120-inch Shane Reflector

Shane Overview & History; Diagram of 3 Foci

: Prime Focus; Prime Focus Instruments; Cassegrain Focus; Cassegrain Focus Instruments; Coudé Focus; Coudé Focus Instruments; Coudé Auxiliary Telescope; Laser Beams from the Shane Dome

Shane Reflector

Completed in 1959 and named for Donald Shane, Lick Observatory Director during its design and construction, the Shane reflector brought Lick Observatory to the forefront of modern astronomy. Constructed around a 120-inch primary mirror, light may be focused at three different locations (foci) along the length of the telescope: prime focus, Cassegrain focus, and coudé focus. Designed for maximum versatility, the three foci accommodate a variety of instruments, used in many different types of research. UCO astronomers often refer to this telescope as the 120-inch or the 3-meter, based upon the diameter of its primary mirror.

The heart of the Shane telescope, the 120-inch primary mirror, was originally a glass test blank cast in Corning Labs for the Palomar Observatory 200-inch reflector. Pyrex, the well known "cooking glass," was in fact invented for these telescope mirrors. CalTech generously sold the 120-inch blank to Lick Observatory at a nominal cost of $50,000. Cast 17 inches deep and weighing 10,000 lbs, the glass was arduously transported to Mt. Hamilton using two flatbed trucks and a crane. On each hairpin turn where it was suspected the glass might shift and break on the flatbed, the glass was lifted by crane from one truck, located below the turn, to the other, located above the turn.

Once on Mt. Hamilton, Lick technicians precisely ground and polished the mirror into a shallow and uniform dish. It is estimated that 95% of the shaping, in terms of glass removed, took place during the first 80 hours, and the remaining 5% during the next 4 years. In the end, 600 lbs of glass had been removed, resulting in a 4.5 ton piece of glass with a shallow "dish" shape measuring ten feet in diameter but only 1.5 inches deep in the center.

To become a mirror, the glass then needed to be aluminized, or coated with a reflective surface of aluminum. First the mirror was cleaned thoroughly. Then aluminization was accomplished in a vacuum chamber in the basement of the Shane dome, by allowing a very thin coating of vaporized aluminum to settle on the concave surface. The depth of the coating is only about as thick as the moisture left by breath on a cold window; indeed only about .5 ounces (the weight of a soda pop can) is deposited over the entire 3-meter mirror surface! When reinstalled on the telescope, the mirror is protected with a specialized mirror cover which is mechanically removed during observing times. Even with this protection, the mirror must be cleaned and re-aluminized about every five years to maintain optimum reflectivity to clearly observe celestial objects.

Click on diagram for enlarged view

Prime Focus

The prime focus is located at the top end of the telescope. Light enters the top of the telescope, bounces off the primary mirror at the bottom, and back up to the prime focus. Originally, astronomers observed directly through an eyepiece at this focus, spending hours in an unheated observers' cage some 60 feet above the observatory floor. Since heat near the telescope would distort the view of the sky, like heat waves creating a mirage in the desert, observers were forced to endure cold conditions to accomplish their research.

Nowadays observations are made remotely, using computers in a warm control room adjacent to the telescope, and the observers' cage has been retired. Note that you can see the observers cage at the top of the telescope in the 1960 and 1980 Shane dome photos, but not in 2004 photo.

View a Quick Time panorama of the Shane control room during an observing run, courtesy of observer Ranier Kohler.

Essentially sitting on top of the telescope, prime focus instruments must be quite small to avoid blocking light from entering the telescope. Notice in the 3 foci illustration above that the light path from the observed object to prime focus is shorter than the light path to the other foci, which means that light is concentrated most intensely at prime focus. Notice also that light at prime focus has been reflected from only one mirror. Because each mirror reduces the amount of light gathered by 10-15%, prime focus instruments also receive the most light. For these reasons, prime focus instruments are typically used to view the faintest objects. Instruments at prime focus also view the widest area of the sky. Thus they are used to observe very faint objects over a wide field of view, such as a cluster of galaxies at the end of the universe. Prime focus instruments include the Prime Focus Camera and the Multi-Object Spectrograph.

Prime Focus Instruments

Prime Focus Camera

The Prime Focus Camera (PFCam) uses CCDs to record light at the Shane telescope’s prime focus. Like most astronomical CCD cameras, PFCam records visible light plus some ultraviolet and infrared radiation—the wavelengths at which most CCDs have maximum sensitivity. PFCam is primarily used to record faint objects, such as distant galaxies, over a wide field of view. See the telescope index webpage for a discussion of astronomical cameras and the differences between astronomical and home camera CCDs.

Multi-Object Spectrograph

Located at the Shane prime focus, the Automatic Multi-Object Spectrograph (AMOS, or MOS), can record the spectra of several different celestial objects at the same time, using fiber optic feeds. The instrument consists of a robotic arm that positions the fiber optics, the fiber optics themselves, and a spectrograph. Astronomers position the fiber optics with the robotic arm using a computer, so that each fiber receives light from a single object. Light travels down each fiber and is recorded by the spectrograph, located on a mezzanine floor below the telescope. The spectrum from each optical fiber is recorded on a narrow band of the CCD, so that spectra of several objects can be seen when the CCD is read out by computer following the observation. See the telescope index webpage for a discussion of spectrographs and spectroscopy.

Cassegrain Focus

The Cassegrain focus is located at the lower end of the telescope. Near the prime focus, light is reflected off a secondary mirror and back down to focus just below the primary mirror. Since the light path is longer than the light path to prime focus (longer focal length), the light received here is less concentrated. Also, the secondary mirror gobbles up another 10-15% of the light, so Cassegrain focus instruments are often used to observe moderately bright objects.

Cassegrain instruments are suspended from the bottom of the telescope and can be relatively large, for two reasons. Since they do not block the light path, a bulky Cassegrain instrument will not compromise the telescope's light-collecting ability. Also, their location close to the dome floor makes Cassegrain instruments more convenient to install and service than prime focus instruments. Many Cassegrain instruments have been constructed during the Shane telescope's lifetime. Currently used Cassegrain focus instruments include the Kast Spectrograph and the Gemini Infrared Imaging Camera.

Cassegrain Instruments

Kast Double-Beam Spectrograph

Used at the Cassegrain focus of the Shane telescope, the Kast spectrograph is actually two complete spectrographs, optimized for different areas of the spectrum. Most spectrographs are sensitive only in one area, but the Kast uses two separate CCDs to analyze and record wavelengths in the blue and red ranges independently and simultaneously. The red and blue spectra are then combined to form a spectrum complete in all visible wavelengths. The Kast also can also produce images of celestial objects, and is often used for analyzing relatively faint objects.

The Kast Spectrograph has been used for many years to research chemical compositions of stars. Other research for which it is employed includes study of massive intergalactic hydrogen clouds and analysis of distant radio galaxies.

Gemini Infrared Imaging Camera

Built at the UCLA Infrared Imaging Detector Lab, this Infrared (IR) camera is used frequently at the Cassegrain focus of the Shane telescope. An innovative beam-splitter increases observing efficiency by allowing astronomers to detect and record two wavelengths at once, rather than making two separate exposures. Gemini is also capable of low-resolution spectroscopy.

Gemini is currently used to observe stars that are not luminous enough in visible light to be imaged well with conventional visible light sensitive cameras, such as brown dwarfs and other low mass stars. It has a wide range of potential use in future research.

Coudé Focus

Sometimes astronomers wish to build instruments too large and heavy to be used at Cassegrain focus. These are typically high-dispersion spectrographs, which spread the light received into its component colors and require a large physical space. Coudé focus instruments are not attached to the telescope at all, and not even located in the same room as the telescope. Because they are located below the Shane dome floor, they can be nearly unlimited in size and weight. The Hamilton Echelle Spectrograph now occupies two entire rooms in the dome basement!

At coudé focus, light from the observed object is focused below the Shane dome floor. A different secondary mirror than the one used at Cassegrain focus is required, along with a third mirror. The Shane 3 foci illustration above shows that the light path is much longer than the light path to the prime focus, so considerably less light can be concentrated at coudé focus. The additional mirrors needed to bring light to coudé focus also reduce the amount of light received here. Because of these factors, coudé focus instruments are used primarily to observe relatively bright objects. Various coudé spectrographs have been used throughout the years. The Hamilton Echelle Spectrograph is now used at coudé focus.

Coudé Instruments

Hamilton Echelle Spectrograph

Unlike most spectrographs, which spread light into a single band of color components, the Hamilton is a high-dispersion instrument, spreading light into an echelle spectrum, or ladder-like arrangement of about 80 bands. Each band represents a precise picture of a small wavelength (color) range. Together the bands create a comprehensive yet highly detailed representation of the observed object.

Occupying two rooms below the Shane dome floor, and used by both the Shane telescope (at coudé focus) and the CAT, the Hamilton Echelle Spectrograph is used mostly to study bright objects.

The Hamilton Echelle Spectrograph is currently used in detecting extrasolar planets, determining chemical composition of stars, and other research. In the photograph above, an astronomer checks the iodine cell before an extrasolar planet observation. As starlight passes through this glass cell, fine iodine lines are imprinted on the stellar spectrum. The stellar lines will shift if the star has a planet, and the iodine lines serve as a stationary grid with which to measure these shifts.

The Coudé Auxiliary Telescope

The Coudé Auxiliary Telescope (CAT) was built in 1969 to maximize use of the Hamilton Echelle Spectrograph. The CAT may be used to observe with the Hamilton Echelle spectrograph any time that Shane telescope observers are using Cassegrain or prime focus instruments. The 24-inch CAT reflecting telescope is in a stationary position at a fixed focus in a metal building adjacent to the Shane dome. (Notice this building in the Shane dome photos from 2004 or 1980, and that it did not exist in 1960.) A flat mirror directs light from the observed object to the CAT, which in turn beams the light to the Hamilton Echelle spectrograph.

Since the 24-inch CAT gathers much less light than the 120-inch Shane, the CAT is typically used to observe brighter stars. The CAT is often used in projects which require many nights of observation or large samples of stars, leaving the more popular Shane telescope available for research which requires less frequent observing time.

Laser Beams from the Shane Dome

The 2004 Shane dome photo above shows a laser beam being shot from the dome. This is part of the Lick Observatory adaptive optics system. Lick was the first observatory in the world to engineer this laser guide star (LGS) system, which is now used in observatories throughout the world.

Adaptive optics (AO) systems essentially focus telescopes images precisely by compensating for the earth's unstable atmosphere which makes stars "twinkle." As you might imagine, the "twinkle" will cause the image of an observed object to be blurry. AO eliminates this by bending the telescope mirror many times per second to focus the observed image and compensate for atmospheric instability.

The adaptive optics system must detect and quantify the distortion caused by the atmosphere in order to compensate for it. Sometimes a bright star is located in the sky nearby the observed object. This star can be used as a "guide star" to quantify the corrections in focus needed.

More often, there is no bright star nearby, so Lick astronomers actually create one. The laser from the Shane instrument excites sodium ions in the upper atmosphere until they glow, creating a laser guide "star." This "star" is then used to detect and quantify the adjustments needed to observe the nearby object with optimum clarity.

Please see the Adaptive Optics webpage for more details about adaptive optics engineering and observation at Lick Observatory.

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