The Observatories of Sacramento Peak
Sunspot, New Mexico
Solar Pioneers and the Greatest Sky Survey to Date
The Apache Point Observatory and the National Solar Observatory (NSO) at Sacramento Peak (Sac Peak) offer a special two-for-one chance to experience the vast progress of astronomy in the latter half of the twentieth century.
The observatories stand in the Lincoln National Forest in central New Mexico’s Sacramento Mountains, a thirty-five-minute drive up one main road from Cloudcroft, a charmingly compact small town sixteen miles east of the iconic city of Alamogordo. The peak is surrounded by a variety of scenic hiking trails and expansive views of the Tularosa Basin, including world-famous White Sands National Monument. The village at the peak is known as Sunspot, a name chosen by James Sadler, the U.S. Air Force officer in charge of constructing the observatory, who went on to international scientific fame as an extreme-weather meteorologist at the University of Hawaii.
| Web site | Driving Directions: |
| www.nso.edu/visitors.html | nsosp.nso.edu/pr/road-directions.html |
| Phone (575) 434-7190 |
The observatories of Sac Peak present an appealing mixture of uniquely shaped solar telescopes that work during the day, the nation’s most active remotely operated midsized optical telescope for nighttime astronomical research, and the single greatest wide-area survey telescope in the world.
The roots of this historic site lie in the once-mysterious, still-unpredictable power of the Sun to disrupt radio communications and related electromagnetic waves in Earth’s upper atmosphere. The U.S. military recognized the serious implications of this interference during World War II, so attempting to understand and forecast these phenomena became a natural focus of work for researchers supported by the air force.
In particular, the dry pristine skies above Sunspot enable telescopes to look at the tenuous hot plasma extending well above the Sun’s surface (the solar corona) and its surface activity (cold sunspots and hot active regions) in great detail. More than five decades after its founding, the National Solar Observatory at Sacramento Peak remains a world leader in high-resolution studies of our nearest star.
The hardy men who founded the solar observatory hailed largely from Colorado, but the rugged isolation of its location caused these men to include their families as residents from its earliest outpost-flavored days.
The first key figure was Leadville, Colorado, native Rudy Cook, who met solar scientist Walter Orr Roberts on a high school class trip during World War II. Cook served a stint in the army, then contacted Roberts during his postwar search for employment. Roberts was in the process of founding the National Center for Atmospheric Research in Boulder. Using funds from the air force, Roberts and solar astronomer Jack Evans hired Cook to scout out and establish a new high-altitude solar observatory station in the Sacramento Mountains in the summer of 1947.
After a few initial reconnaissance trips to the isolated 9,200-foot peak using a military weapons carrier as transport and a railroad boxcar as living quarters, a crude road was constructed. Cook went back to Colorado and returned with his wife and three-year-old daughter in 1948. A second pair of newlyweds, Lee and Rosemary Davis, were also betrothed that fall and also began their new life together on Sac Peak.
Local cattle ranchers Jean and Bill Davis (unrelated to Lee) had a self-sustaining settlement about eight miles from the peak that saved the early crew of the observatory more than once, according to the fascinating anecdotes of Joanne Ramsey, wife of the third major staff member, Harry Ramsey. Early life was a rugged, often invigorating, sometimes muddy mess, exemplified by premanufactured Quonset huts and gasoline-powered wringer washing machines. Living at the edge of oxygen deprivation in such rough conditions sometimes led to some petty disputes among the residents, but the day-to-day challenges of life on the peak let these incidents pass quickly and led to incredible personal bonding among the early residents.
By 1952, a series of redwood houses and a combination cafeteria/ administration building/community center had been built for staff and their families; the buildings survive and continue to serve staff well today. These first signs of any “luxury” were followed by a post office stop in Sunspot established in 1953, a milestone that signified the arrival of mainstream civilization.
The first permanent solar telescope was built in a grain bin ordered from a Sears catalog (the same handy sourcebook used to purchase the first garage workshop on the rugged mountain). The grain bin was rebuilt by air force machinists to rotate and to have a door. The plainly named Grain Bin Dome began work in 1950 with a 6-inch (0.15-meter) telescope designed to look at large active tendrils of hot gas on the limb (edge) of the Sun known as solar prominences. From March 1951 through 1963 daily images seeking new solar flares were taken from the Grain Bin, until its duties were finally handed off to the newly built Hilltop Dome Telescope.
The Grain Bin continued to be used for coronal observations until 1969. It survived a period of disuse (including a roof collapse from heavy snow) to be revived today as the home of a small telescope for nighttime viewing by mountain residents.
Another Sac Peak facility that breaks the mold of the typical research telescope shape is the Big Dome—also known as the Evans Solar Facility, in honor of solar astronomer Jack Evans. Completed in 1953, this squat pyramid-like building resembles a visual twist on a 1950s flying saucer.
The telescope on the large spar (or column) inside the Big Dome is mostly used as a coronagraph. It helps scientists study the faint outer layers that lie beyond the easily visible solar disk (the corona) of the Sun by blocking the light from the bright circular body of the Sun using a round disk inside the telescope—thereby permitting observations of the shapes and physical characteristics of the hot plasma comprising the solar corona. In effect, a coronagraph creates its own solar eclipse whenever one is needed. It is only effective as a science instrument in clear, thin air free of large amounts of dust, which is why the Evans facility is located atop Sac Peak.
The Big Dome Telescope does not sit straight on the big concrete spar. Instead, it is mounted at an angle of 35 degrees relative to the horizon. If you were to draw a line from the spar through the telescope and extend it into the sky, then you’d end up near the North Star, around which the whole sky (including the Sun) seems to rotate once a day. This means that the telescope can simply follow the Sun along its daytime track in the sky by rotating in one direction only, parallel to the imaginary line projected outward to the North Star, using a motor that works at constant speed. The machinery of the Big Dome facility shows that this elegantly practical configuration, called an equatorial mounting, works as well during the daytime as at night.
A small building attached to the Big Dome uses a movable two-mirror system called a coelostat to feed a stationary image of the Sun to a suite of instruments that monitor the Sun all day for transitory activity such as energetic solar flares that eject hot plasma outward from the solar surface. A small observation room at the left-hand side of the Big Dome (as seen from the road) is open to the public.
The other multifunction facility in Sunspot is the Hilltop Dome. From the outside, it resembles a mini version of the 200-inch dome at Palomar, with a service building attached to the side. Inside, the dome houses another large, equatorially mounted spar to track the Sun, this one with eight sides capable of holding a scientific instrument. The instruments are a mixture of experimental devices and longstanding workhorses that take regular images of the whole Sun in white light (basically the range of wavelengths visible to the human eye) and in selected wavelengths that highlight the formation of solar flares. The resulting photographic record runs decades long. Recent images are displayed on one of the TV monitors for the public inside the lobby of the Dunn Solar Telescope.
The telescope spar in the Evans Solar Facility at Sacramento Peak.
The Dunn Telescope is the signature building on the solar side of Sac Peak. Visible from White Sands and other distant points, the Dunn Telescope is a striking blend of the angular beauty of an ancient pyramid with white-painted modern scientific functionality. The Dunn is 136 feet tall and extends nearly 230 feet more than that underground. The main function of all this external structure is to support an innovative entrance window at the top, where two mirrors guide an image of the Sun down and through a 329-foot-long tube that has been evacuated of air: essentially a vacuum chamber.
The vacuum inside the telescope tower serves to pass the image of the Sun through a turbulence-free tunnel, feeding the pristine image to a variety of imaging cameras and spectrographs to dissect the incoming sunlight into its component colors. In recent years, the staff of the National Solar Observatory and collaborators at places like the New Jersey Institute of Technology have developed some of the most advanced adaptive-optics systems in the world for solar science—enabling imaging and spectroscopy of the Sun at resolutions comparable to that achievable in space by correcting for the distorting effects of turbulence in the upper atmosphere in real time.
High-resolution image of a sunspot produced with a modern CCD camera attached to the Dunn Solar Telescope’s adaptive-optics system. The dark black area (the umbra) is roughly two Earths in size.
Originally known—for obvious reasons—simply as the Vacuum Tower Telescope, it was renamed in 1998 for Richard B. Dunn, a legendary figure in solar astronomy who conducted observations for his doctoral thesis at Sac Peak in 1951 and caught the eye of Jack Evans, who was the director of the observatory at that time. Dunn’s specialty was making observations of solar prominences and other surface features at the sharpest-possible resolution. Dunn used the telescope to make historic observations of transient jets of solar gas known as spicules and thin filigrees of magnetic fields in the Sun’s photosphere at the edge of the telescope’s capability to resolve separated features, also known as the diffraction limit. (The diffraction limit is proportional to the wavelength of light being observed divided by the diameter of the telescope. The bigger the telescope, the smaller the diffraction limit and the smaller the distance between objects that are at the limit of the telescope’s resolution.)
The primary mirror of the Dunn Telescope is 1.6 meters (64 inches) in diameter, comparable to the bigger nighttime telescopes in the world when it was dedicated in 1969. The telescope is equipped with instruments that work in visible light and into the near-infrared region of the spectrum (wavelengths between 1 micron and 2 microns). It can produce an image of the Sun that is 20 inches in diameter with enough sharpness to resolve features on the surface of the Sun as small as 90 miles in diameter when magnified. This is a relatively small area, considering that the Sun is a ball of incredibly hot gas and plasma that is 9,600 times wider than that, with a volume that could hold more than a million Earths.
The central vacuum tube and everything connected with it—a mass of 200 tons—must rotate to compensate for the apparent motion of the Sun (as seen from the telescope) as our nearest star completes its arc across the sky each day. Instead of being supported from the bottom, as one might imagine, the entire structure is hung from a bearing near the top, which resides in a large tank of mercury. The balance of the system is so fine that a person can rotate the tube pushing with one foot on the main floor and one on the turntable. The Dunn Telescope remains at the forefront of high-resolution imaging of the Sun a half-century after it was conceived.
The National Science Foundation took over full responsibility for operating Sac Peak from the U.S. Air Force in 1976, formally changing its primary character to pure research, though a core air force staff remains today. The Dunn and Evans telescopes continue to enable studies of sunspots, active regions, flares, and the solar corona, toward the overall goals of understanding the basic physical processes that control energy transport throughout the Sun and monitoring (and beginning to forecast) the processes that affect the transport of energetic particles from the Sun to Earth.
In the early 1980s, Sacramento Peak astronomers such as Jacques Beckers recognized that the time was ripe to add nighttime telescopes to the mountain. Site testing with some of the solar telescopes verified the high quality of the conditions at night, thanks to the naturally smooth flow of air rising upward from the Tularosa Basin below. New Mexico State University and the University of Washington in Seattle hatched plans to install a 2-meter (79-inch) telescope, which later drew interest from the University of Chicago, Princeton University, and Washington State University. This group, later augmented by Johns Hopkins University and the University of Colorado at Boulder, formed a partnership called the Astrophysical Research Consortium (ARC).
Frustrated by insufficient observing time on national facilities, ARC engaged Roger Angel of the University of Arizona to use his new mirror-making laboratory to construct a 3.5-meter (138-inch) mirror by a creative new technique called spin casting. Developed by Angel and his team, this technique used a rotating oven to melt glass over a form of ceramic inserts. The spinning motion gives the mirror its basic parabolic shape; once it has cooled and hardened, the ceramic cores are blasted out with high-pressure water, leaving just the mirror blank, ready for polishing. The resulting hollow honeycomb spaces allow the mirror to cool much more quickly to match the surrounding air temperature, greatly improving the quality of images that it can produce.
The ARC 3.5-meter telescope has been a pioneer in the field of remote observing, where the astronomers remain at their home institutions and control the telescope and its instruments from a distance. This mode, with its obvious appeal in terms of saving travel time and costs, had been explored at places like Kitt Peak years earlier, but ARC was the first to make it work effectively, thanks in part to major advances in control technology and the power and speed of computer networks.
The 3.5-meter telescope has made a variety of discoveries, many recent ones in combination with its more famous neighbor on the mountain, the Sloan Digital Sky Survey (see below). For example, in 1999, a near-infrared camera mounted on the 3.5-meter telescope carried out observations of multiple deep-space objects first imaged by the Sloan survey, which led to the discovery of some of the most distant galaxies then known.
The 3.5-meter has also been used to carry out observations of cool, gaseous, starlike objects known as brown dwarfs, which are cooler than stars and hotter than planets. Brown dwarfs are defined as having a mass too low (less than 8 percent of the mass of the Sun) to support nuclear reactions at their centers. Astronomers using the ARC 3.5-meter found the first brown dwarf to exhibit evidence of the methane molecule, which is characteristic of extremely cool objects (with only one-tenth the surface temperature of the Sun) whose masses are only a few times those of the most massive planets.
In 2007, the 3.5-meter began science observations with the Apache Point Observatory Lunar Laser Ranger Operation (APOLLO), which makes use of the retro-reflectors installed on the Moon by the NASA Apollo mission astronauts in the late 1960s and early 1970s. APOLLO sends laser pulses toward the lunar retro-reflectors and makes precise measurements of the time it takes for the return signal to arrive back to Apache Point in order to measure the relative distance of the Earth-Moon system to a precision of 0.04 inch over a distance of 240,000 miles.
This remarkable capability enables astronomers and physicists to track the slow, nearly imperceptible outward motion of the Moon’s orbit away from Earth and to make sensitive tests of the fundamental properties of gravity, such as whether Newton’s gravitational constant is indeed a constant or might change with time on a scale of one part in one thousand billion.
However, the most widely known and referenced research to emerge from Apache Point has flowed primarily from its other major telescope and a tremendously vast survey it conducted, the Sloan Digital Sky Survey.
The Sloan Digital Sky Survey 2.5-meter telescope with the Sacramento Mountains in the background.
The 2.5-meter (100-inch) telescope used for the Sloan Sky Survey is not particularly advanced in itself. However, it is equipped with a 125-megapixel digital camera that enables deep imaging of the sky over a huge area (27 lunar diameters) as well as a spectrograph that allows simultaneous observations of hundreds of targets, which can be selected for further detailed study following analysis of deep images. Somewhat unusual among large research telescopes, it is housed in a roll-off building—suspended over the edge of its mountain cliff 40 feet above the ground—that slides back at night to expose the facility to the open night air.
But the true uniqueness of the Sloan rests on its landmark effort to observe a huge swath of the northern sky with great fidelity and consistency. Using creative teamwork and funding support from the Alfred P. Sloan Foundation (along with taxpayer contributions from the National Science Foundation), a coordinated group of three hundred astronomers and engineers at twenty-five different institutions has used the Sloan Telescope to extend the legendary Palomar Sky Survey to a new frontier.
The Sloan telescope enclosure building rolled over the telescope, with its instrument room (below), juts out from the mountainside. The facility was designed this way to provide the calmest and coldest possible air above and around the survey telescope.
The first five-year round of the Sloan survey, 2001–2005, took images of 300 million celestial objects and spectroscopic measurements of 800,000 galaxies, 300,000 stars, and 104,000 quasars (the bright cores of active galaxies that contain black holes). These surveys have deepened our understanding of how galaxies are arranged in space (see pages 89–91) and how they evolved from approximately a billion years following the Big Bang to the present.
The Sloan Telescope has also played a major role in probing the population of stars in our own galaxy, searching for rare brown dwarfs, and charting the distribution of various “stellar populations” (stars spanning a range in chemical composition, age, and kinematic properties) in the Milky Way—a study that is key to understanding how our parent galaxy was assembled.
Astronomers proposed, and received funding for, a second round of experiments with the Sloan Telescope, which ran from 2005 to 2008. Sloan-II was aimed at refining the initial survey, filling in some gaps, and extending the range of time over which variable objects are observed.
A third round of experiments, now under way, is slated to run until mid-2014. Sloan-III includes four coordinated surveys that are studying the expansion of the Universe, mapping the outer reaches of our Milky Way galaxy, quantifying the nature of more than 100,000 stars using infrared light observations to peer through the dust that usually obscures them, and searching 10,000 stars for evidence of planets around them. Sloan-III will cost approximately $7 million, a large sum but only a fraction of the cost of a new telescope.
The Sloan team has also undertaken some creative public outreach, with a pathfinding online citizen-science experiment called Galaxy Zoo (www.galaxyzoo.org) that has enabled more than 200,000 people to participate in research by visually classifying the types of galaxies in the Sloan Sky Survey images. When averaged over the millions of characterizations of the total sample, the Galaxy Zoo campaign produces scientifically useful statistics—innovative enough to trigger follow-up observations at places like the WIYN 3.5-meter telescope on Kitt Peak. Galaxy Zoo has been such a successful experiment in citizen science that a more elaborate second-generation version known as “Zoo 2” was released in early 2009.
For the Public
The National Solar Observatory at Sacramento Peak lies in New Mexico at the southern end of NM Scenic Byway 6563, about 18 miles south of Cloudcroft (on NM 82) and 40 miles southeast of Alamogordo (on NM 70 and 54), in the village of Sunspot.
The drive from Alamogordo in the Tularosa Basin to Cloudcroft in the Sacramento Mountains involves a curving, 16-mile climb spanning 4,500 feet in altitude, and thus takes more time to navigate than a comparable distance on a flat highway. Expect the drive to take an hour from Alamogordo to Sunspot and about thirty minutes from Cloudcroft to Sunspot. Sunspot has no public gas station, grocery store, or restaurant, so make sure you have enough gas and food to return to Cloudcroft. Water, soft drinks, and snack food items are available at the Sunspot Visitor Center.
From Cloudcroft, take NM 130 East (the junction with NM 82 lies at the western edge of town) and drive about 2 miles to the junction with Sunspot Highway (NM 6563). Follow the highway all the way to its end, about 14 miles.
In 2008, the NSO built a 1:250-million scale model of the solar system along this route. Eight signs along Sunspot Highway mark the orbits of the planets, and models at Sunspot depict the Sun (an 18-foot dome on this scale) and the planets (Earth is just 2 inches wide). As you arrive, note two tiny signs: PENUMBRA (“almost” a sunspot) and UMBRA (the center of a sunspot). Once inside Sunspot (through the stone gate), take the first turnoff to the left (marked “Visitor Center”) and park in front of the center.
The Sunspot Visitor Center and Museum first opened in July 1997. The center is a collaboration between NSO/Sacramento Peak, Apache Point Observatory, and the U.S. Forest Service. It contains a variety of exhibits about astronomy and the tenants of the mountain, free maps for the self-guided walking tour, and a well-stocked gift shop. Perhaps the highlights of its collection are found outside near the entrance, where a beautiful sundial and an ancient model of the celestial sphere known as an armillary sphere are displayed.
The Sunspot Visitor Center.
The visitor center is accessible by wheelchair and is the start point and end point of guided and self-guided walking tours of the observatory. It is open every day, except major holidays, from 9 A.M. to 5 P.M. (weather permitting). The walking tour and restrooms are available for visitors at all daylight times throughout the year (weather permitting). Call ahead if necessary to (575) 434-7190.
Guided tours of the solar observatory are offered daily for a small fee at 2 P.M. Reservations are not required.
Groups of at least ten visitors, of all ages, can arrange for private guided tours any day of the week, year-round (depending on the availability of a guide). Beyond the general walking tour and visits to the Dunn Telescope (about forty-five minutes each), the standard private guided tour usually includes a slide show of comparable length.
For more information and reservations, call (575) 434-7003 or fill out a request form at nsosp.nso.edu/pr/tour-form.html.
The visitor center includes exhibits on Apache Point Observatory, which does not offer regular tours but welcomes visitors to stroll its grounds between 7 A.M. and 5 P.M. Special tours can be arranged by contacting the observatory at (575) 437-6822. For the latest information on Apache Point, see www.apo.nmsu.edu.
For Teachers and Students
The National Solar Observatory maintains an active Research Experiences for Teachers (RET) program, where high school teachers work with NSO staff scientists over the summertime to develop classroom exercises and to bring the flavor of research back to their students.
It also offers online learning activities for the middle school and high school levels, based on more than twenty-five years of measurements of the Sun’s magnetic field—see the Data and Activities for Solar Learning (DASL) project at eo.nso.edu/dasl.
A Talk with Steve Keil
Director, National Solar Observatory
Emblematic of the long-term hold that Sacramento Peak Observatory holds over many of its residents, Steve Keil came to the peak as a summer student in 1973 and 1974, returned again for postdoctoral work via the University of Colorado in 1975, then became a member of its U.S. Air Force scientific staff through 1999, when he became director of the National Science Foundation’s National Solar Observatory (NSO). Today, Keil has expanded his research on the interaction between the magnetic fields near the surface of the Sun and the convection of its heat to include the complexities of the outer region of the Sun’s atmosphere known as the corona, as he simultaneously guides the NSO toward construction of an advanced solar telescope four times larger in diameter than any in operation today.
Steve Keil, director, National Solar Observatory.
What are the earliest roots of Sacramento Peak Observatory?
Right after World War II, the air force realized that solar storms were causing interference in radio communications, especially long-range communications that went over Earth’s poles. So they became very interested in trying to understand the causes. The Air Force Cambridge Research Lab got Harvard College involved, and it hired the High Altitude Observatory in Colorado to find and establish a site. The air force had a solar coronagraph at Climax Observatory in Colorado, which was a difficult place to observe from during the winter. They were looking for a dust-free site and, at the time, the Southwest was going through a wet period, so New Mexico looked very attractive to relocate the telescope from Climax.
In the early 1950s, the air force built the world’s largest coronagraph at the time and installed it at what we now call the Evans Solar Facility on Sac Peak, figuring that if you could understand the behavior of the Sun’s outer atmosphere, you could understand how these storms escaped to influence interplanetary space. Then they decided they wanted a predictive capability, which led to the Hilltop Dome and the flare patrol instruments. This helped people realize that, because of the Sun’s rotation, the flares that affect Earth appear first on the western hemisphere of the Sun.
The Dunn Solar Telescope is probably the most iconic facility at Sac Peak. What led to its unique design?
In the late 1950s and early 1960s spectroheliographs [instruments to image the Sun at a particular wavelength, or color] were being employed to measure the surface of the Sun and see active regions like sunspots, and then surface magnetograms. This led to the belief that the Sun’s magnetic field was interacting with the plasma comprising the corona on smaller scales than we had been able to observe.
There were large solar telescopes at the time, but their images were bad because of heating of the turbulent air inside them. Solving that problem led to the design and construction of the Dunn Solar Telescope with its evacuated light path. The design by Dick Dunn became sort of the standard. It enables you to get the entrance of the telescope up high to avoid the natural air turbulence stirred up near the ground. You couldn’t turn the telescope in traditional ways because of the big vacuum chamber, so turrets at the top became the way to direct the light. The subsequent German and Swedish tower telescopes were based on that concept. The big pyramid shape of the Dunn is essentially a way to hold up the top of the telescope with a very long focal length, which gives you a very flat image of the Sun.
Why is it still important to have ground-based telescopes in this age of very high-tech satellites?
Primarily, it’s the flexibility to quickly change instrumentation to keep up with the scientific need. You can use multiple instruments simultaneously, and you can fix them more easily if they break. If you discover something new on the Sun, you can rapidly change the configuration that you have to observe it. In space, you’re stuck with what you launch. Data rates are also a big thing—you can handle much higher ones on the ground, which is very important for the multi-wavelength, multi-camera observations that we do today.
The other element is money: It costs about ten times as much to get the same size aperture in space as on the ground. The disadvantage is that there is no access to x-rays or ultraviolet light, which we can get in space. It’s really the combination of high-energy observations from space and high-resolution spectroscopy from the ground that drives solar science today.
What is the connection between the current telescopes at Sacramento Peak, such as adaptive-optics technology on the Dunn, and the 4-meter Advanced Technology Solar Telescope (ATST) that you hope to build in Hawaii?
When we achieved the diffraction limit of 1-meter telescopes using adaptive optics (the theoretical best image quality that can be produced for a given aperture size), it became clear that there were some very interesting features that were still unresolved. Resolving these features requires a larger telescope with a smaller diffraction limit.
The other problem is not being able to gather enough photons of light. In order to measure polarization of small features on the Sun [which is diagnostic of magnetic field direction], you need lots of photons in a shorter timeframe than the lifetime of solar features; again, that requires a larger telescope that collects more photons. Short exposures also mitigate the effects of changes in Earth’s atmosphere [also known as “seeing,” which results from the passage of turbulent elements of air above the telescope, with different effects at different altitudes]. A large photon count also produces a strong enough signal against the background noise of the observation to allow accurate measurements of solar magnetic fields.
So, you need more light, which is hard to believe given that it’s the Sun we’re talking about, and you need to be able to collect it in a much shorter amount of time.
How do solar telescopes overcome the heat generated by sunlight so that it does not melt your instruments?
So far, the technique has been to build telescopes with really long focal lengths, so you don’t concentrate the heat in a small area. The Dunn Telescope gives you an image of the Sun about 20 inches across and, as a result, the light is spread out and does not raise the temperature of the focal surface by a significant amount. For the ATST, thermal control is a big cost driver. The telescope admits a much smaller field of view, and we have to cool the primary and secondary mirrors about two degrees below the ambient temperature so the heat flows down and does not produce turbulence above the mirror. Dust is also a big issue—scattered light is actually a bigger concern than the quality of the mirror. So we have designed systems to actively clean the mirror.
It seems that family life has always been a significant element of the observatory. What is it like to live on Sacramento Peak?
We’ve always had a real strong community group, and it’s still going strong today. It’s really the heart of our social interactions. They meet once a month, and have a potluck dinner once a month. They give out a scholarship every year to local high school students. Some people come for a year and leave. But if you stay for two years, you tend to stay forever.
Life at Sac Peak can be a mixed blessing, because it is isolated. Some people really prefer city life. It can be difficult for spouses to find work, unless they are teachers or self-employed. Some of the spouses have been drawn into work with Apache Point Observatory, such as helping to install the optical fibers in the spectrographic plates used in the Sloan survey, or serving as spotters for their laser. But it’s a great place to raise kids—the phrase “it takes a village” is very applicable to life there. I’ve never locked my door in the thirty-five years that I’ve lived there. And it continues to be a joint facility with the U.S. Air Force, with each of us having about a half-dozen scientists based there, so in terms of scientific vigor, it’s still a good place to work.
The Dunn Solar Telescope is popularly rumored to be the burial site of the alien bodies from the infamous Roswell incident of 1947. What can you tell us about this situation?
Well, it’s true that the Dunn Telescope is only a three-hour drive from Roswell, New Mexico, and that we are very nearby Holloman Air Force Base, which has been known to conduct some highly classified “black” programs over the years. One of the key figures in the founding of Sacramento Peak Observatory, Donald Menzel of Harvard College Observatory, is often considered to be part of a secretive society of astronomers who offered Cold War–era advice to the military. Finally, why else would the air force dig a 230-foot hole in the ground at such a remote wooded place? Hmmm. Unfortunately, the aliens are usually out gallivanting around causing UFO sightings, and we have not seen them.
Science Highlight
The Complexity of Sunspots
Sunspots are apparently dark regions visible on the surface of the Sun. They were first studied in a systematic way by Galileo in the early seventeenth century. His pioneering observations in 1612 revealed the vast size of sunspots, their impressive persistence, and their intriguing eleven-year cycle. The largest sunspots span thousands of miles in diameter, wider than the diameter of Earth, and can sometimes last for several thirty-day-long complete rotations of the Sun.
The true nature of sunspots remained a mystery until twentieth-century measurements demonstrated that they are relatively cool regions, with temperatures about 1,500 degrees lower than the 9,800-degree temperature typical of the solar surface region, called the photosphere. The characteristics of sunspots are deduced from measurements of the relative strength of features in the spectrum of the Sun that arise from the gaseous form of elements such as iron, calcium, and magnesium. These elements absorb or “block” certain wavelengths of light, causing features known as absorption lines. First observed by Newton using a large prism, spectral absorption lines are now measured precisely using instruments known as spectrographs.
Why are sunspots cooler than their surroundings? The key to answering this question was the discovery that sunspots are regions characterized by strong magnetic fields. Powerful spectrographs on telescopes revealed the detailed shapes of the absorption lines. These shapes show “broadening” and “splitting” of individual spectral lines that are produced when strong magnetic fields thread through hot, gaseous plasmas.
From measurements in terrestrial laboratories and theoretical calculations of line broadening and splitting, it was possible for astronomers to translate measurements of line shapes and splitting observed within and outside sunspots into measurements of magnetic field strength. Astronomers at Sacramento Peak and at their sister office in Tucson that runs the National Solar Observatory telescopes atop Kitt Peak have pioneered techniques for producing regular images of the Sun’s magnetic field—images produced from measurements of line broadening and splitting and available for use by astronomers and other scientists throughout the world.
The fact that sunspots are sites of enhanced magnetic field strength results in a significant decrease in the efficiency of energy transport from the hot interior regions of the Sun in the vicinity of sunspots as compared to the surrounding regions of the solar photosphere, where heat transport proceeds unimpeded. As a result, sunspots are cooler than nearby regions of the photosphere.
Sunspots have long been known to be the origin of bursts of energetic activity. The most prominent of these are solar flares. Such flares, which occur most frequently during the part of the eleven-year cycle when the number of sunspots reaches a maximum, are generated as the energy stored in the twisted magnetic field “loops” above the spots is released suddenly, launching hot plasma into space. Some of the charged particles from the released plasma can reach Earth and penetrate our protective magnetosphere, sometimes interrupting radio communications and disrupting electrical power grids. The charged particles from flares can also produce auroras, as the energetic electrons emanating from flaring events excite atoms in the upper atmosphere (such as oxygen) and create glowing sheets of light.
Sunspots have thus pointed the way toward understanding the complex interactions of gas motions and magnetic fields that transport energy from the interior of the Sun (where nuclear reactions unleash energy sufficient to heat the gas to temperatures of millions of degrees) to its surface. Gaining a deep understanding of energy transport both inside the Sun’s hot plasma and across its complex outer atmosphere is vital to understanding the factors that influence the total energy per second reaching Earth as well as the spectacular “solar storms” created by flaring events. These factors are in turn crucial elements of the external forces that drive long-term climate changes on Earth, as well as far shorter-term terrestrial responses to solar activity, dubbed space weather.
Astronomers at Sacramento Peak Observatory have designed and constructed a variety of sophisticated instruments aimed at making detailed measurements of the physical properties of the outer layers of the Sun: temperatures, magnetic field strengths, and the velocity of the hot gases, both those in the deceptively quiescent regions of the solar photosphere and those around the sunspots that fascinated Galileo almost four hundred years ago.
Along with their Kitt Peak colleagues at the Tucson office, Sac Peak astronomers have carried out systematic studies of gas motions and magnetic field structure in the Sun’s outer layers. Combined with international efforts to study the structure of the solar interior (via a technique analogous to sonograms) as well as hot plasmas above the photosphere extending into interplanetary space (using a variety of remote sensing and spacecraft-based measurements), National Solar Observatory astronomers and their colleagues have begun to develop an empirical and theoretical framework for understanding the basic workings of our parent star.
Further progress in sharpening our models of solar behavior and making them reliably predictive requires observations of features on the Sun at a far smaller scale than has been possible in the past.
Current solar astronomers at Sac Peak, such as Thomas Rimmele, have pioneered techniques to use adaptive optics during the daytime to compensate for the blurring effects of Earth’s atmosphere, without the benefit of the bright stars or laser beams used by nighttime astronomers. Over a period of fifteen years, NSO astronomers have used the Dunn Solar Telescope to produce images of a clarity unprecedented for any solar telescope in the world. These observations have led to breakthroughs in understanding how sunspots work and, more generally, produced a deeper understanding of how energy is transported in the presence of a magnetic field—both the weak fields permeating the “quiet” photosphere and the strong fields found around sunspots. Solar scientists are now applying this experience to the design and development of the next great solar telescope, the planned 4-meter Advanced Technology Solar Telescope, to be built on Mauna Loa in Hawaii.
The Hobby-Eberly Telescope as seen from the air.