Palomar Observatory



One of the benefits of being an astronomy student at Caltech in the 198os was access to Palomar Observatory, which boasted the largest telescope in the Western world. The 200-inch telescope, so named for the diameter of its main mirror, is informally known as the Big Eye and has produced a steady stream of astronomical discoveries since its completion in 1948. Palomar Observatory also boasts a a 60" telescope for less demanding observations, and a 48" Schmidt telescope, used for wide-angle astrophotography, and some smaller telescopes.

I used the 60" telescope for my dissertation, since it had a superb echelle spectrograph, perfect for my work on hot stellar atmospheres; and it was not in as much demand as the 200", so it was easier to schedule. Construction of the 60" was funded by the Oscar Meyer Foundation, and the more irreverent graduate students called it the Pig Eye. It was a friendly telescope, located on a knoll at the south end of the observatory complex, with a well-stocked kitchen and comfortable control room. Like the control room of any modern telescope, this was insulated to prevent the heat from the astronomers' bodies disturbing the air around the telescope and ruining the "seeing." The kitchen had the usual assortment of frozen dinners for midnight snacks. There were only two things wrong with the 60", really: The area was infested with field mice (my war with the vermin goes back a long ways), and a prominent Protestant denomination had built a large convention center just a few miles down the road that they insisted had to be brightly lit at night. This produced significant sky glow at the 60".

Now that I think about it, there was a third thing wrong with the 60": The night assistant. No observatory director in his right mind allows the astronomers to operate the telescope themselves. Most astronomers are at an observatory for only a few nights out of the year and don't know the equipment; besides, an astronomer with a big shiny telescope is like a 10-year-old with a loaded pistol. Adult supervision is required. This is supplied in the form of a night assistant who is in charge of "moving" the telescope onto its target and performing all other major telescope operations. The astronomer gets a paddle with four buttons that can only be used to make fine adjustments to the pointing. This is called "guiding" the telescope, and it is usually a little like playing the world's slowest and most boring video game for hours at a stretch.

The 200" had its own night assistant, who was superb. The 48" also had its own night assistant, who doubled as a darkroom technician. (The 48" was the only one of the three telescopes that still used photographic plates.) Two other night assistants shared duty on the 60" and gave the 200" and 48" assistants an occasional night off. One of these was a fairly competent and agreeable woman that I did not mind working with. The other was an ex-motorcycle hippie and short order cook who knew his job but visibly resented taking orders from overeducated young gentlemen like myself. Working with him was an ordeal, particularly since I am not the kind who is comfortable giving orders in the first place.

Palomar Observatory sits in a pocket of wilderness in the middle of Southern California, on a site that was carefully selected for its steady atmosphere. The surrounding cities have mostly adopted dark-sky ordinances that have kept the sky glow down, and on a good night the coastal cities are fogged in anyway. It can be very dark at Palomar on a moonless night. There is also significant wildlife in the area: Besides the vermin, it is routine to encounter herds of deer, and on one lucky occasion another student and I spotted a pair of cougars stalking the deer.

Like many older observatories, Palomar had the atmosphere of a university club. The sleeping quarters at Palomar, "The Monastery", was equipped with surplus World War II blackout curtains so the astronomers could sleep during the day. There was a small library with a collection of Scientific American going back to the days when it was still a great magazine. Dinner was served in a cozy little dining room by a cook that reminded me of my mother, only with more attitude. (My mother was also a better cook.)

The engineering staff at the observatory were excellent. They also tried to be as helpful as possible. On one trip to the observatory, my Chevette snapped a timing belt halfway up the mountain. I got a tow in to the observatory from a local driving a pickup and parked my stone-cold-dead vehicle at the main garage. About halfway through dinner, one of the staff wandered in and asked for the owner of the busted car. He told me that it was against the rules for the engineering staff to work on the car for me, but I was welcome to use the garage facilities myself; the problem was obviously the timing belt, and he had taken the liberty of ordering a new one for me that should be in the next day. It took me twelve hours' work (I am no polished mechanic) but I got the car running again.

All of this is secondary to my experiences with the Big Eye.

Though I did my dissertation on the 60", I had the opportunity to assist some more senior astronomers on the 200", including Ann Merchant Boesgaard, my advisor. Ann was studying a peculiar deficiency of the light element lithium in stars somewhat brighter than the sun. (We also took a stab at studying beryllium, but couldn't get enough ultraviolet light through the 200" Coude optics.) My duties were to take shifts with the control pad, keeping the telescope pointed at the right star while the spectrograph accumulated data, and to fill the spectrograph with liquid nitrogen when necessary.

Almost all astronomical instruments use CCDs in place of photographic film, like a digital camera. But back then this was still cutting-edge technology. A really good CCD can count individual photons with an efficiency of better than 50% at long wavelengths. To eliminate thermal noise, the CCD is bathed in liquid nitrogen, and part of the morning ritual before turning in for the day was to refill the dewar. This usually generated large clouds of condensation, particularly if you spilled any liquid nitrogen on the floor.

This was an opportunity for me to be a smart aleck. Because the Big Eye was twice as large as any other telescope in the world when it was originally completed, the 200" has a large visitors' gallery. Visitors enter the telescope dome through a special entrance with lots of faux marble, busts of famous astronomers, and nuggets of wisdom carved in the walls. (The astronomers use the service entrance.) The visitors' gallery itself is completely enclosed in glass, and gives a most impressive view of the telescope, which weights 40 tons and sits in a 490-ton yoke. The average visitor spends at least a few moments staring up with jaw agape at the huge mass of telescope overhead. It is a most impressive sight.

One winter night, we worked until seven in the morning before the rising sun forced us to quit for the day. It then took considerable time to tidy up our records and secure the telescope before I went out to refill the dewar on the spectrograph. By then, there was already a handful of visitors in the gallery gaping at the monster telescope. On impulse, I shuffled across the observatory floor in my best imitation of a hunchback, and I made sure I spilled plenty of liquid nitrogen while refilling the dewar. I then shuffled back to the control room and my highly irritated advisor. By then the visitors' stares were directed at me rather than the telescope, as intended.

Of course, ordinary visitors don't get to see all the neat stuff that the astronomers get to see. The 200" is a masterpiece of early 20th-century engineering. The telescope and yoke combined weigh 530 tons, but they are so perfectly balanced that you can move the telescope with a push from your hand. (Or so I'm told -- no one ever let me try.) The observatory dome weighs 1000 tons but is moved using a quarter-horsepower motor. (It moves rather ponderously.) The back of the mirror has an array of mechanical analog computers that apply small forces to keep the mirror from sagging under the force of gravity. The controls for the telescope go through a bank of electromechanical relays that is great fun to watch -- the relays snap and spark as the controls are operated. The telescope can be aimed with a precision of a few arc seconds, which is equivalent to a precision of about a sixteenth of an inch out of 100 feet.

We couldn't build another Big Eye today. Of course, we wouldn't try. Modern telescopes use advanced borosilicate glasses that are superior to the Pyrex used in the 200" mirror, and the mirrors are spun rather than cast. This makes for a much lighter and cheaper mirror. Any sagging in a modern telescope mirror is compensated using digital actuators rather than handcrafted mechanical analog computers. The overall structure is lighter and easier to move and control. But the pointing accuracy and quality of optics are not significantly better than in 1948.

Here is a nice collection of photographs of the Big Eye.

Another thing I liked about being an astronomy graduate student was climbing around inside the dome. There are ladders and catwalks everywhere. The catwalk that runs around the outside of the dome is a favorite place to take a break and look at the weather. It's about the equivalent of five stories off the ground, if I remember right. There is an urban legend that a prankster (probably a graduate student) painted part of the catwalk midnight black, so that in the dark it looked like a section of catwalk was missing, and scared the pants off one of the most distinguished astronomers in the country. There are a number of urban legends floating around most observatories. I have an even better one involving a French astronomer, the 200" mirror, and a slice of pizza, but I'll save it for another time.

I am proud to make a boast that few living astronomers can: I have guided the 200" telescope using my actual eyes for the better part of a night. Usually the telescope is guided using a television camera, so the astronomer is looking at an image on a TV screen in the control room. But Ann was using the Coude spectrograph -- very unusual by the late 1980s -- and no one had ever installed a guide camera on it. So I have seen the Hyades cluster through the 200" telescope, an impressive sight indeed.

(Part III is here.)