GBI

In the 1960s, we wanted to build an array of radio telescopes called an interferometer that simulates a larger telescope from the combined observations of strategically-aligned smaller ones. When the waves of these separated telescopes are combined, the signals get stronger and the noise flattens out, creating sharper views of radio objects.

However, before such a large-scale project could be funded, we needed to prove our design’s hardiness and our ability to operate it.

From 1959 in Green Bank, we had been running a successful and busy 85-foot telescope, called the Tatel Telescope, that we bought as a kit from Blaw-Knox Corporation. Blaw-Knox was still making these kits in 1963 when we ordered another 85-footer from them to create a two-telescope observing system.

By February of 1964, we had assembled this near-twin 85-foot radio telescope that we named the 85-2. The 85-2 differed slightly from the Tatel in that it had 80 14-ply truck tires permanently mounted to its frame. Using a D-7 Caterpillar and an Army surplus aircraft towing tractor, we could haul the 85-2 closer or farther away from the static Tatel to change the pair’s combined resolution and sensitivity.

In anticipation of our new movable arrival, we built a mile and a half-long track leading away from the Tatel. We placed the 85-2 at the end of the new track for maximum distance and mounted six staging stations at 1000-ft intervals along the track. Each station had three piers for stability and sockets for data and power lines that ran the length of the track, some buried, some suspended above on trays.

Cabling between the 85-2 and the 85-foot Tatel took several months, and, with the first digital autocorrelator to combine their observations, the two finally became a working interferometer on June 1, 1964. The interference patterns of the telescopes’ combined radio wave observations told astronomers about position, size, and strength of objects in space.

Renamed the Green Bank Interferometer, this pair’s interference (fringe) patterns were very good, showing astronomers the periodic changes in radio signals from objects that dimmed and brightened over time. Within the first year of GBI observations, the major astronomy report from the National Academy of Sciences recommended the immediate funding and building of a large-scale radio telescope array as a national science facility. We began experiments and discussing designs for what would become that national facility, the Very Large Array in New Mexico.

We built for the GBI a new control room and added a third 85-foot Blaw-Knox telescope to the middle of the interferometer track in spring of 1967, greatly increasing the sensitivity of the Green Bank Interferometer. The 85-3, too, had truck tires mounted on either side, and we used it to test multiple baselines – but still only in one, short direction, running roughly northeast to southwest. For a true test of a high-resolution array, we needed a telescope located far outside of that track.

Therefore, that year, a fourth portable telescope was added to the Green Bank Interferometer. The dish of this portable kit telescope was half the size of the others, 42 feet. This unique telescope could be dismantled and hauled away on the truck that also ran its control desk and motorized mount.

The 42-foot telescope was hauled first to a location 8 miles from its larger siblings. It later was relocated to a site 11 miles north-northeast,  a mountaintop called Spencer Ridge. (The distance is the same as an arm of the Y-shaped VLA, still under design at the time.) Finally, it was sited on a ridge in Huntersville, West Virginia, 26 miles from Green Bank, the same distance of the longest baseline of the VLA.

Its observations were wirelessly transmitted as a microwave data link down the mountains to a receiver that sent the signal into the autocorrelator, still in the little building along the GBI track.

By 1972, the aging surface of the 42-foot portable was too inaccurate for our increasing research needs, and the unique telescope package was given to Cornell for interferometer work at its Arecibo observatory in Puerto Rico.

A new, 45-foot portable replaced the outgoing 42-foot in 1973 and served through 1983 on the Huntersville mountaintop. It was brought back to the Green Bank site as a stand-alone telescope, still in use, but now as a solar radio telescope.

A new 46-foot portable dish telescope was placed on Point Mountain to complete the VLA test work. In 1988, with the VLA nearly a decade into its reign as the best interferometer on Earth, the 46-foot was given to the NOAA, moved to Fortaleza, Brazil in 1991, and served as an element in their Earth-mapping VLBI project.

The GBI continued doing important scientific observations into the 21st century.