The OH survey observations of the Bulge region were taken with the
Australia Telescope Compact Array (ATCA) during 14 days in 1993 October
and November. The ATCA consists of six radio telescopes each
22 m in diameter, located along an east-west track, at a geographic latitude
of 
. At a wavelength of 18 cm,
the primary beam of each antenna has a full width at half maximum (FWHM)
of 
.
The array was used in the 6A configuration, which has 15
baselines ranging from 0.34 to 5.94 km. For sources
in the Bulge region (at declinations of 
) the longest
baseline of 33 k
(5.94 km) corresponds to an angular resolution of
approximately 6
in right ascension and 12
in declination.
The observations of the Bulge region consisted of
a total of 539 pointing centres
in the region 
and 
.
The grid
contains 13 rows of constant galactic latitude with an offset of
in galactic longitude between adjacent positions within a row.
Adjacent rows are offset by 
in galactic latitude and are
shifted by 
in galactic longitude. For the 30
primary
beams of the antennas, the arising honeycomb grid pattern
provides an almost complete coverage of the survey region (see Fig. 1 (click here)).
The data were taken in two linear polarizations, using a
total bandwidth of 4 MHz and 1024 spectral channels (channel
separation 3.9 kHz, correlator frequency resolution 4.69 kHz).
The spectral band
was centred at 1612 MHz, offset by 0.231 MHz from the rest frequency
of the OH groundstate transition at 1612.231 MHz
(
). No Doppler tracking
(to correct for the Earth's motion around the Sun) was used during the
observations. During the observing period of 14 days spread over 5 weeks,
the velocity range covered by the observations
varied over 12 
, between (
) and
(
). Doppler corrections (radio definition)
to the observed frequencies
were applied off-line, and the data were Hanning smoothed to give a velocity
resolution of 1.45 
(7.8 kHz). After Hanning smoothing,
every other spectral channel was discarded, as well as all channels within
9 
of the edges of the spectral bandpasses.
In the resulting data the channel separation exactly equals the
intrinsic velocity resolution. All velocities
are given with respect to the local standard of rest (LSR),
assuming a velocity of the Sun of 19.7 
towards
right ascension = 18:07:50.3, declination = +30:00:52 (J2000.0).
Figure 1:
Grid showing the distribution in galactic coordinates
for the pointing centres used in the survey. The diameter of the symbols
reflects that of the images (42
). This corresponds
to a primary-beam attenuation of 0.25 (Sect. 5.1). The dashed line indicates
a point at the intersection of 3 fields. That point has the largest possible
offset (19
) from all
surrounding pointing centres in the inner regions of the survey
To optimize the u-v sampling for each pointing centre and to minimize the telescope drive times, the observations were taken using a mosaic\ procedure in the following manner. Each day a single row of the grid was observed, together with calibration sources, for a total time of approximately 12 h. Each pointing centre on a row was observed for 50 s, after which the telescopes were driven to the adjacent position. After completing the scans on the row, the secondary calibrator source 1748-253 (approximately 1.15 Jy at 1612 MHz) was observed for 5 min. This procedure was then repeated cyclically, so that each pointing centre in the row was observed typically 9 times during the 12 h period, giving a total on-source integration time of typically 7.5 min. For absolute flux density calibrations, the calibrator sources 1934-638 and/or 0823-500 were also observed at the start and end of each 12 hour period. The flux density of 1934-638 at 1612 MHz was taken to be 14.34 Jy (Reynolds 1994). The flux density of 0823-500 was taken to be 5.82 Jy.