At 8:45 p.m. on a warm, clear night, the last rays of the setting sun lit up a remote mountaintop a thousand feet above the small Chilean town of Vicuña. Mother Nature’s show, however, was just beginning.  I saw Sirius, the brightest star in the night sky, first. Later came the Tres Marias, as the three glittering studs in Orion’s belt are known here. Then as violet hues faded to black, there were dozens, then suddenly hundreds of stars. An hour later the Milky Way blazed across the dome of the sky, a glimmering helix of light and darkness.  Emilio Lepeley, our guide for the night at Mamalluca Observatory, aimed a green laser into the sky. “The Incas had dark constellations,” he said, referring to the practice of finding forms in the “empty” areas between stars. “Can you see the llama?” Outside, 50 or so people milled about at the observatory doors, the usual summer crush waiting their turn for a peek through the telescope inside.  Mamalluca is one of about a dozen observatories in northern Chile’s Elqui Valley region that cater to astrotourists (once known simply as stargazers). An oasis in an otherwise arid landscape, the roughly 90-mile-long valley stretches from the Pacific Ocean eastward into the Andean foothills. Its geography, with the cloud-blocking Andes to the east and the bone-dry Atacama Desert to the north, produces exceptionally clear conditions, optimal for stargazing.  Starting in the 1960s, a string of major international research telescopes were built here to take advantage of that, including Cerro Tololo, La Silla and Gemini South, and in 2014 work began on the planet’s most powerful space camera. Within five years, an estimated 70 percent of the world’s astronomical infrastructure will be in Chile, much of it in the Elqui Valley.  Increasingly, would-be astronomers and stargazers are getting in on the act, too, drawn by the region’s famously clear night skies and a host of charms equally evident by daylight. Growing popularity among Chileans and international visitors, however, has posed a challenge of its own: As towns sprawl onto surrounding hillsides and resulting light pollution spreads, will those pristine night skies last?  To reach Vicuña, the biggest municipality inside the valley and a hub for astrotourism, I had driven an hour east from the coastal city of La Serena, climbing slowly along the aptly named Ruta de Las Estrellas, or Route of the Stars. Vineyards and orchards along the valley floor eventually gave way to cactuses stippling the flanks of steep ridges. The region has long been a getaway for Chileans, who flock to a string of small towns filled with centuries-old Spanish churches and distilleries that produce pisco from local grapes. After dark, however, attention turns to the skies.  CreditRemy Scalza for The New York Times  While Mamalluca was among the region’s first observatories for casual stargazers, others now offer a more intimate peek into the universe, with better equipment and smaller groups. The following night, I headed down to Vicuña’s main plaza for a ride to the area’s most powerful tourist telescope, situated on a hard-to-reach mountaintop outside town. In the plaza, a restaurant named Halley (after the comet) did a brisk business in empanadas, while stray dogs lazed in the shade of an ornate Catholic church.  At 9 p.m., a battered S.U.V. rumbled up, and I got inside with a half-dozen other stargazers. On a moonless night, we barreled up rutted dirt roads for nearly an hour, climbing along switchbacks before reaching our destination, the privately run Pangue Observatory, which was seemingly the only structure for miles.  We were met by Eric Escalera, a retired French astrophysicist who formerly worked at nearby La Silla Observatory. “Government telescopes are actually very boring,” he said, leading us to an outdoor viewing platform and a 10-foot-tall telescope silhouetted against the sky like a piece of artillery. “They’re just for collecting data — you never actually get to see anything. This is more fun.”  On the isolated ridge, conditions were inky black, and it was impossible to see more than a foot or two ahead without a flashlight. Mr. Escalera motioned to the Milky Way, even gaudier up here, and pointed out two faint puffs of light nearby — distant galaxies more than 150,000 light years away known as the Magellanic Clouds.  To actually use Pangue’s 25-inch telescope (enormous for hobbyists, though puny, we were told, by research standards), it was necessary to climb a stepladder and look into an eyepiece at the top. Mr. Escalera first focused in on the Tarantula Nebula, a nursery for stars located inside the Large Magellanic Cloud. Through the lens, the mass of gas and dust did indeed look like a giant spider, its body outlined by the nebula’s pale glow. (Don’t expect those brilliantly colored images of outer space that show up on screen savers and dorm-room posters; the real thing turns out to be much more subdued.)  As the night wore on, we took turns gazing at a creamy mass of stars called a globular cluster; Jupiter, with four of its moons and bands of tropical clouds; and an aging pair of stars called Eta Carinae, racing toward a violent end as a supernova. But the real show, at least for a novice like me, didn’t require a telescope at all. As midnight approached, the southern sky was a dense canopy of stars, packed tighter than I’d thought possible. A distant thunderstorm in Argentina sent up flashes of light from the east, while the screech of night birds cut through the darkness.  The next day, on a hot, cloudless morning, I continued my journey east on the Ruta de Las Estrellas. After Vicuña, the valley floor drops away dramatically and the drive turns into a twisting, roller coaster ascent. I passed adobe houses clinging to cliffside ledges and fruit stands selling fresh tuna, the fruit of the local cactus, before reaching the village of Pisco Elqui, a traditional favorite among Chilean vacationers that’s been enjoying a mini-boom in recent years.  In the late afternoon, I climbed the village’s cobblestone streets with Bayron Muñoz, the guide on a trekking tour that takes stargazers on foot into the local mountains. “Does anyone suffer from vertigo?” he asked, before we turned onto a steep rocky trail. As the sun dipped behind a ridge, we shuffled past the ruined remains of adobe huts and centuries-old canals, carved into the mountainside by the indigenous Diaguita people and still pulsing with water.  It was nearly dark when we toed along the lip of a yawning canyon and scampered up a rise — only to discover a tour van, which had evidently arrived via a less harrowing route. The advance group had been nice enough to start a campfire, however, as well as mix up a round of pisco sours, which were passed around as we turned our view to the darkening sky.  Laser pointers flitted through the darkness, tracing over constellations (Orion, Taurus, Cancer), the glowing orb of Jupiter and the intergalactic dandelion puffs of the Magellanic Clouds. Afterward, we looked into a small, eight-inch portable telescope, hauled up the mountain on our account, at the famous red supergiant star, Betelgeuse, and Sirius, which glittered like a diamond under a jeweler’s loupe.  But it was hard to miss the dull glow of lights from the valley below, which blotted out all but the most brilliant stars in one corner of the sky. “Pisco Elqui is getting bigger all the time,” Mr. Muñoz said. “More construction means more light pollution.”  Indeed, unblemished skies are becoming harder to find throughout the valley. The yellowish glare from the growing twin coastal cities of La Serena and Coquimbo — whose combined populations have nearly doubled in the last 20 years to more than 400,000 — is a constant on the western horizon, while light pollution in Vicuña has already raised flags at the major international observatories.  Meanwhile, the influx of domestic and international travelers means Elqui Valley’s stars now have more competition than ever. With billions of dollars in astronomical infrastructure at stake, the Chilean government has had to go on the defensive. In 1999, a law to protect the integrity of night skies in places like the Elqui Valley was implemented, which included provisions to shield streetlights and limit the power of public lighting. Beefed-up requirements went into effect in 2014, taking into account billboards and new sources of pollution like LEDs and other so-called white lights, big offenders in terms of sky glow.  But late that same night on the edge of Pisco Elqui, it would have been hard to imagine a more brilliant sky. Several miles outside town, near where this leg of the Ruta de las Estrellas dead ends against the vertical wall of the Andes, a collection of mini geodesic domes has been built on a hillside — part of Elqui Domos, a hotel catering to stargazers. Each PVC-wrapped dome has a raised bed inside, with a zip-away panel in the ceiling — a private window onto the universe. (The domes do turn into ovens once the sun rises, however, a factor to consider if you plan to sleep in.)  By the time I got back from my mountain trek, most domes had gone dark, but the silhouettes of a few were still glowing softly. I stepped inside mine, climbed to the loft and unzipped the skylight. Orion stared back at me, bow tensed and ready. As my eyes adjusted to the dark, more and more stars came into view, stars packed upon stars. I watched a gleaming planet climb higher in the sky before falling asleep.  The city of La Serena, gateway to the Elqui Valley, is a one-hour flight from Santiago. From the airport, rent a car and follow Ruta 41, part of the Ruta de Las Estrellas, which extends east to Vicuña (a roughly one-hour drive) and other astrotourism hot spots.  Solar de los Madariaga, Gabriela Mistral 683, Vicuña; solardelosmadariaga.cl; double rooms in a late-1800s Spanish colonial home (and museum) from 36,000 Chilean pesos (about $60 at 613 pesos to the dollar) per night.  Refugio Misterios de Elqui, Arturo Prat (no street number), Pisco Elqui; misteriosdeelqui.cl; private cabañas for two on landscaped garden terraces from 60,000 pesos per night.  Elqui Domos, Camino Público Pisco Elqui Horcón Km. 3.5, Pisco Elqui; elquidomos.cl; mini geodesic domes with zip-away skylights for up to four people from 85,000 pesos per night.  To see the most dazzling skies, it’s best to visit around the new moon, the time each month when the moon is not visible. Astronomictourism.com, a privately run website, compiles information on local offerings.  Mamalluca Observatory, astronomictourism.com/mamalluca-observatory.html. Two-hour guided tours to the region’s oldest and most popular tourist observatory depart nightly from Vicuña’s main square. 4,500 pesos.  Pangue Observatory, observatoriodelpangue.blogspot.ca. An hour from Vicuña is the region’s most powerful tourist telescope, run by a retired astrophysicist. 21,500 pesos.  Astro Trekking, astronomictourism.com/astro-tour-pisco-elqui.html. A one-hour dusk hike from the village of Pisco Elqui leads to a mountaintop lookout for stargazing (and pisco sours). 20,000 pesos.  An earlier version of this article described one part of a telescope at the Pangue Observatory incorrectly. It is the mirror of the telescope that is 25 inches, not the lens. (Modern reflecting telescopes do not have lenses.)  A version of this article appears in print on June 28, 2015, on page TR11 of the New York edition with the headline: A Heaven Made for Astronomers. Order Reprints| Today's Paper|Subscribe
Astronomers and stargazers are flocking to the Chilean destination, where night skies offer brilliant views despite the growing threat of light pollution.