Technical Challenges & Development Path

Technical Challenges:

Conventional feed horn arrays are built by packing traditional feeds tightly to maximize the number of pixels on the sky per unit area. A typical feed-horn array can achieve a pixel spacing of ~2.5×beamwidth in the focal plane, so multiple pointings of the array are needed to cover an area on the sky completely. At NRAO we are investigating the concept of integrated feedhorn designs. To meet the scientific goals a conventional feed horn array must achieve the following: Low instrumental noise, dual polarization, wide instantaneous bandwidth (10-20 GHz or greater), closely packed feed horns (≤2.5 beam width separation on the sky), stable baselines at the μJy level, and high spectral resolution (≤2 km s-1 at 115 GHz).

These requirements pose a number of significant technological challenges:

Integrating and packaging the receiver systems: Efficient integration and packaging of the feed horn array is a difficult problem. Focal plane arrays to date have evolved from single pixel receivers, and existing arrays with up to a few tens of elements have naturally been assembled from individual receivers and components, albeit efficiently packed together. This is sufficiently expensive, time consuming, and difficult to maintain that as one moves to the next level – arrays with many tens of pixels – the approach must be modified and multi-pixel modules employed. Additionally, in order to maximize the scientific potential of a feed horn array one must typically place the feed horns as close together as possible. This allows for a more efficient imaging of compact regions on the sky, the removal of uncertainty in the telescope pointing (e.g. due to wind) through permitting re-gridding, and permitting the use of the multiple feeds for image calibration and reduction.

Achieving low noise, dual polarization across a wide bandwidth: Typical scientific requirements of a receiver system include very low noise with instantaneous bandwidths of greater than 20 GHz and dual polarization with a high isolation between the polarization components. This often cannot be achieved using a single technology. As an example, Monolithic Microwave Integrated Circuits (MMICs) do not typically have the noise performance necessary for the scientific needs of the array. As a result, a hybrid system, involving a combination of MMIC and discrete Field Effect Transistors (FETs) may be the proper approach. However there is still significant research which must be done to achieve the optimum solution for a focal plane array (FPA).

Dewar systems: Cooled, large N feed horn arrays can require very large windows for the dewar system. Research is necessary to determine the prectical size of any single dewar window as well as the possibilities for multiple windows which overly smaller-sized modules of the larger array.

Broad Bandwidth: It is advantageous to maximize the possible bandwidth for the majority of feeds, thereby maximizing the scientific possibilities. However, determining the bandwidth possibilities involve amplifier research and development, research into LO design possibilities, and determining the noise trade-offs, gain slope, ripples, etc for all hardware components.

Thermal management: There are a number of areas of research which need to be solved for any large N feed array and which can be classified as "thermal management". These include window heat loading, thermal transitions at the LNA outputs, and mechanical expansions of the various componants.

Current Development Work:

The NRAO has built and commissioned one conventional feed horn array, a 7-pixel, 18-26 GHz array. Current work is focusing on the research and development areas outlined above, with the goal of building a 100+ pixel receiver covering at least the 92-110 GHz range. Both projects are outlined below.

7-pixel K-band (18-26.5 GHz) array : Observations in the K-band frequency spectrum (18-26.5 GHz) of extended sources in the spectral line require inordinate amounts of telescope time with a single pixel receiver. Focal-plane array instruments improve telescope efficiency with additional sampling of the aperture plane. In 2010 the NRAO commissioned a 7-pixel conventional feed horn array which covers the 18-26.5 GHz frequency range. The array is the first feed horn array to be built for the GBT, and it does not use an integrated design.

100 pixel W-band Focal Plane Array : With the recent improvements to its surface rms, the GBT is the most sensitive single-dish telescope at Wband (3mm), and with a spatial resolution unmatched by any other single-dish telescope operating at 3mm. Combining this excellent sensitivity and resolution with a large spectroscopic focal plane array would provide unprecedented mapping ability and sensitivity. This will be a uniquely capable instrument for the investigation of the astrochemistry and physical properties and processes associated with molecular clouds and star formation within our Galaxy, nearby galaxies, and objects in the distant Universe.