Arecibo Observatory

Angular resolution, diffraction limit etc.

The ALFALFA survey exploits the Arecibo L-band Feed Array, dubbed ALFA. L-band is a radio engineering term referring to the frequency (wavelength) range at which the instrument works. ALFA detects radio radiation at frequencies from 1225 to 1525 MHz which is part of the designated "L band". The frequency (for sources at rest, i.e. not moving) of the atomic hydrogen HI line is 1420.4 MHz, or a wavelength of 21 cm.

The angular resolution of an image produced by a telescope depends on the telescopes diffraction limit or, in the case of ground-based optical images, the atmospheric seeing due to turbulence in the Earth's atmosphere above the telescope site. The latter is not an issue for the 21 cm radiation we detect with ALFA. Instead, the ability of ALFALFA to resolve cosmic structures is dictated by the telescope's diffraction limit, which depends on the wavelength of radiation and the telescope's size.

Radio telescopes have relatively large beam sizes. Even though Arecibo is a huge antenna, 305 meters in diameter, its angular resolution at L-band is about 3.5 arcminutes.
Advanced:     Calculate the expected beam size of Arecibo at a wavelength of 21 cm. You will get a different number than 3.5 arcminutes. Since you know the wavelength, what must be true about the "effective diameter" of the telescope? Can you understand why this is so?

If we were to observe the Moon with an optical telescope which had the same angular resolution as Arecibo, we would not be able to make out its craters. The image to the far right shows how the Moon would appear if we observed it with the same resolution that Arecibo gives at a wavelength of 21 cm.
Because of the limitations of its angular resolution, we do not use Arecibo to make images of the detailed structures of individual galaxies. Instead, we use it to map large areas of the sky by combining the measurements made at single pointings. Each beam be thought of to contribute a single pixel to the final image, as illustrated in the image to the right. While ALFALFA images are limited in their ability to reveal small-scale structural detail, they allow us to detect and catalog sources even if they are smaller than the beam. And there are plenty of interesting sources which are so big that ALFALFA sees greater large-scale detail missed entirely by interferometers.

ALFALFA is specifically designed to exploit the wide area mapping capabilities available with ALFA at Arecibo. The limitations of its beam size are compensated by its mapping speed. And, unlike previous blind HI surveys, ALFALFA allows adequate centroiding of detected sources so that their most probable optical counterparts can be identified.

Useful external links

Back to ALFA for ALFALFA

Last modified: Sat Oct 6 15:58:01 EST 2007 by martha