HET flybyFly-by of the telescopes on Mt. Locke.

Harlan J. Smith Telescope

The 107-inch (2.7-meter) Harlan J. Smith Telescope is the largest "full-purpose" telescope at McDonald Observatory.

The 107-inch telescope was designed to accommodate a variety of instruments that study the universe in different ways, and it has a full range of motion to allow it to view any part of the West Texas sky. It remains a workhorse today, studying everything from our own solar system to the most distant galaxies.

The Smith telescope is a reflector, so it uses mirrors to gather and focus starlight. Its primary mirror measures 107 inches (2.7 meters) in diameter. That giant collecting area gathers about 250,000 times more light than the human eye.

Additional mirrors can reflect the light gathered by the primary mirror to instruments attached to the bottom or side of the telescope, or in a dark room one floor below the telescope.

107-inch Harlan Smith Telescope

The Harlan J. Smith Telescope dome basks in the West Texas sunshine, with the Hobby-Eberly Telescope in the background. [Bill Nowlin Photography]

The Smith telescope uses an equatorial design, which means it can rotate 360 degrees and tilt from almost horizontal to straight up. That allows it to see any portion of the sky, and to track its targets for hours at a time.

Smith telescope mirror

The Harlan J. Smith Telescope’s primary mirror is at the right end of the tube. The framework at the left end of the tube (the top) holds an instrument at prime focus. The small gray circle at right is a port for other instruments. [Marty Harris]

The mirror is hoisted back into position. alum 2 alum 3Looking down the tube at the primary mirror. A hatch in the floor is removed, revealing the aluminizing room below.A view of the aluminizing chamber The mirror sits on the observing room floor.An empty telescope tube. The mirror is lowered to the aluminizing chamber.Workers maneuver the mirror The mirror is loweredMcDonald employees prepare to reassemble the telescope. The mirror is at right.The mirror is hoisted back into position.

Over time, the reflective aluminum coating on the telescope's primary mirror, which measures 107 inches (2.7 meters) in diameter, becomes tarnished. To keep its views sharp, technicians periodically remove the mirror from the telescope tube and seal it in a vacuum chamber one floor below, where a fresh aluminum coating is applied. [All images by Kevin Mace]

Room Without a View

As with most large, modern telescopes, the astronomer operates the telescope from inside a heated, air-conditioned control room with no outside windows. The room is adjacent to the telescope on the observing floor.

The astronomer arrives at McDonald Observatory with a list of target objects. These objects are entered into the telescope’s computerized control system, which then automatically turns the telescope to the appropriate star, galaxy, planet, or other target object. The astronomer sees the region of the sky the telescope is viewing through a television monitor, and can use a hand controller to adjust the telescope’s position. Even though the entire telescope weighs about 160 tons (145,000 kilograms), it is so finely balanced that it requires only a tiny amount of energy to move it.

As Earth rotates on its axis, the telescope automatically turns to keep its target object in view.

The telescope looks through a narrow slit in the 435-ton (395,000 kg) dome, which rotates with the telescope. Wind screens inside the slit can be configured to minimize vibrations from wind gusts.

The telescope operates during most clear nights. The dome must be closed if the humidity is too high (moist air can condense on the telescope surfaces), if winds exceed certain limits, or if too much dust is in the atmosphere.

During the day, the shutters are generally kept closed to prevent the telescope from getting too warm. If it is warmer than the nighttime air, “heat waves” rise from the telescope (like a desert mirage), distorting the view.

Observations are recorded electronically and either emailed to the astronomer or copied onto CDs or other physical media.

A Symphony of Instruments

The Smith telescope directs light to spectrographs, photometers, or cameras. A spectrograph splits light from the astronomical object into its component wavelengths or colors. A photometer measures the intensity of an object’s light, which is especially useful for studying objects that change in brightness. Cameras, which are used infrequently these days, provide beautiful images of the universe.

Some instruments are mounted at the top of the telescope, which is a configuration known as prime focus. Others are mounted to the back or side, using the Cassegrain focus. And some spectrographs are located in a large room one floor beneath the telescope itself, at the Coude focus. This configuration allows spectrographs to split the light into smaller slices, providing more details about the target object.

The reflective aluminum coatings on the mirrors must be reapplied periodically to maintain clear, bright views of the sky. Realuminizing the primary mirror takes about two days. A crane removes the mirror from the telescope tube and lowers it through a slot in the floor. It is then placed in a large chamber, the air is pumped out, and a small amount of aluminum is vaporized. The vapor coats the mirror’s reflective surface. A few hours later, air is allowed back into the chamber and the mirror is removed and lifted back to the observing dome, where it is reinstalled in the telescope, ready for another couple of years of service.

diagram of Cassegrain focusdiagram of Coudé focus

Cutaway images show light reflecting off the various mirrors in the telescope. Cassegrain focus is at left, with Coude at right. [McDonald Observatory]