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1. Introduction

The continuum radio emission of the sky background is a mixture mainly of thermal (free-free) and non-thermal (synchrotron) components, in a proportion that depends on the observing frequency. At frequencies higher than about 1 GHz the largest fraction of the radiation is thermal, and this fraction decreases with frequency. At intermediate frequencies, roughly a few hundreds of MHz, the nonthermal radiation begins to dominate and continues to do so with decreasing frequency until the absorption by ionized hydrogen sets in at about a few tens of MHz. Studies of many astrophysical processes in our Galaxy rely on the separation of the two components. For example, since the synchrotron emission results from the interaction of the galactic cosmic ray electrons with the galactic magnetic fields, its study is important to learn about the distribution and energy spectrum of the energetic electrons, as well as about the distribution of the magnetic fields. The identification of the components has recently received special attention in the study of anisotropies of the cosmic microwave background (Kogut et al. 1996; Davies et al. 1996). The separation of the thermal and nonthermal components has been a problem not satisfactorily solved so far. In spite of the advantage that low-frequency observations (tex2html_wrap_inline1217 MHz) offer in this respect, very few surveys have been made in this frequency range, probably because of the many inherent difficulties. For example, it is necessary to build very large arrays in order to achieve good angular resolution, this usually limits the telescopes to be only transit instruments; the interference (man-made and natural) is a severe problem; the ionosphere can produce considerable changes in the incomig signal; depending on the design of the array the observations may be strongly affected by meteorological conditions (fog, rain, etc.). The design of the array imposes also limitations on the amount of sky that the instrument can cover. Ideally a survey should cover the whole sky, the observations should be made with instruments of similar characteristics and observing at the same frequency.

Most of the available galactic continuum surveys cover the galactic plane only. The few surveys covering the whole southern sky below 408 MHz and with single frequency observations are listed in Table 1 (click here). It is seen that half of the surveys have a very low angular resolution (tex2html_wrap_inline1219). The fourth column lists the temperature step between adjacent contours lines near a minimum at tex2html_wrap_inline1221, the fifth column shows the percentage of the background temperature corresponding to that step. This number intends to estimate the sensitivity of the system.

 

tex2html_wrap_inline1223

Coverage (tex2html_wrap_inline1225) Resolution (tex2html_wrap_inline1227) tex2html_wrap_inline1229 tex2html_wrap_inline1231 Instrument Reference
(MHz) (tex2html_wrap_inline1233) (tex2html_wrap_inline1235), (tex2html_wrap_inline1237) (K) (%)

200

-85 - +45 tex2html_wrap_inline1241 16 100 array Allen & Gum(1950)
153 -90 - +10 tex2html_wrap_inline1245 20 13 dish Hamilton & Haynes (1969)
100 -90 - +30 tex2html_wrap_inline1249 50 9 array Bolton & Westfold (1950)
85 -90 - tex2html_wrap_inline1253 tex2html_wrap_inline1255 100 10 dish Yates et al. (1967)
45 -86.1 - +19.1 tex2html_wrap_inline1259 250 6 array This survey
30 -90 - 0 tex2html_wrap_inline1263 1800 25 dish Mathewson et al. (1965)
16.5 -90 - 0 tex2html_wrap_inline1267 tex2html_wrap_inline1269 25 array Ellis (1982)
to to
2.1 tex2html_wrap_inline1271 tex2html_wrap_inline1273 25

Table 1: Continuum surveys covering the whole southern hemisphere below 408 MHz

 

Realizing the need of low-frequency observations the University of Chile begun in 1970 the construction of a large array tuned at 45 MHz (Reyes 1977), with the purpose, among others, of making a sky survey. Preliminary results were reported by Bitran (1981), and Bitran et al. (1981). The first results of the survey were presented by Alvarez et al. (1988).


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