and now to the heated observing room, has stolen the romance of lonely nights aloft in the telescope, with the heavens laid out beneath the astronomer in the great mirror. Astronomers eagerly trade romance to reach ever farther into the universe.
In the 1970s an experimenter at the Bell Labs working on a new memory device for computers discovered that the silicon chips he was using were sensitive to individual photons of light. More experiments disclosed that these CCDs, or charge-coupled devices, were phenomenally sensitive, recording up to 90 percent of the light that hit them, compared to the 3 percent that even the most sensitive photographic emulsion recorded. The detectors were tiny, a few hundred pixels in each direction, compared to the millions of grains on a photograph emulsion. But a grain of a photographic emulsion is black or white. Each pixel of a CCD can record hundreds or thousands of shades of gray, as the sensitive chip electronically integrates the photons falling on its surface. The military promptly developed CCDs as detectors for spy satellites. The astronomers had to wait, until an enterprising astronomer and tinkerer from Princeton and Caltech named Jim Gunn found sample chips at an electronics surplus shop that had bought up rejected CCDs that didn’t meet military specifications.
Today CCD detectors are available even for amateur telescopes and video cameras. The electronic chips have revolutionized telescope research. Faint objects that could once be detected only in the two-hundred-inch telescope are now within the range of a one-meter telescope with a sensitive CCD detector.
In the basement shops in Robinson Hall, where instruments are built and maintained for the two-hundred-inch telescope, the electronic wizards stretch the capabilities of these new detectors. They combine supersensitive new CCD detectors with tiny Schmidt cameras and dispersion gratings. Another instrument uses fiber optics that can be manipulated so that in a single exposure, a large CCD can record spectrograms of hundreds of galaxies simultaneously, accomplishing in a fraction of an evening work that would have taken Hubble and Humason years. The new imaging cameras and spectrographs are so sensitive that the reach of the Hale telescope has doubled and doubled again, letting astronomers reach out to ever-more-distant galaxies and quasars, until they bump up against what seems a wall of detection, as if the telescope had finally reached the edge of the universe.
The CCDs feed their signals directly to a computer. The newest instruments collect so much data, so rapidly, that for the next generation of detectors the coaxial cables that connect the instrument cage on the telescope with the computers will have to be replaced with a fiber optic pipe to provide adequate bandwidth for the data. With no need for an observer to change photo plates or manually focus the images for the plates and spectra, and with the image from a video camera to guide the telescope, observations are now done from a warm, brightly lit room on the mezzanine of the observatory. The night assistant and the observers sit in swivel chairs, controlling the telescope and the detectors from computer keyboards. Bags of Oreos and ever-available coffee have replaced the treasured midnight breaks when observers came down from their perch on the telescope to warm themselves with hot coffee or tea and share their sausages and cheese with the night assistant. A tape library that ranges from the Grateful Dead and Led Zeppelin to Bach and Mozart has replaced the AM radio the night assistants could pipe through the intercom to the prime focus, that sometimes mistakenly got stuck on a southern station broadcasting fundamentalist sermons for hours at a time.
Yet for all the new detectors and other improvements to the telescope, and the substitution of the warm data room for the cold perch in the observer’s cage, much of the operation of the telescope hasn’t changed. The strictest rule at the observatory, from the days when Ben Traxler became the first night assistant on the two-hundred-inch telescope, is that observers cannot move the telescope. The great machine is too valuable to trust to an astronomer. The night assistants aren’t necessarily engineers. One started out as a barber before finding his way to Palomar; another was a librarian. What they share is a respect for the instrument entrusted to their care, fascination with life on a lonely, beautiful mountain, and the skills to maneuver five hundred tons of machine.
The old control panel for the night assistant, developed from Sinclair Smith’s plans, is still in place, and still works, although for most use now, the motion of the telescope is controlled by a computer. At his console in the data room, the night assistant has gauges to monitor wind speed and direction, humidity, and the temperature at various points on the observatory and the telescope. Instead
of turning dials to the observer’s coordinates, the night assistant types them into a computer keyboard.
It’s a long-standing tradition for the night assistant to make a ritual, several times each night, of going up to the balcony around the inside of the dome and then through the doorway to the balcony on the outside. From there, five stories off the ground, he (or she—even that has changed) can feel the humidity and view any threatening weather or run a hand along the brass handrail of the balcony. If the rail is wet with condensation, it is too damp to observe. It is then the night assistant’s responsibility to close the diaphragm over the mirror until the air is drier or to shut the shutters of the dome if threatening weather appears.
Observers still go to the dome early in the evening, before dark, to calibrate equipment. Instead of hypering photographic emulsions, they refill the liquid nitrogen dewars (insulated flasks) on the CCD detectors, carrying a vacuum jug of super cold nitrogen up to the Cassegrain cage on the telescope. They get to the cage on a ladder that was modified from a war-surplus C-54 boarding platform. To make sure the ladder is out of the way when the telescope moves, it carries a plug that has to be secured in a wall outlet. Before the interlock system was installed, observers in the data room one night heard loud screeching during a run. When they finally came down to inspect, they discovered that the telescope had been dragging the heavy wheeled ladder with it as it slewed to different positions. There was no damage—a tribute to the robustness of the telescope—but no one likes to risk a priceless machine.
The Perfect Machine Page 58
