Progress on the Ka-Band Focal Plane Array for the GBT
In February, the GBT K-Band focal plane array (KFPA) program successfully completed its conceptual design review, which addressed system specifications and component design details, with an emphasis on obtaining specific scientific goals. Mapping nearby star-forming regions to study the density and temperature in star-forming cores, as well as the role of turbulence and inflows and outflows, are prime examples. The committee also reviewed the expandability of the design from seven to approximately 61 pixels, and the approach to data analysis and M&C software plans. They recommended proceeding with the system specifications given in Table 1. They endorsed the approach of designing an expandable array without sacrificing the seven pixel array schedule. Appreciating the complexity of the mechanical design for a larger array, many helpful comments were forthcoming.
Table1. Baseline Instrument Specifications
We have in place a letter of intent (LoI) with the University of Calgary. This collaboration has provided a framework for developing a data analysis pipeline. The LoI proposes 0.3 to 0.5 FTE support for component development. Their experience with the Arecibo\GALFACTS data pipeline will expedite the development for commissioning and prototype tools that support an expanded array. Equally important, but with more technical details to be addressed, is the LoI for a collaboration with a Canadian group that will deliver a correlator capable of processing the entire 1.8 GHz bandwidth.
Results comparing the theoretical and measured feed performance with the associated GBT aperture efficiencies were presented during the review. An analysis of mapping strategies of ammonia in star-forming cores, with sizes comparable to the GBT beam (about 30 arcseconds) at the frequency of ammonia (24 GHz) and distributed on scales of a few arcminutes, determined a close-packed hexagonal feed layout as optimum. The KFPA will also be a powerful tool for interstellar chemistry searches for new complex molecules that tend to be diffusely distributed on scales of a few arcminutes.
Simulated mapping areas ranging from 9 to 144 square arcminutes have shown that a simple raster map in any celestial coordinate system, with a Nyquist sampling perpendicular to the scan direction, provides relatively uniform coverage while spending less than 30 percent of the observing time outside of the area of interest. This is true for a range of source hour angles and declinations. For chemistry experiments, the feed pattern will rotate on the sky, but the observations will still provide for deep integrations with some information on the radial distribution of the molecules being studied.
S. D. White and D. J. Pisano