The Instrument

• Introduction

  A seven feed, dual polarized K band focal plane array receiver is proposed for the Green Bank Telescope. This instrument will enhance the scientific capabilities of the GBT, improve its K band observing efficiency, and be the first step in a long-term development program of focal plane arrays. The current system characteristics have largely been determined by the desire to have an instrument ready for observing in three years by focusing almost entirely on front-end development and defer any improvements to the analog IF and spectrometer systems. The current IF system also limits the maximum number of beams to seven. The observing modes will be determined by the spectrometer: Observing Modes. A receiver rotator may be included to improve observing efficiency by optimizing the beam placement on the sky.

• Receiver Specifications

  The feed arrangement is a closely packed hexagonal pattern for maximum sampling of the focal plane. A 3.5 inch feed diameter coupled to the main reflector through the Gregorian Optics produces a 2.9 half power beamwidth spacing. A maximum efficiency (illumination and spillover) of 68% is predicted for the on-axis feed with the surrounding feed's efficiency being nearly the same. The cooled, 15 K, components consists of a an OMT, a Noise Calibration Module (NCM) and a CDL K-Band LNA based on the WMAP design and using a CRYO3 wafer first-stage HFET. The design allows for a phase shifter providing circular polarization measurements; however the current plans are to omit this capability for simplicity. The NCM will be a new integrated block consisting of a 30 dB directional coupler and a noise source. The noise source could be either a diode or a MMIC amplifier with calibrated ENR versus bias current.
  • Noise Temperature Plot
  • Receiver Specification Table
  • Downconverter Block Diagram
  • LO Distribution Block Diagram

Science Case The main scientific driver for this instrument is the need to map extended sources in the spectral line, although some continuum capability might also be possible. The array is expected to be used for mapping of molecules such as NH₃ and CCS in molecular clouds, for studies of very extended molecular regions which would require inordinate amounts of telescope time with a single pixel receiver, for large-area searches for H₂O masers, for studies of chemical variations across molecular clouds. It might also be used for high redshift studies of clusters of galaxies. In addition, the array will increase the efficiency of telescope use, especially in times of good weather, and will improve the mapping speed of the GBT and thus improve the map calibration.

Collaboration

• The team is actively seeking collaborators. During the initial proposal discussions, incorporating high speed samplers with a digital transmission system and a new spectrometer increased the scientific capabilities by adding more pixels, widening the bandwidth and adding spectral windows. However, these are currently outside the scope of the instrument for budgetary reasons, but remain high priority items for collaborative efforts.
• If you are interested in collaborating contact Glen Langston, Jay Lockman, or Steve White.

Team Members

• Steve White, Project Manager
• Jay Lockman, Project Scientist
• Glen Langston, Project Scientist
• Matt Morgan, Project Engineer
• Eric Bryerton, Engineer
• Karnaljeet Saini, Engineer
• Roger Norrod, Engineer
• Bob Simon, Engineer
• Gary Anderson, Engineer