Observations were made in several modes during the working life of the telescope. In the early part of the observation period the telescope formed only one beam which could be moved in declination by operating the NS phasing switches. Long scans at fixed declination were made to observe the background emission as well as point sources, and short scans were made to measure the flux density of point sources with the beam switched frequently between different declinations. Only the scans of long duration were used to assemble the data presented in this paper. At a later stage, more automated phasing switches were added to permit rapid time-shared observations at five adjacent declinations. A large fraction of the present data was obtained from long scans with this equipment.
Scans were made at a set of standard declinations, chosen to sample the emission at half-beamwidth intervals. Since the NS beamwidth increased with increasing zenith angle, the standard declinations were not evenly spaced.
The observations were made mostly in the years 1965 to 1969 which
covered a period of fairly low solar activity. Nevertheless,
because of the low frequency of operation, the influence of the
ionosphere on the observations was often large. Observations of
point sources were affected by refraction, scintillation, and
absorption in differing degrees. A correction factor for
ionospheric absorption (primarily a daytime phenomenon) was derived from an
on-site 22 MHz riometer and was applied to the data.
Refraction amounting to a significant fraction of the (EW) beam
was detected only near times of sunrise in the ionosphere when
large horizontal gradients of electron density were present.
Scintillation
of "point sources'' (sources less than 15
diameter) was
a frequent occurrence. Due to lack of correlation of the
phase and amplitude variations over the extent of the telescope
array during times of severe scintillation, flux densities could
be seriously underestimated. To overcome this problem, many observations
of each source were made and measurements of flux density were derived
from only those observations judged to be the least affected.
By contrast, most observations of the extended emission were
largely unaffected by scintillation and, after
correction for absorption, could be reliably averaged together.
The data presented here are the result of averaging at least two
observations, and in many parts of the sky up to five good observations
contributed to the average.
Thus, the basic data set is a data array assembled from the averaged scans at the standard declinations. Because neighbouring averaged scans contained data observed at different times with varying conditions, the array displayed significant scanning artifacts. These were greatly reduced by the application of a Fourier filtering process in the declination dimension. The data were then interpolated onto a grid sampled at 1 minute intervals in right ascension and 15' in declination.
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