The LA area: Mt. Wilson and Palomar


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Mt Wilson Observatory

Mt Wilson Observatory was founded by George Ellery Hale in the early years of this century, initially as an out-station of the University of Chicago's Yerkes Observatory. Hale had his eyes set on developing a solar observatory at a more favourable site than William's Bay, and with the acquisition of 60-inch mirror blanks, those plans expanded to include what would be the largest reflecting telescope constructed to date. Southern California was the preferred location, and once Hale acquired funding from the Carnegie Institution, founded by Andrew Carnegie in 1902, Mt. Wilson, in the San Gabriel mountains above Altadena and Pasadena, was chosen as the site. Hale broke away from Chicago in 1904, and the Carnegie Observatories, located in Santa Barbera street, Pasadena, were founded with the express aim of running this new observatory. The first observations from Mt Wilson, using the Snow solar telescope, shipped from Yerkes, were made in 1905, and work on the 60-inch started in the same year, led by George Ritchey. (See Don Osterbrock's "Pauper and Prince" for a fine account of Hale and Ritchey's interactions and the development of Mt. Wilson Observatory.)

Hale and Carnegie next to the 60 inch (Caltech archives)

The 60-inch telescope was completed in late 1908, the same year as the 60-foot Solar Tower went into operation, and quickly proved its worth. Hale didn't stand still, however - two years previously, he had received an offer from John D. Hooker of funding for the purchase of a 100-inch mirror blank, and over the next 10 years, his efforts were devoted to making the 100-inch Hooker telescope a reality. The telescope finally went into operation in 1919, and served a key role in both the extragalactic studies of Hubble and Humason, and in Walter Baade's creation of the concept of stellar populations. Carnegie Observatories ran operations at Mount Wilson until the early 1980s. During the 80 years of its existence, the Los Angeles conglomeration had increased vastly in size, with a corresponding increase in light pollution. OCIW decided to concentrate on developing its southern out-station at Las Campanas, handing Mt. Wilson the same fate that Hale handed Yerkes. Observations have not ceased, however: the optical Observatory is currently run by the Mount Wilson Observatory Association , and is the focus of considerable work in adaptive optics, including the optical interferometric array, established very recently by Georgia Tech. Follow this link for some fine recent pictures of Mt. Wilson.

The Mt. Wilson shuttle in the early 1900s (Caltech archives): the un-named driver is on the far left; the passengers are Hale, George Ritchey, the main architect of the 60-inch and 100-inch telescopes, and John D. Hooker, the funding source for the 100-inch Hooker reflector.


Palomar Observatory

Following the completion of the 100-inch on Mt. Wilson, Hale's attention moved to the possibility of an even larger telescope - but at a different site, remote from the lights of LA which, even then, were starting to threaten Mt. Wilson. Skating briefly over a long, complex story (which is well covered in Ronald Florence's excellent book, "The Perfect Machine"), the site chosen was Mt. Palomar, about 120 miles south of Pasadena in the mountains north of San Diego, and one of the sites considered originally in 1903; and the telescope was the 200-inch reflector. The funds for the construction of the telescope were provided by the Rockefeller Foundation. Hale had originally envisaged the new telescope being run from Santa Barbera Street, but Rockefeller balked at the notion of funding Carnegie Observatories, which is why Caltech has an astronomy department.
The first telescope to be built on site at Palomar was the 18-inch Schmidt, a wide-field camera, based on a design by Bernhard Schmidt. Fritz Zwicky is the astronomer most associated with this instrument, and he used it extensively for supernova searches and galaxy catalogues. The credit for its construction, however, rests with Walter Baade (my vote for greatest astronomer of the 20th century), who was a colleague of Schmidt's at Hamburg prior to his (Baade's) move to the US, and, with Schmidt, tested the prototype. The 18-inch camera went into operation in 1936, and its success paved the way for the inclusion of the 48-inch Schmidt as a complement to the 200-inch.

The dome of the 18-inch Schmidt

The 18-inch Schmidt at Palomar
both pictures by R. Danner & D, Hogg

A Porter sketch of the 200-inch: Caltech archives

Before any of the construction work was started on the 200-inch telescope, and the 48-inch Schmidt, the engineers and scientists had a good idea of what they were going to build. That was thanks to Russell Porter, an artist, designer and amateur-telescope masker, who had joined the 200-inch project in 1928. His amazing ability to visualise the final instrument from a set of blueprints is amply illustrated by his numerous sketches of both telesopes, all made well before cutting any metal.

Constructing the 200-inch: Caltech archives

Construction work at the Palomar site began in 1934, but work on the dome for the 200-inch could not get underway until 1936. The exterior, and most of the interior, was complete by 1938. The telescope, made by Westinghouse, was completed in the same year and shipped, via Pasadena, to Palomar. The mirror, made by Corning Glass Works, was already in Pasadena, being polished in the optics lab at Caltech. The projected finishing date was 1942 - but WW II intervened, and it wasn't until 1948 that the mirror polishing was completed, and the telescope dedicated as the Hale 200-inch on June 3, 1948. Engineering tests, mainly centringon the mirror support system, continued for a year before the system was finally handed over for observational use in late 1949.

Palomar mountain in the 50s: Caltech archives
The administration building is at 8 o'clock; the monatery lies at upper end of the main track; the 48-inch domes is just outside the field of view at about 9 o'clock; and the ridge where the 60-inch will be constructed is at about 11 o'clock from the P200 dome

The completed 200-inch dome and telescope (Caltech archives)

A fisheye view of the interior of the 200-inch dome (Alain Maury)

The 200-inch at work, with the Milky Way in the background (Hogg/Danner)

The 48-inch and Palomar Sky surveys

The dome of the 48-inch Oschin Schmidt

At the same time as work was completed on the Hale 200-inch, a smaller, but equally important, telescope went into operation on the other side of the mountaint top. The successor to Baade's 18-inch Schmidt, the 48-inch Schmidt was designed to provide moderately-deep, wide-field photographic images to complement the deep, but limited angular coverage of the larger telescope.

Apart from anything else, the 48-inch would provide targets for the 200-inch to observe, but in the late 40s the notion of a more ambitious undertaking, a sky survey, surfaced. Caltech approached National Geographic, and the Palomar Sky Survey was born. The first plates were taken in late 1949, and the initial schedule was for a 3-year project; things didn't quite work that way, of course, and the final plates weren't taken until 1958. Those plates not only provided the first serious glimpse of large-scale structure (George Abell, a Caltech grad student, was one of the prime movers in the completion of the survey), but also served as a rich database for identifying radio sources (Minkowski graded every POSSI plate) and, later, infrared and X-ray sources,
Over the last decade and a half, the same telescope, refurbished with a new corrector plate and improved guiders, and using improved emulsions, has been taking plates for the second Palomar Sky Survey (POSS II). POSS I was a two-colour survey, blue and red plates taken successively on the same night; POSS II is three-colour, blue, red and near-infared, with the plates taken on separate nights. Like POSS I, the initial estimates for POSS II were for a three year project; like POSS I, those estimates were optimistic. Nonetheless, the blue and red passbands are now complete (and scanned - see STSCI's DSS ), and the over 85% of the near-infrared plates have been take. See this link for more details.

The Oschin Schmidt - Alain Maury

The gun barrel-like Schmidt: the plates are loaded into (weighty) plate holders in the small room visible behind the main mirror of the telesope; the plateholders are raised into position, midway within the telescope tube, with the telescope horizontal, at the parked position; then point and fire.

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