× 0. Palomar Observatory
M. Visitor Center (Museum)
H. Hale Dome

The 18-inch (0.46-m) Schmidt Telescope

The 18-inch Schmidt telescope was the first instrument at Palomar Observatory and was the only operational telescope on site between 1936 and 1949. Beginning in the late 1940s, this instrument was used in conjunction with the newly-built 48-inch Schmidt (later to become the Samuel Oschin Telescope) to provide targets for the 200-inch Hale Telescope. Between the 1970s and 90s, the 18-inch proved to be a workhorse in the systematic search for minor bodies in the Solar System. The telescope was decommissioned in the mid-1990s and was removed from its dome to make room for a robotic atmospheric turbulence monitoring system in 2006. Thanks to the generosity of the Eleanor and Ronald Helin Trust, the 18-inch Schmidt was re-assembled in 2013 and is currently on display as part of the Eleanor Helin Commemorative Exhibit at the Greenway Visitor Center.


The Palomar 18-inch Schmidt Telescope was the first operational telescope on site, and a proof-of-principle technical demonstration that evolved to have an impactful scientific legacy. As originally conceived, Palomar Observatory was to be focused solely on the Hale Telescope—the idea of broad sky reconnaissance was suggested by astronomers Walter Baade and Fritz Zwicky. Zwicky in particular became a strong advocate for a novel telescope design proposed by Bernhard Schmidt. The Schmidt telescope (or camera) design delivers a usable field of view hundreds of times larger than that of the Hale. The concept was first demonstrated on sky at Palomar in 1936 with the 18-inch Schmidt telescope, currently on exhibit in the Greenway Center. Performance of the 18-inch was so encouraging that Caltech committed to building a larger version, the Samuel Oschin Telescope, which saw first light in September 1948.

After successfully demonstrating the concept, the 18-inch Schmidt was used extensively by Zwicky to survey the sky for explosive transient events known as “supernovae.” The term had been introduced in seminal theoretical work by Baade and Zwicky in 1934, and Zwicky was enthusiastic to find observational support for the concept. Zwicky’s 18-inch supernova survey was highly successful until it was interrupted by the US entry into World War II. Zwicky also used the under-construction Hale dome as a twilight “test subject” as he was preparing for the night’s observations.

In the 1970s the 18-inch telescope became a workhorse in Solar System surveys. Separate teams led by Eleanor Helin, and Gene and Carolyn Shoemaker, respectively, used the 18-inch to search for undiscovered asteroids and comets. Among many objects discovered by the 18-inch, the most notable was the famous Comet Shoemaker-Levy 9 discovered by the Shoemakers and David Levy in 1993. This comet was found in orbit around the planet Jupiter and is thought to have fragmented in the capture process. The fragments of Shoemaker-Levy 9 impacted Jupiter over several days in July 1994, and detailed study of the collision events with many different telescopes facilitated new discoveries both of Jupiter’s atmosphere and the chemical makeup of the comet.

The Shoemaker-Levy 9 spectacle was a fitting capstone to the 18-inch scientific legacy and the telescope was decommissioned in the mid-1990s. The 18-inch was relocated to the Greenway Center in 2013 as part of the Helin commemorative exhibit, a gift to the observatory by the Helin family trust.

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