The idea of including coelostat in Griffith Observatory so that the public could view a projection of the sun’s disk and a direct view of the sun’s spectrum could be viewed was that of the Observatory’s donor, Col. Griffith J. Griffith (1850 – 1919) – presumably after visiting Mt. Wilson Observatory (ca. 1910). Less than a decade after Griffith’s death in 1919, the founder of Mt. Wilson Observatory, George Ellery Hale (1868 – 1938), invented a device – the spectrohelioscope- for viewing the eruptions in the outer layers of the sun that previously could only be observed in the fleeting moments of total solar eclipses. The spectrohelioscope was added as a feature when plans for the observatory began to drawn up by Hale and his scientific colleagues when approached for help by Griffith’s trust committee in 1930.
The challenging task of creating a coelostat to feed the three separate telescopes fell to Russell W. Porter (1871 – 1949), one of the important founders of the hobby of telescope making, and an important instrument designer, architect, and illustrator connected with the 200-inch telescope project started by Hale in 1928. Porter’s 1931 concept for Griffith Observatory resulted in both the moving tracking mirrors and the relay mirrors being mounted in parallel on separate polar axes, an innovation that subsequently affected the designs of coelostats at Caltech’s Astrophysical Laboratory and at the McMath-Hulbert solar observatory at the University of Michigan. The detailed working drawings of Griffith’s coelostat were made by E.C.Nichols of Mt. Wilson Observatory, and the device was fabricated by the Gaertner Scientific Co. of Chicago in 1935. As is true of the general layout of Griffith Observatory and many of its architectural features, the original concepts for the rotunda of the solar telescope were developed by Porter and his colleagues at Caltech and followed by the architectural consortium of John C. Austin and Fredrick M. Ashley.
While the purposes of the solar telescopes are the same today as they were in 1935, there have been some interesting variations and additions to their use since they were first installed.
From the late 1930s and until World War II, it was decided that the central telescope of the coelostat would be used for moon viewing at night. After sunset on evenings when the moon was visible, the hinged frame of the solar projection screen was opened and an enormous eyepiece was aimed through the opening so that the moon could be viewed from the rotunda, freeing the 12-inch Zeiss telescope in the east dome for observing other celestial objects.
In the 1940s a 5X7-inch camera was added to the room above the museum ceiling so that light can be diverted from the spectroscope objective to photograph the entire solar disc. This camera was used by the Observatory’s staff to photograph sunspots, solar eclipses, and other solar events though 2001.
The Hale spectrohelioscope was originally installed in a 12-foot-deep pit under the south side of the rotunda, and used a simple 8-inch (20-cm) objective of 18-feet (5.5-m) focal length in the middle story of the building. Using whirling prisms the device depended on the persistence of vision to give the illusion of a continuous image. An improved spectrohelioscope using optical interference was installed in the 1950s, and in turn the filter and telescope was replaced in the 1980s by a compact and efficient commercial Daystar interference filter and a 40-mm refractor lens, all contained within the exhibit on the museum floor.
As part of the Observatory’s Renovation and Expansion, two small telescopes and television cameras were added to the southern beam of the coelostat. These provide live televised images to a wheelchair accessible station in the Ahmanson Hall of the Sky.