A tour of the Hale Telescope

My parents recently moved to San Diego, and I flew out for Mothers’ Day weekend. On Saturday, I suggested that we all go up Mt. Palomar, in the very northern part of San Diego County, to take the public tour of Palomar Observatory, home of the 200-inch Hale Telescope, once the world’s largest optical telescope. The photo above shows the view of the telescope dome from the observatory’s visitor center. A decade ago, my mother was living in Carlsbad and we tried to do this during the winter holidays, but had to turn back without seeing anything. This time it was a nice warm sunny May day, and we knew that the Observatory grounds would be open and that tours were being held twice daily, so we drove up, stopping along the way to buy lunch. (By the way, I must apologize for any errors in the presentation here, which was mostly written several days after I got back from the trip and had a chance to forget some details.)

We got to the visitor center just in time to buy the last tickets available for the second tour of the day. The tickets are only $5, which is pretty reasonable, but tours are limited to about 30 guests. (Non-ticketed visitors are welcome to picnic on the grounds, see the visitor center, and enter the telescope’s visitor gallery, but the gates close at 4 PM to ensure that all the tourists are off the property and out of the way of the working astronomers.)

Here’s what we saw, in words and images:

The Hale Telescope was constructed in 1939, but the construction and delivery of the primary mirror was delayed until 1948 by World War II. This cast-concrete dummy was made to have the same size and weight as the actual mirror, and was used to during the initial setup of the telescope mechanics.

My mother is standing in front of the Hale Telescope building for scale. My parents always complain that I never take pictures of people — not true! (As I scroll this photo up and down in the browser window, I’m suddenly going “whoa! perspective distortion!” Not sure how I didn’t see that before.)

A line of shrubs surrounds the Hale Telescope building along most of its circumference. I don’t know what they are, botanically speaking anyway, but they have flowers that look like this.

Here’s a broader view of one of the shrubs (or maybe it’s a dwarf tree?).

The guided tours of the telescope enter the building at the basement level, from the staff parking lot, through the open door seen here. All of the components of the telescope were delivered through the large rolling door surrounding the staff entrance.

Off to the right just inside the basement door is a vacuum-deposition chamber used for resilvering the smaller mirrors at Palomar (the 200-inch mirror obviously wouldn’t fit). The white room behind it houses part of the adaptive optics system, and behind that are two spare gears (never needed) for the telescope’s mechanism.

Our tour group of about 30 people stands on the chilly (40°F) observing floor looking at the telescope mount (which is in its daytime “rest” position, pointed straight up. to minimize mechanical stress on the mirror). The inside of the telescope building is kept at nighttime temperature all day long, so that observing time is not wasted waiting for the mirror’s shape to stabilize in response to temperature changes — a problem with the Hale’s predecessor, the 100-inch telescope at Mount Wilson.

There are two locations for instruments on the Hale: one at the prime focus, and one behind the main mirror, shown here. An optical system at the prime focus reflects some of the incoming light back down towards the center of the mirror, which is hollow, where it can be processed by another instrument. Our docent said that the current, and most common, configuration was to have imaging equipment at the prime focus and a two-prism spectrograph at this location.

The Hale’s main mirror was made by Corning Glass Works out of borosilicate glass (Pyrex). The voids you see in the model are to reduce the weight of the mirror; the first time Corning tried to cast the mirror, the refractory brick used to form the voids came loose and the mirror had to be scrapped — an embarrassing failure for Corning’s PR department, which had arranged for press coverage of the event.
200 inches is 5080 mm; the practical limit for a one-piece glass mirror like this is about 8.6 meters (achieved by the optical lab at the University of Arizona). All larger telescopes (like the Hale’s younger sister, the Keck, in Hawaii) use multiple mirrors which are automatically computer-aligned in real time.

The telescope mount rides on fluid bearings.

Our tour went up a flight of stairs to the second floor of the telescope building, where there is a gallery with a good view of the telescope and the observing floor. We’re looking straight out over the railing at the actual main mirror of the telescope; the red tube runs down through the hollow center of the mirror to the secondary instrumentation hanging below.

I took two pictures of the telescope in sequence; the photo on the left is closer to the true lighting condition inside the building, whereas the one on the right has been digitally lightened to bring out more detail. (I could have done it with two copies of the same picture, but since I already had two exposures from this exact spot, it was simpler to do this.)

Two women (members of our tour group) stand at one end of the telescope mount, showing the telescope’s massive scale.

Up at the top of the telescope is the prime focus. In days of yore, the observing astronomer (or an assistant) would have to stand up here for an entire observing run, changing photographic plates. In modern times, the main sensor is a CCD (the same sort of device used in video and digital still cameras), so access is required much less frequently. There’s an elevator at the north end of the observing floor, so those astronomers were not usually required to carry their heavy stacks of plates up this ladder.

The white metal plates cover the interface between the telescope platform, which is fixed, and the dome, which rotates.

Back in the 1930s, big industrial concerns like Westinghouse would actually design and build one-off machines like gigantic telescope mounts. The Hale was to be the world’s largest optical telescope; there was a lot of prestige for a company like Westinghouse in building the mount, and the company had a great deal of experience in building very large electromechanical systems like generating stations, so a huge telescope mount was definitely in their wheelhouse at the time. (Westinghouse Electric & Manufacturing is now known as CBS Corporation.)

The telescope dome rides on sets of railroad-car-type wheels, which sit on a circular track. A window in the visitor gallery provides a view.

Inside the main entrace to the Hale Telescope is a bust of George Ellery Hale, who died in 1938 while the telescope was still under construction.

The Rockefeller Foundation provided most of the funding to build the Hale Telescope.

The main entrance to the Hale Telescope is now little used except by tourists visiting Mt. Palomar.

We took a detour on the way back down to San Diego so I could stop by the Pala Indian Reservation and get a few pictures of their radio station:

KOPA is the Pala tribal radio station. It’s located next to the recreation center and tribal government offices just off Pala Mission Road.

I’ve seen this type of antenna before, but I don’t know what it’s actually called.

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