Radio telescopes operating at centimeter wavelengths have historically used just a single feed system, equivalent to a single spatial pixel on the sky. This makes for a very slow and laborious data-gathering process. In contrast, modern instruments at optical and infrared wavelengths, and more recently those at millimeter and sub-millimeter wavelengths, are able to gather data in parallel from multiple (and in some cases many) independent pixels at once. The gains in observing efficiency and scientific capability can be enormous. Multiple pixels improve the ability to remove signal background and greatly increase the speed at which large-area surveys can be carried out. An initial step at the wavelength of 21 centimeters is the Arecibo Observatory 7-pixel Arecibo L-band Feed Array (ALFA), installed on the 305-meter telescope in 2004.
A future direction with even more impact on observing power is a planned cryogenically-cooled 40-beam phased array feed (PAF) for Arecibo, also operating at L-band. Feasibility studies have shown that Arecibo optics can support all 40 beams on the sky, and a demonstration dewar system that will house a 19-element array is nearly complete. To further develop the technology needed to build large arrays of the future, Drs. G. Cortes-Medellin and D. Campbell of Cornell University plan to add low noise amplifiers to the cryogenic dewar and deploy the full system for use on the Arecibo telescope. The goals will be to demonstrate that a system noise temperature of less than 35 K can be achieved with a multipixel array, investigate optimal beam-forming algorithms for Arecibo's optics, and determine the system's overall temporal stability. The new, more efficient generation of multi-element L-band receivers being advanced by this project will mesh with phased array feed signal-processing backends under development at Brigham Young University. Together, the studies will enable the design of a larger, full-scale phased array feeds that will greatly accelerate the capabilities of cm-wavelength surveys. Students and young scientists employed throughout the project will help augment the next generation of hardware-savvy researchers with expertise in signal detection and propagation for the important radio regime.
Funding for this work is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.
Modern optical telescopes use digital cameras with thousands of pixels, similar to the ones that could be bought at an electronics store, but more sensitive, to form images of stellar objects. On the other hand, single dish radio telescopes normally use just a single pixel to form these images at radio wavelengths. This is accomplished by pointing the radio telescope towards a particular astronomical object, and then forming the image one pixel at a time, a lengthy process. At radio wavelengths, adding more pixels is more expensive and complicated than for optical counterparts. More pixels means faster mapping. ALFA is a 7-pixel camera that has been used on the Arecibo radio telescope for more than a decade. An expression for the speed for which a telescope could map a region of the sky at a given level of sensitivity depends of the number of simultaneous pixels used, the size of these pixels (solid angle), the bandwidth used, and the sensitivity of the telescope squared: (Aeff/Tsys)2, where Aeff is the effective aperture of the telescope for capturing radio waves and Tsys is the system temperature, i.e., the noise level in the system. For fixed aperture radio telescopes, the survey speed then could be increased by increasing the number of simultaneous pixels used, increasing the bandwidth or decreasing the system noise temperature. Cryogenic phased array (Cryo-PAF) cameras increase the survey speed by creating a large number of pixels simultaneously in the sky and reducing the system noise temperature by cooling the detectors to cryogenic temperatures (See Fig. 1). Addressing these two issues poses difficult challenges. A future Cryo-PAF instrument for the 305 m (1000ft) Arecibo telescope with 40 simultaneous pixels (or beams) on the sky will increase the survey speed by a factor of 24 when compared to current single pixel detectors and a factor of 4 when compared to ALFA; a survey project completed recently by ALFA took 7 years, a similar survey could be completed in a little more than a year with a Cryo-PAF camera with 40 beams. Our project demonstrated that it is possible to address these two issues by using a large number of cryogenically cooled detectors enclosed inside a cryostat with a large radio-transparent vacuum window and then combining their signals to synthesize many number of independent beams in the sky. Our camera demonstrated that system temperatures of the order of 35K or less are feasible, which is competitive to current technology used in single pixel radio telescopes today (27K to 30K) at L-Band (1.2-1.6 GHz). We developed an ingenious approach to having large vacuum windows that are also transparent at these wavelengths. Under vacuum, the window is supporting the atmospheric pressure, 4 metric tons for the 75 cm diameter window of the system we developed. (See Fig. 2). We also developed re-insertable dipole/low noise amplifier assembly that is simple to extract and replace from the front of the camera without fully opening the cryostat. This enables future cameras with a large number of pixels to be serviced more easily in the field. We also developed a noise injection coupler to individually calibrate each dipole/low noise amplifier assembly. (see Fig. 3) Figure 4 shows the camera installed in the rotary receiver room floor of the Arecibo radio telescope during the test observing campaign in July-August 2013. Once the Cryo-PAF camera system is calibrated, it is possible to map the area of the sky covered by the formed pixels, in a just single shot. Figure 5 shows the image of the galaxy M87 made with 9 pointing of the telescope, after calibration of the Cryo-PAF array, covering a total field of 30 arcmin x 30 arcmin. If a single pixel instrument had been used, more than a 1000 pointings would have been necessary to obtain an equivalent image with similar sensitivity.
