Imaging the Radio Universe
Camera Development on the GBT
The NRAO camera development program is a collaboration between the NRAO and more than 20 university, college, and industry groups to design, develop, and build a suite of radio cameras that will increase the GBT’s capabilities dramatically. Three types of instrumentation are planned: conventional feed horn arrays, phased array receivers, and bolometer arrays. The science achievable with these new instruments on the GBT is extraordinary and varied: rapid, sensitive maps of outflows from comets and of molecular clouds in nearby galaxies; studies of pre-biotic molecules and chemical processes throughout the Galaxy; study of hot gas in galaxy clusters to compliment X-ray images and reveal how cosmic structures form and evolve; and uncovering the most distant galaxies from the emission of cold dust and redshifted molecular lines. In addition, the cameras will improve capabilities for pulsar searches and very deep observations of HI that trace past galaxy interactions. The power and flexibility of these instruments will make possible experiments that cannot now be done on any telescope.
Conventional Feed Horn Arrays: 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).
Phased Array Receivers: In contrast to the conventional feed arrays, phased array receivers are composed of a number of small elements whose output is added digitally to yield complete sampling over some area of the focal plane. A number of talented groups around the world, notably in Australia, The Netherlands, and Canada, are actively working on instruments, variously referred to as active, phased, beam-forming, or smart arrays to distinguish them from the more conventional independent-pixel feed-horn arrays which sample less than 1/16th of the available sky area within the array's field-of-view. A focal plane phased array feed (PAF) can electronically synthesize multiple, simultaneous beams on the sky for complete coverage of the field of view without loss of sensitivity in each beam. However, a substantial amount of signal processing is required to form each PAF beam and phased arrays need considerably more development work to achieve system temperatures comparable to the best single-beam and conventional horn arrays. For survey and mapping applications the higher system temperature penalty of a non-cryogenically cooled PAF can be compensated by forming more beams and trading off the required increase in integration time for greater sky coverage per pointing, but this makes sense only when post-beam-forming signal processing requirements are relatively light, such as modest bandwidth spectral line observations. In applications where single-beam or horn array systems are already starved for signal processing power and data storage capacity, such as pulsar and transient searches and high-redshift HI surveys, the trade-off of more beams at higher system temperature does not make economic sense.
Bolometer Arrays: A bolometer consists of an "absorber" connected to a heat sink through an insulating link. Any radiation received by the absorber raises its temperature above that of the sink. Like phased array receivers, bolometer arrays can be built to provide complete sampling of the focal plane over a given sky area with a large instantaneous bandwidth. Large format bolometer arrays offer the prospect of revolutionary strides in sensitivity over the next decade so are the focus of extensive development activity throughout the millimeter and submillimeter instrument communities. The ability to implement background photon-noise limited, robustly operable detector arrays will be crucial to the success of future projects such as CCAT and space-borne CMB polarization experiments. Their ability to sensitively map large areas of sky will complement ALMA in defining systematic samples of objects, and extreme instances of them, for further study. The GBT development effort will occur in synergy with these enterprises, and will facilitate testing and refining.
Camera Development Portfolio
- FLAG - Focal Plane L-band Array for the GBT - In research phase
- W-band phased Array Feed - In research phase
- MUSTANG - Released for general use
- MUSTANG2 - In research phase