Stacey Falconer BB&C Member Visits this Magnificent Structure on Palomar Mountain
Hale to the Telescope
by Stacey Falconer
’m looking into a wonderfully clear, moonless, star filled sky, just down the road from the Palomar Mountain Observatory in Northern San Diego County. A few minutes ago I turned on the radio in the camper and George Noory’s guest on "Coast to Coast AM" was speculating about ‘real live aliens’ from across the unimaginable distances of interstellar space. Time for me to go outside and look at the real stars. Tomorrow I will fulfill a childhood dream and see the 200 inch Hale Telescope, which was the largest in the world for 25 years. But for now, I’ll just stare into this magnificent starry void. The Trapezium in Orion has rarely looked better. Sorry George, I’ve got other things on my mind tonight.
Meanwhile, up in the observatory, coordinates are entered into the computer controlled tracking system and the 530 ton telescope is slewed towards its target. Sometimes this target is as close as the moon, though more often it is several billion miles further out into the cold reaches of our outer solar system, or some unimaginably super-massive object thousands, if not billions of light years out into space. When the two horsepower motor is finished slewing the telescope, the dome rotates, the two massive 125 ton shutters slide open, and the astronomy begins.
Down in the old Coude focus room at the south base of the telescope, a special six watt sodium laser shoots up through a complex set of mirrors and out along the optical axis of the telescope. At about 100 kilometers up, the laser excites the sodium atoms in the rarefied atmosphere causing them to glow, creating an artificial guide star. At the bottom of this long invisible tube of turbulent, seething air this guide star is analyzed by a device called a wavefront sensor, and a special flexible six inch mirror at the bottom of the optical path is deformed by 241 piezeo electric actuators up to two thousand times a second, deforming and restoring the wavefront close to the theoretical limits of the telescope. The Hale is one of only three large telescopes in the world currently using this type of adaptive optics system, and in the near infrared, now routinely produces images of planetary bodies and deep sky objects with a resolution better than the Hubble Space Telescope. In the near future this will also include the shorter visible spectrum.
Let us now go forward a few hours in time to the floor of the Hale Telescope dome and the realization of my childhood dream. My guide is W. Scott Kardel, Public affairs Coordinator for the Palomar Observatory. Scott, like myself, started as a teenager building his own telescope, grinding the optics by hand. Our conversation was very informative; Scott’s history of the observatory and the changes in technology makes for a very interesting story.
Scott explained that the observatory has several telescopes, each with its specialized function and on the cutting edge of modern computerized technology.
The mountain bears witness to three completely automated telescopes. One is the Samuel Oschin 48 inch Schmidt telescope and its 112 CCD, 161 mega pixel QUEST camera. This telescope does routine sky surveys and was used to discover the solar system’s 10th planet. It photographs each four degree star field three times per night, and sends its data back to Cal Tech in the morning where a computer program sifts through the data looking for moving objects.
Then there is the 60-inch telescope in the Oscar Mayer Memorial Building that became operational in 1970, and is now also a fully robotic, remotely operated telescope. It was originally built to take some of the load off of the big Hale telescope, and is now used primarily for observing the locations of gamma-ray bursts and other transient phenomenon as soon as possible after their detection.
Probably the strangest instruments on Palomar Mountain are the three small telescopes that make up the 130 meter Palomar Testbed Interferometer. This instrument has an astonishing relative astrometric resolution better than 100 micro-arc seconds; a milli-arc second (1/1,000 arcsec) would be the angular size of Neil Armstrong standing on the moon, as seen from the earth. Although the telescopes are small, this technically is the largest instrument on the mountain. This instrument measures the wobble of stars caused by the effect of orbiting planets like our own planet Jupiter.
Also robotic, the small SLEUTH telescope is dedicated to searching for transiting planets around other stars. It is the smallest instrument on the mountain with an aperture size of only 10 cm. but photographs roughly 10,000 stars a night over a four degree field, every night for three months looking for stars that subtly change in brightness from eclipsing planets. It is totally automated through a Linux work station, and FTPs its data back to Cal Tech at the dawn of each morning.
The Hale Telescope is the best known telescope though, and for 25 years it was the largest telescope in the world, becoming fully operational in 1949. The Hale Telescope was named for George Ellery Hale, the astronomer primarily responsible for its planning and development. Hale never lived to see the telescope’s completion. The 200 inch mirror was cast in Pyrex glass, which has a very low co-efficient of thermal expansion, and weighed 20 tons. It took two attempts to successfully pour the giant disk, took eleven months to slowly cool it down, and 12 years to grind and polish the optical surface, removing some 5.5 tons of glass.
The Hale Telescope is mounted in an horse-shoe type equatorial mount, and both the lower south end and the the giant north end horse-shoe bearings use high pressure cushions of oil, making the mount virtually friction free. The 530 tons of the telescope only needs a 1/12th horsepower motor to track its astronomical target’s journey across the night sky—a child could probably even move the massive telescope.
Up to about twenty years ago, these big telescopes used photographic plates to capture the faint light of the planets and stars. As Scott explained, the astronomer would often ride in the prime focus cage high above the floor of the observatory for up to ten hours at a time to take an all night exposure, or sometimes even one exposure over several nights. You see, photosensitive film looses its sensitivity over long exposures, a process called photographic reciprocal failure. Scott recounts an astronomer who took an all night exposure and then discovered he hadn’t opened the camera shutter. He also recounted an astronomer who took a single exposure spanning several nights, and then dropped and broke the glass photographic plate!
About 20 years ago astronomers started replacing film with photo sensitive electronic sensors, and this revolutionized modern astronomy. There are now amateur backyard astronomers using CCD cameras and specialized software to create images of planetary objects that resolve amazing detail, often surpassing the Hale telescope’s photographic plates prior to 20 years ago. The Hale though, in addition to its new adaptive optics system, has a huge light gathering power exceeded by only a very small handful of telescopes in the world. This light gathering power combined with the adaptive optics camera requires a small fraction of time to take images that used to take up to several days. These new cameras use the same kind of CCD technology as the average digital camera, but the sensors are refrigerated with liquid nitrogen to eliminate the noise level and make the camera sensitive to even just a few single photons of light. Scott explains that sometimes small heaters are used to bring up the temperature if the camera gets too cold.
Each specialized instrument has its own computer package, and they are often changed several times a night. The guiding system is now also computerized, and the astronomer no longer has to sit for hours in the prime focus cage at the top of the telescope making fine tracking adjustments, having to wearing surplus WW II electrically heated bomber clothing to keep them warm.
Let us now go back to the starry sky the night before. After my eyes became used to the moonless, midnight sky, I realized that I could easily make out the silhouettes of the Bristle Cone Pines against the starry firmament. Scott Kardel explains to me that one of his major tasks is dealing with the light pollution from Los Angeles and San Diego. Since the telescope first opened its massive shutter doors, light pollution started to become a problem for the old giant. Our own Centre of the Universe here in Victoria is not immune to light pollution; we live in a sea of light, night and day. In the not too distant future we might have to relegate these old giant telescopes to being silent monolithic museum pieces, and the new telescopes will continue to be in far away places such as Hawaii and the Andes, or even in space like the planned James Webb Space Telescope. Cal Tech is now planning a 30 meter ground based optical telescope, with a massive multi-faceted mirror more than ninety feet across.
It is hard to predict what next change in technology will revolutionize astronomy. But for now, I’m cherishing the realization of a childhood dream
