85-1 (Tatel Telescope)

Tatel Telescope (85-1)

Tatel Telescope

When the NRAO was first founded, and the Green Bank site was chosen, the United States’ new national radio observatory had no radio telescopes. Plans for a giant telescope were underway, but no working telescope was on site. An order was placed with the Blaw-Knox Corporation in Pittsburgh, Pennsylvania for the purchase of one of their new 85-foot dish telescope kits. In the meantime, a 12-foot replica was used to test receivers and feeds for the 85-foot’s imminent arrival.

The kit arrived in summer of 1958, and the telescope was completed in early 1959. It was dedicated on October 16, 1958 and named in honor of Howard E. Tatel. Tatel designed the giant gearing of the telescope, presenting a scale model to Blaw-Knox that used his wife’s cereal bowl in place of the 85-foot dish. The new design increased the precision of the telescope’s movement and also made the kit much more affordable.

The Tatel Telescope began 24-hour a day observations on February 13, 1959, starting with the brightest radio objects known: Cygnus A, Cassiopeia A, and Taurus A. One of the first major observing projects performed was the first detailed mapping of the center of our Milky Way Galaxy by staff astronomer Frank Drake. He discovered that the heart of our Galaxy contains several sources of radio waves and is a lot more complex than ever seen before.

Green Bank Interferometer

The 85-3 and the 85-2 twin 85-foot telescopes working together with the 85-1 (not shown) in the Green Bank Interferometer, the NRAO’s first array. The GBI operated from 1967-2006. From 1978-1996, it was operated in support of USNO and NRL geodetic and astronomy programs; after 1996 in support of NASA High Energy Astrophysics programs. In the background of this photo, the 300-foot telescope ghosts through the trees.

In 1960, Drake initiated a two-month observing program on the Tatel which he called Project Ozma, after the Queen of Oz, to aim the big telescope on two of the nearest, Sun-like stars in our galaxy to listen for possible signals from extraterrestrial civilizations. He and his team observed stars Tau Ceti and Epsilon Eridani with a receiver tuned to the 21-cm line of hydrogen, the most common radio signal in the Universe. Despite a brief excitement over what later turned out to be an airplane, the project did not pick up any signs of intelligent life around those stars.

The Tatel quickly gave astronomers more accurate positions and brightnesses for known radio objects. Astronomers also used the Tatel to measure surface temperatures for Venus and the Moon. Studies were done of Jupiter’s radiation belts, the envelope of charged particles that are trapped in the enormous magnetic field of our Solar System’s largest planet.

In the mid 1960s, two more 85-foot telescopes (photo to right) were built to the same design as the Tatel to become the three-element Green Bank Interferometer (GBI). A succession of smaller, portable telescopes was added to the interferometer to give it another axis and turn it into an array prototype for what would become the Karl G. Jansky Very Large Array.

Radio interferometers help astronomers study the fine structure and positions of radio objects, because the separation of the telescopes creates a powerful binocular vision.

The precision of the GBI allowed it to make the first radio measurement that confirmed, to high accuracy, the prediction by general relativity of the bending of light (i.e. any electromagnetic radiation) near a massive body.

From 1978 to 1996 the Tatel, as part of the GBI, was operated by the USNO for studies of Earth rotation and monitoring of variable radio sources. From 1996 until October 6, 2000, the Tatel Telescope was in continuous use as part of the GBI, funded partly by NASA, for radio studies of X-ray and Gamma-ray sources.

Tatel Stats

  • Reflector:  85-foot diameter paraboloid;  Surface is 0.125 inch thick aluminum panels;  Surface area is 5700 square feet with better than 0.125 inch RMS tolerance.
  • Focus:  36 feet above reflector surface and 115 feet above ground;  Carries 600 pounds of receiving equipment; position relative to paraboloid stable to 0.25 inch.
  • Mount:  Equatorial (polar and declination axis, mutually perpendicular)
  • Declination Axis:  Shaft is 40 feet long, 16 inches diameter;  gear is 40 feet diameter; Travel is 132 degrees total, 48 degrees north of stow and 84 degrees south of stow.
  • Polar Axis:  Shaft is 23 feet long, 28 inches diameter; Gear is 48 feet diameter;  Travel is about 90 degrees either way from stow.
  • Drive Rates, Both axes: Slew is 20 degrees per minute; Scan is up to 4 degrees per minute.
  • Material:  Painted steel superstructure.
  • Brakes:  Electrical set and spring set hydraulic release.
  • Total Weight:  210 tons.
  • Pointing Precision:  About 30 arc seconds (about a quarter at 600 feet).