NANOGrav astronomers use the world's most sensitive radio telescopes to observe and discover millisecond pulsars: the William E. Gordon Telescope located at the Arecibo Observatory in Puerto Rico, and the Robert C. Byrd Green Bank Telescope located at the National Radio Astronomy Observatory in Green Bank, West Virginia.

Arecibo Observatory

The 305-meter dish of the William E. Gordon Telescope. Image credit: NAIC

Arecibo is the largest radio telescope in the world, with a diameter of 305 meters, which is the size of 3 football fields. Because it has so much collecting area, Arecibo can detect extremely weak radio signals, such as those from millisecond pulsars. In fact, Arecibo is the most sensitive radio telescope in the world, by a wide margin, contributing to the search for gravitational waves using pulsar timing. The dish is so large, however, that it must remain fixed in position. This limits the parts of the sky that Arecibo can access—it is capable of seeing the sky directly overhead and 20 degrees to each side. It accomplishes this by using a spherical reflecting dish (as opposed to the more common parabolic shape) and a special optical system housed in its large Gregorian focus dome.

The Gregorian focus dome of Arecibo corrects spherical abberation and allows it to observe a larger portion of the sky at high radio frequencies. Image credit: NAIC

NANOGrav uses Arecibo to observe 25 millisecond pulsars. In addition, Arecibo is used to search for new pulsars through two large-area pulsar surveys: the Pulsar Arecibo L-band Feed Array (PALFA) survey and Arecibo 327 MHz drift scan pulsar survey.

The Green Bank Telescope

The 100-meter Green Bank Telescope, the world's largest fully steerable telescope. Image credit: NRAO

The Green Bank Telescope is smaller than Arecibo, but is capably of pointing in any direction. As such, it can see 85% of the total sky, and is therefore critical to achieve the sky coverage necessary for our project. It is located in the only U.S. National Radio Quiet zone, in which radio transmissions are regulated to limit the amount of interference that might adversely affect our observations. NANOGrav uses the Green Bank Telescope to observe 25 millisecond pulsars, and to search for possible new additions to our pulsar timing array through the Green Bank North Celestial Cap pulsar survey.

Telescopes Under Threat

Unfortunately, the futures of both the Green Bank Telescope and Arecibo are uncertain. In 2012, the National Science Foundation, which funds both telescopes, announced a plan to "divest" (dramatically decrease its financial support) the Green Bank Telescope by 2017 due to budget constraints. NANOGrav is one of many organizations that are looking for alternative sources of funding to support this world class instrument (read our official response to the NSF plan). The future of Arecibo is also insecure, with NSF exploring the closure of the facility without the contribution of additional external funding.