San Diego History · Palomar Mountain
San Diego County built a road around the turning radius of one piece of glass.
Poured at Corning in 1934, on the mountain in 1947. All records sourced and cited.
Poured at Corning in 1934, on the mountain in 1947. All records sourced and cited.
The road up the south side of Palomar Mountain is county route S6. People drive it every weekend without knowing that its curves are the shape they are because of the dimensions of a mirror.
The San Diego History Center states it plainly. "The road up Palomar Mountain was designed and engineered so the crated mirrors, and earlier the sections of the telescope mounting, could make the turns."
Not widened for them. Engineered for them. The county pledged to build the road when Caltech chose the mountain, and the thing that set the radius of the turns was a twenty ton disk of Pyrex that had to get to the top in one piece.
Three things before the story runs. That disk was poured in New York in 1934 and did not reach the mountain until 1947. The steel for the dome went up a different road, the old grade on the back of the mountain named for Nathan Harrison, who was born into slavery in Kentucky and homesteaded there. And on the day the mirror came up, a man rode on top of the crate shouting at men walking ahead of the truck, because nobody could see the edge of the road.
Caltech bought a hundred and twenty acres on Palomar Mountain on September 21, 1934, for twelve thousand dollars. The money for the telescope had come from the Rockefeller Foundation in 1928. The reason for the mountain was dark sky, because the lights of Los Angeles were already reaching Mount Wilson.
Corning Glass Works poured the disk in March 1934 and it failed. The firebricks holding the bottom of the mold melted and floated up through the glass.
They poured again on Sunday, December 2, 1934, and that one held. Then it had to cool slowly enough not to crack, which took ten months, and in July 1935 the Chemung River came up and the power went out and the disk cooled on its own for three days. It survived that too.
On March 26, 1936 the disk left Corning on a railroad car built for it. It arrived in the rail yard at East Pasadena on April 10.
Then it sat in the Caltech optical shop while men ground it into a parabola, which means they took several tons of glass off it a fraction at a time. The job was expected to run six or seven years.
The war stopped it. Work halted in the spring of 1942 and did not start again until December 17, 1945. In October 1947 the mirror was declared finished.
Thirteen years from the pour.
While the glass was being ground, the mountain had to be made reachable.
The county road, S6 up the south grade, was known at the time as the Highway to the Stars, and it was cut to carry a load that could not be tilted, flexed or taken apart. The observatory's own doorway was built to the same requirement.
It was not finished in time for everything. When Consolidated Steel of Los Angeles hauled the dome's steel frame up, the new road was still unbuilt, so the trucks went up the old Nate Harrison grade on the back of the mountain instead. One tractor trailer tipped over backward on the pitch. The observatory crew pulled it and its load upright with a chain and an old Caterpillar.
Nathan Harrison was born in Kentucky around 1833 and came west with the man who held him during the Gold Rush. He mined in the north, then homesteaded, taking a patent on a hundred and twelve acres near the base of Palomar in 1879, selling it for three hundred dollars, and moving up the mountain in 1882 to claim forty five and a half acres on the west side. He filed on a spring in 1892 and gave the water away to anyone who came up. He died on October 10, 1920 and is buried at Mount Hope.
The steel for the largest telescope in the world went up a road named after him.
The mirror left Caltech and went south on Highway 101 toward Oceanside.
At the Galivan Bridge, five miles north of San Juan Capistrano, the crew bolted on extra dollies with sixteen wheels each, so the forty ton crated load crossed on fifty eight wheels.
The mirror was set on sponge rubber over two I beams welded to the trailer bed. The cell was fastened at three points, two fixed and one that could move, so the flexing of the trailer would not reach the glass. A radio crystal detector was mounted on top of the disk and wired to an indicator in the cab of the lead tractor, and an assistant driver watched that needle the whole way.
The convoy stopped overnight in Escondido.
The next day it went up the Highway to the Stars, one tractor pulling and two more behind it pushing. It was cold and raining. Byron Hill, who ran construction on the mountain and later became the observatory's first superintendent, rode standing on top of the mirror crate and shouted orders down to men walking on foot, marking the edges of the road for the drivers, because the visibility was that bad.
The mirror was installed in the telescope at Christmas 1947.
Before it could see anything, the surface had to be coated with aluminum, and before that it had to be cleaner than almost anything had ever been.
John Strong, the physicist who worked out the process, covered the face of the mirror with Wildroot Cream Oil hair tonic and then burned it off with heating coils.
His explanation is the best sentence in the whole project. "In order to get glass clean, you first have to get it properly dirty."
On January 26, 1949, Edwin Hubble made the first exposure, on an object called Hubble's Variable Nebula.
Two roads get up Palomar Mountain and they exist for two different reasons. The south grade was engineered to a specification set by a scientific instrument. The back grade is a nineteenth century homestead road that still carries a homesteader's name.
That is the general condition of North County inland. Access out there was not planned around subdivisions, it was cut for mines, ranches, water and in this one case a telescope, and what got built afterward followed the roads rather than the other way around. If you are looking at land up there, the road is usually older than anything on the parcel and it explains more about the property than the parcel map does.
The nearest market coverage is Valley Center, and the wider area is in North County Inland.
The mirror's weight. Caltech gives the finished mirror as fourteen and a half tons and the blank as it arrived as twenty tons, and separately says four and a half tons of glass were removed. Those three figures do not reconcile to the ounce. This piece uses twenty tons for the disk as poured and fourteen and a half for the finished mirror, which are the two figures Caltech states directly, and says "several tons" for what came off.
A mirror, not a lens. The 1936 photograph is catalogued by the newspaper archive that holds it as a "200-inch, 20-ton lens." It is a mirror. Light does not pass through it. The archive's wording is kept in the credit line because that is the record's own title, and corrected here.
The road's build date. The sources establish that the county pledged the road, that it was engineered around the cargo, and that it was still unfinished when the dome steel went up. They do not give a start date, a completion date, a cost or a length, so none are given here.
The family name on the land. Caltech names the sellers of the 120 acres as the Beach family. The San Diego History Center's photograph captions refer to the Beech family having run cattle on the same ground. One of those is a misspelling and the sources do not agree, so this piece does not name them.
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