ALFA is the Arecibo L-band Feed Array, a new seven feed (pixel) system mounted on the Arecibo telescope in 2004. Seven pixels may not sound impressive given that a run-of-the-mill digital camera has three million or more pixels. However, before ALFA was commissioned, only single feed detectors were available on the Arecibo telescope. We are now able to map the sky seven times faster. This might not seem like much but consider this: the ALFALFA survey is projected to take 6-7 years; if we were using a single pixel detector it would take 42-49 years to attain the same coverage of the sky.
The picture below shows a schematic view of the ALFA beam pattern. The colors indicate the sensitivity of the beams of the telescope. The center beam is brighter, indicating it is more sensitive. This is because it sits at the center of the focus of the telescope; the other beams are slightly offset and are slightly less sensitive.
The L-band in the name of ALFA refers to the wavelength coverage of the receiver. Radio astronomy owes many advances to the development of radar during World War II. This is reflected in many ways, including the use of the radar frequency bands to describe the response of radio receivers. L-band is an important frequency range in radio astronomy as it includes the 21 cm line radiation of neutral hydrogen.