The 3-meter Shane Reflector (and its CAT)

The Primary Mirror Optical Configurations Instruments The Control Room The CAT
By the end of World War II, after more than fifty years of operation, Lick Observatory still relied primarily on two 19th-century telescopes, the Great 36-inch Refractor and the 36-inch Crossely Reflector. These instruments had once been considered giants, but fifty years had seen them slip from dominance as the world's largest, to a modest position among the larger telescopes at other observatories that had sprung up in the first half of the 20th-century. Mount Wilson, 300 miles to the south, had become the major force in astrophysics with its 60- and 100-inch reflectors. The 200-inch reflector at Mount Palomar was nearing completion. Lick astronomers were finding it increasingly difficult to compete at the scientific forefront.

It was clear that a large reflecting telescope was needed on Mount Hamilton to reinvigorate observational astronomy at the University of California. The answer lay in the 3-meter (120-inch) Shane Reflector. Nearly fifteen years of planning, design, construction, and testing would be needed before the telescope went into operation in 1959, but on completion it was second in size only to Mount Palomar. The 3-meter Reflector catapulted Lick back onto the scientific frontline, where it remains today. Though now again surpassed in size by a number of others, the 3-meter remains an important telescope, due to its state-of-the-art instruments and the productivity of its user community.

The Shane 3-meter Reflector The Dome of the Shane 3-meter Reflector
In this early picture of the 3-meter, the telescope is configured for prime focus. The large cylinder at the top end is the "prime focus cage," at the center of which a small compartment with a small, well-worn chair carried the night's observer. The fixture has been replaced with a remotely operated prime focus top end. This recent picture, taken from the southwest under a full moon, shows the Lick Adaptive Optics laser, beaming skyward from inside the dome. The laser is part of sophisticated instrument which corrects the effects of atmospheric turbulance on observations. (photograph by Laurie Hatch)
The parts of the telescope and its equatorial mount can be seen in the picture at at left, above. The upward-facing primary mirror (hidden under the closed mirror cover in this picture) lies at the base of the yellow openwork "tube." The tube itself is held between the arms of a huge steel "fork," in turn connected to the polar axle--so called because it is aligned to the north celestial pole (the axle is hidden from view inside the traingular aluminum housing below the fork). Rotating the fork around the polar axle sweeps the telescope from east to west. North-south motion is accomplished by swinging the tube within the arms of the fork. Watching 145 tons of steel and glass in motion is an awesome sight, yet the entire structure is so smoothly borne and so finely balanced that once pointed at its target, the telescope's continuous tracking motion is driven by a 1/25th horsepower electric motor, small enough to hold in the palm of one's hand!

The Primary Mirror
At the heart of every reflecting telescope is the primary mirror. This component, more than any other, determines a telescope's characteristics; the primary's diameter often giving the telescope it's name, e.g. the Shane 3-meter. The Shane's primary is a 3-meter Pyrex disk with a honeycomb backside designed for lightness and rigidity. Its front is an ultra-precise, ultra-smooth, parabolic dish, accurate to within a millionth of an inch of a perfect geometric figure. The shaping of the 3-meter primary consumed three years of painstaking grinding and polishing.
Realuminizing the 3-meter Primary Mirror (See also the complete pictorial account of the 2000 aluminizing.)
At left, the mirror, removed from the telescope, is stripped and cleaned in preparation for the new coating. At center, the clean mirror is lowered toward the vacuum chamber in which it will be recoated. At right, the newly coated mirror emerges from the chamber with a pristine new surface.
To make polished glass into a highly reflective mirror, a thin film of aluminum is deposited on its front surface. Exposure to the elements dulls the aluminum coating, making it less reflective as time goes by. The old coating must be stripped and replaced every three to five years. The three pictures above were taken during the 2000 realuminizing of the 3-meter primary. (See also the complete pictorial account of the 2000 aluminizing.)

3-meter Configurations: Prime, Cass, & Coude
The 3-meter was built to satisfy the diverse research requirements of the rapidly growing community of University of California astronomers. It was given the necessary versatility to meet the needs of a variety of observations. Not only can different instruments be mounted on the telescope for different types of observations, but the telescope itself can be optically reconfigured in a few hours. It is really three telescopes in one. Light gathered by the primary mirror may be brought to a focus at three different locations: the efficient, wide-field prime focus, the long focal-length coudé for high precision spectroscopy, and the intermediate cassegrain focus. The choice of focus, like the choice of an instrument, depends on the requirements of each observing program.
The Three Optical Configurations of the 3-meter Reflector
The primary mirror is common to all three configurations. In each case, parallel beams of starlight bounce off the mirror and converge to a focus, forming an image. In the case of the prime focus, the primary is the only mirror, and the image is formed at the top of the telescope. The cassegrain focus indtroduces a second, much smaller mirror at the top end, bouncing the light back toward the primary where it passes through a small hole at the center of the primary before forming the image behind the telescope. Like the cassegrain, the coude uses a second mirror to reflect the light back down the telescope tube, but intercepts it with a third mirror which reflects the beam down the polar axle and into the basement. Each focus has unique properties that lend themselves to particular kinds of instruments and observations.
All three configurations begin with the 3-meter primary mirror. Starlight falls on the primary's gently curved surface--slightly deeper at its center than at the edges--and is reflected upward and inward in a converging cone, toward the top center of the telescope. Depending on the telescope configuration--prime, cassegrain, or coudé--the light is allowed to come to a focus at the top of the telescope, reflected back through a hole in the primary mirror, or bounced all the way to the basement.

The job of reconfiguring the telescope, called a "focus change," is accomplished by literally replacing the top end of the telescope. Alternate top ends--heavy steel rings, the same diameter as the telescope tube--are stowed around the dome floor. Each supports optics which, when in place atop the telescope, modifies its characteristics. Using a large crane mounted at the highest point of the dome, a technician hoists the appropritate ring into place. A small crew, starting in the morning, can complete a focus change by lunchtime, including rebalancing the telescope and attaching new instruments. Focus changes are typically made several times each month.

For more on each configuration, read the popup sidebars for prime, cassegrain, and coude.

3-meter Instruments
For a telescope to be a useful tool for astrophysical research, it must be provided with an instrument or instruments for manipulating, measuring, detecting, and recording the light it gathers. The Shane 3-meter is equipped with a suite of instruments designed and built by Lick Observatory or its partner institutions. Known as "facility instruments," they are available to the University of California astronomical community for use with the 3-meter, and cover a full range of applications for different kinds of investigations.

3-meter instruments can be divided into two classes: spectrographs and direct imaging cameras (some functioning as both). 3-meter instruments can be further divided into those which detect light at optical wavelengths (more or less centered on the colors our eyes can see) and at near infrared (IR) wavelengths (redward of the visible). The principal instruments in use on the 3-meter are:

  • The Prime Focus Camera (direct imaging in the optical)
  • The Kast Dual Channel Spectrograph (spectroscopy and limited imaging in the optical)
  • IRCAL(adaptive optics direct imaging and limited spectroscopy in the infrared)
  • Gemini (direct imaging and limited spectroscopy in the infrared)
  • Hamilton Spectrograph (high resolution spectroscopy in the visible)

    Non technical descriptions of these instruments appear (or will appear) in these pages. For those interested, technical information for 3-meter instruments, mostly in the form of user's manuals, can be found on this website.

  • The 3-meter Control Room
    Long gone are the days of the astronomer, huddled at an eyepiece in a cold dark dome. Observing with a research telescope now takes place in a warm, well lighted room, adjacent to the telescope, or even by remote connection at another location altogether. In the case of the Shane 3-meter, though it is in prinicple possible to observe via internet link from a remote location, observers invariably come to Mount Hamilton to carry out their work.
    The 3-meter Control Room
    The picture above was taken in the 3-meter control room in the small hours of the night, during an adaptive optics observing run. The astronomer, seated at far right, operates the instrument's infrared camera (IRCAL) from a computer terminal. At far left, the telescope operator sits at the console from which he points and guides the telescope, and monitors its many systems. This picture is part of a 360-degree panorama (140K).
    The night's work takes place in the 3-meter control room or--as it is known on the mountain--the "Readout Room" (we think the name originated in the pre-digital days of electronic astronomy, when data were "read out" on long rolls of chart recorder paper). Racks of electronic equipment and a flock of computer terminals allow remote control of telescope, instrument, and detector. Virtually all the work of observation is done without leaving the Readout Room. Of the various telescopes on Mount Hamilton, only the 3-meter has a full-time operator. Astronomers are taught to operate the smaller telescopes themselves.

    ... and the CAT (The 0.6-meter Coudé Auxiliary Telescope)
    The coudé spectrograph, though permanently mounted in its basement cave, must share the telescope with other instruments at other foci, leaving only about a third of the nights available to astronomers who wish to use the coudé focus. The excellence of the coudé spectrograph and its suitability for the study of bright stars led to construction of the Coudé Auxiliary Telescope (CAT) in 1969. The CAT is a 0.6-meter reflector that can be focused on the entrance to the coudé spectrograph when the 3-meter telescope is configured for cassegrain or prime focus.

    Unlike a conventional telescope, the CAT is fixed vertically just inside the wall of the 3-meter dome, above the entrance to the spectrograph. Light is fed to the CAT by a steerable flat mirror called a sidereostat, housed in a shed attached to the south side of the dome. When the CAT is in operation, the roof of the shed is retracted and the sidereostat pointed to an object in the sky. As the sidereostat tracks a star, it reflects light through an opening in the dome wall, into the fixed telescope, and finally into the spectrograph. (See a diagram of the CAT.) The CAT and coudé spectrograph can be operated completely independently of the 3-meter telescope, from a small control room in the basement.

    Because there is less demand for CAT time than for 3-meter time, the CAT can be used for scientific programs which require many nights of observation, such as studies that monitor changing phenomena or those that require observations of large samples of stars. Though only 0.6 meters in diameter, and with far less light-gathering power than the 3-meter, the CAT has proven its worth many times over for the study of bright stars.