When one first reads about or visits Palomar Observatory it’s hard not to be caught up in the scale and grandeur of the Hale Telescope; building the Hale was the animating vision for the observatory. But a singular focus on the Hale and its many accomplishments misses a critically important part of the observatory’s legacy: namely that of the many sky surveys accomplished at Palomar—a discovery arc that continues even today.
Palomar astronomers did not invent the idea of sky surveys; before the observatory was created many astronomers contributed to our understanding of the sky’s demographics: such as astronomical source populations, their spatial distribution, and their evolution over time. At the time conventional telescopes had relatively narrow fields of view. Some astronomical questions require larger statistical samples of objects to answer, and rare but astrophysically interesting objects are hard to find with limited sky coverage. Palomar astronomers were fortunate in timing—during observatory development a new kind of telescope was imagined that would have a much larger field and thus greatly enhance sky survey efficiency. This new telescope is called a Schmidt Telescope (or camera) after its creator optician Bernhard Schmidt. A prototype 18-inch Schmidt telescope was completed in 1936, and was a spectacular success as proof of principle of the Schmidt design and its capabilities; going on to make its own important scientific contributions with some of the first transient-event, galaxy-clustering, quasar, and Solar System surveys.
The 18-inch Schmidt success motivated the creation of a larger and more capable telescope—the Palomar 48-inch Schmidt that saw first light in 1948, and was in 1987 rededicated as the Samuel Oschin Telescope. The Oschin’s capabilities were transformational, enabling a broad spectrum of survey methods for different purposes, and inspiring complementary facilities around the world. The Oschin’s early defining success was the Palomar Observatory Sky Survey or POSS sponsored by National Geographic Society and completed in 1958. POSS was among the first comprehensive astronomical imaging surveys, panoramically photographing the full sky accessible from Palomar in two colors and in almost 2000 individual exposures. POSS was also the first image survey to make its imagery widely available to the astronomical community. These aspects made POSS broadly impactful—enabling scientific studies covering topics as diverse as Solar System demographics to Milky Way structure to galaxy clusters and virtually everything in between. POSS was so successful that it was repeated in a second epoch survey in the 1990s with improved telescope optics and photographic methods. Between these two general imaging surveys, more specialized surveys focusing on topics such as asteroid populations, nearby stars and stellar luminosities, and supernova searches yielded astrophysical insights that defined our understanding on important topics for decades.
Palomar remains at the forefront of astronomical survey discovery. In 2018 The Zwicky Transient Facility project installed the latest in a series of increasingly larger format cameras on the Oschin Telescope. The ZTF camera focal plane has nearly 600 million individual detectors (or pixels), and a single exposure images nearly 50 square degrees of the sky in optical wavelengths. The Oschin Telescope robotically scans the sky every clear night looking for transients—astronomical sources that appear to change in brightness or position with time. When transient sources are detected the companion Palomar 60-inch telescope is robotically tasked to measure source colors to perform rough classification and initial assessment. During the first five years of operation ZTF discovered and classified over 8000 supernovae, including over 3000 Type Ia events; hundreds of near-Earth asteroids, and tens of rare transients like tidal disruption events when a star is violently ripped apart by the gravity of a black hole. A second complementary survey project called WINTER began operations in late 2023, and robotically searches the skies for transient sources in near-infrared wavelengths invisible to ZTF. Both projects received development and operations support from the U.S. National Science Foundation, and both make data available to the astronomical community and public. Never before has Palomar data been as broadly relevant and accessible.