Las Campanas Observatory: Establishment and Early Observations

The Carnegie Institution’s Mount Wilson and Palomar observatories already had large Northern Hemisphere telescopes, but researchers hoped to extend their observing programme southward. In 1963, the institution approved funds for a Southern Hemisphere site survey and reached an agreement with the University of Chile. The programme organised by Horace Babcock investigated Chile and also conducted tests in New Zealand and Australia.

The site survey used specially developed seeing monitors to assess how atmospheric turbulence affected stellar images. The number of clear nights, image stability and practical conditions for establishing an observatory all mattered. Funding for the originally envisaged Southern Hemisphere 200-inch telescope was not secured in time, and the team considered first establishing a station with smaller instruments.

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Babcock initially considered building on Cerro Morado near Tololo, but land arrangements and room for future expansion proved obstacles. Continued surveys brought Las Campanas to the fore. Its ridge, approximately 2,300 to 2,500 metres above sea level, offered several telescope locations, dark skies and test wells indicating an available water supply. The team chose the site in 1968.

In November, Babcock met Chilean President Eduardo Frei in Santiago to seek support for acquiring the land. A large property provided construction space and a buffer around the observing environment. Selecting a site therefore involved roads, water, long-term use and expansion as well as finding a summit with clear views of the stars.

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A donation from astronomer Henrietta Swope enabled the early construction to proceed. Engineers built an access road from the valley to the west, established water and diesel power supplies, and provided accommodation, storage and maintenance facilities. The El Pino office in the coastal city of La Serena handled administration and logistics, connecting the mountain observations with supplies and support in the city.

The 40-inch reflecting telescope, with an aperture of about one metre, began operating in June 1971. It later took Swope’s name. A 10-inch photographic refractor previously used at Mount Wilson was also installed on the ridge. Another agreement, reached in 1970, allowed the University of Toronto to install and operate a 24-inch telescope on the same site, making cooperation between institutions part of the early observing facilities.

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Ira Bowen and Arthur Vaughan designed the optical systems for the 40-inch and subsequent 100-inch telescopes, prioritising clear photographs over a large area of sky as well as aperture. Their 1973 design paper compared the two modified Ritchey–Chrétien systems and their corrective lenses. The 40-inch instrument used a flat focal plane, while the 100-inch allowed photographic plates to be curved moderately to match the curvature of its focal surface.

These choices affected the area of sky recorded in a single exposure and the measurement quality of stellar images near the edges of the plate. Field of view, focal-surface curvature and aberrations were central optical design considerations; actual mountain images also depended on atmospheric conditions. Site tests assessed the observing environment, while instrument design adjusted how light formed images within the telescope.

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In the same year, Bowen and Vaughan also published a grating–prism design for the 40-inch telescope, exploring how spectra of multiple objects in the field could be obtained together on photographic plates. They evaluated the design through ray tracing, discussed ways to reduce aberrations, and warned users about crowding caused by different spectral orders of bright stars.

The proposed device was intended as an accessory for the 40-inch telescope to record spectra across a wide field. Researchers wanted data for classification and comparison over a large area of sky, and therefore considered the grating, prism, focal-plane position and overlapping spectra together. The paper translated this observing requirement into performance that could be tested.

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When the 40-inch telescope began observing, the larger du Pont telescope was still under development. Following a donation by the Greenewalts in 1970, Carnegie decided to supply the remaining funds for a 100-inch instrument. An observatory committee formed in 1971 divided responsibilities for mechanics, electronics, optics, auxiliary instruments and Chilean administration, with Babcock coordinating the work.

The observatory held its dedication ceremony on 6 October 1976. By then, observations begun in 1971 had accumulated over several years, while work on larger instruments was continuing. Through phased construction, early Las Campanas gradually expanded its observing capabilities, cooperating institutions and supporting facilities.

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