|Left: Telescope pointing on May 27, 2008 vs. Atlantic Standard Time, giving azimuth, zenith angle (90o - elevation), right ascension, declination, Galactic longitude, and Galactic latitude. Only ALFA's central beam track is shown. Right: Corresponding scan tracks for the same date (red) and previous dates (blue), which cover about 2/3 of the map area (the field was completed on June 15). The prime target area is marked in green.|
The Arecibo dish is not steerable, but the instrument platform housing the ALFA receiver can be moved to catch radio waves from any direction within about 1.2o - 19.6o of the zenith. As the Earth rotates, this allows us to cover almost 1/3 of the total sky -- a strip 360o long and nearly 40o wide! The I-GALFA survey area is divided into several fields with different declination ranges. Each night, the receiver platform is slewed back and forth as the Earth turns, tracing out a zig-zag pattern (see figure). By starting each subsequent night at a different position, we can cover the entire map area by ``weaving'' many such tracks together to build up a complete image of the sky brightness at all positions in the field. The figure illustrates this process: the left plot shows the telescope pointing on a single night measured in different ways, while the right plots show how these pointings accumulate over many nights to form a map.
The telescope pointing positions are closely monitored during each observing session. Graphs for each night like those at right are kept on the daily position plots page.
You can also view a movie of the observations made during the entire 2008 observing season (May-Oct; 69 frames; 2 MB animated GIF). This steps through a sequence of position plots like those on the right side of the figure above. Three different fields (A, Z, D) were observed in 2008. The plot centers change when a new field is begun.