2. Observations

The observations were made with the 45-MHz array of the University of Chile at the Maipu Radio Astronomy Observatory (). Since the instrument has been described in detail elsewhere (May et al. 1984; Alvarez et al. 1994), only a brief description will be given here. The main parameters of the array are presented in Table 2 (click here).

 

Polarization E-W linear Effective area at the zenith

Sensitivity at the zenith 3.9 K/Jy

Bandwidth 1 MHz

Receiver noise temperature 300 K Array coordinates

 

The array is made up of 528 full-wavelentgh dipoles oriented E-W and distributed in six groups, where the signal after passing through a balun-preamplifier is fed to a matrix of hybrid rings. This matrix produces a set of beams staggered along the meridian and separated approximately by . The beams produce simultaneous and independent outputs and can be steered in declinations by electric phasing. Since only the four central beams were used, the array scanned the sky along four declinations spanning a strip about wide at a time. The sky was covered by moving the set of beams to adjacent positions separated by (called ``nominal" positions), in such a way that the end-beams overlapped thus producing the anchorage of the observations. The antenna behaves well approximately within from the zenith so, in all, 19 equidistant nominal positions covered the range . At the beggining of a new declination run we checked for possible errors in the electric phasing of the array by observing intense radio sources or the transit of the galactic equator, if there was no source. Since low-frequency observations are very sensitive to ionospheric conditions, as well as to man-made and natural interference (the solar maximum around 1989 was particularly devastating), each position was observed for several weeks in order to obtain the best possible data. Also, because of increasingly adverse observing conditions each position was observed several times. To help in eliminating diurnal and seasonal effects, each position was observed in epochs separated by odd multiples of six months. The observations began in 1982 and ended in 1994.

An internal calibration signal from a noise generator was injected automatically every hour through the balun-preamplifiers located at the array site, and the temporal drift in gain of the preamplifiers and receivers was corrected by linear interpolation. In order to make a reference calibration we observed a region of the sky () well covered with observations at different frequencies. The best fit to the spectral data gives 30000 K which agrees well the 29500 K we measured. The bandwith and sensitivity (at the zenith) were 1 MHz and 3.9 K/Jy, respectively. Data points in right ascension were acquired every 10 seconds and were integrated over one minute. The integrated temperature value was asigned in time to the middle of the minute.