Optimized Feeds for the GBT

While the GBT Camera development program will ultimately provide the best possible instruments across the wide frequency range of the GBT, is it expedient to simultaneously provide receiver systems which are optimized to one, or a limited few, science cases and which can be placed on the GBT in a reasonably short period of time. These instruments are typically built to accommodate a single high profile science case for the GBT.

Currently there are three such instruments underdevelopment - a sensitive 1.7-2.5 cm receiver for pulsar and spectral line studies, a 4mm two-pixel system for molecular line and VLBA studies and a 15cm wide bandwidth instrument optimized for the NanoGrav experiment.  All three instruments are further described below.

Wideband Ku Receiver (15-18 GHz): The existing GBT Ku receiver will be replaced with a new system with wider bandwidth and lower system temperature, using many of the components designed for the EVLA Ku receiver.   The primary science driver for the upgraded system is the detection of pulsars at the center of the Milky Way.  Timing of these pulsars could be used to test the theory of General Relativity in the strong field regime.  The instrument will, though open other areas of scientific interest for the GBT, including probing changes in the fundamental physical constants, constraining the physical and chemical environment of star forming regions, and allowing for "blind" searches for redshifted CO (1-0) in the z=5.4-8.6 regime.

4mm Two Pixel Feed: With the recent improvements to its surface accuracy, the GBT is the most sensitive telescope operating at 90 GHz. To take advantage this, NRAO is building a traditional dual beam feed-horn receiver which operates at the lower frequency end of the 3-4mm atmospheric window (68–92 GHz), designated as the 4mm Receiver. The science goals are focused on molecular spectroscopic studies of star formation and astrochemistry within our galaxy and beyond, but the new receiver will be built to enable VLB observations as well. On technical grounds, the 4mm receiver will permit further improvements to the surface accuracy and tracking performance of the GBT.

Wide Band Pulsar Timing System: Pulsar research has made phenomenal advances using the GBT. These include confirming the birth process of millisecond pulsars, discovering the fastest pulsar, discovering more than 60 new pulsars in nearby globular clusters, and providing rigorous tests of strong-field gravity with double pulsar timing. These studies provide probes of nature in some of the most extreme environments known. Recently, the GBT has become a crucial instrument for the North American Nanohertz Observatory (NANOGrav), a consortium of scientists seeking to detect gravitational waves via the precise timing of a network of radio pulsars.  To ensure NANOGrav's success and to enhance pulsar detection and timing capabilities more generally, NRAO is working with our university colleagues to develop a wide bandwidth front-end optimized for pulsar science.