The angular resolution of the TGSS will be around 20 arcsecs and it will be
superior to several existing surveys as shown in the figure.
TGSS will be several times better than the 7C survey at the same frequency and several
times more sensitive than the VLSS at 74 MHz as shown in the figure.
The pointing centres of the fields to be observed at 150 MHz are shown in a figure.
We follow Becker et al. (ApJ, 1995, 450, 559) in placement of the pointing centers
of the fields.
Observing Strategy :
Our optimised observing strategy was arrived upon by
conducting several experiments using test time allocated by the
Centre Director, NCRA and Chairman, GTAC.
The observing run starts with a 10m scan on an amplitude calibrator (one of
3C48, 3C286 or 3C147). Then we observe the chosen phase calibrator.
Then a chosen set of eight neighbouring pointings are observed (each for 3.5m)
before the phase calibrator is reobserved. Depending on the declination of
the pointings, either another set of eight pointings is observed or else
the same eight pointings are reobserved to get good coverage over five
Thus, in a typical 13h observing run, we are able to observe on average 32
Reduction in some of the overheads such as slew time and other control commands
can result in a larger number of pointings being accomodated in the same time.
We select the pointing centres such that the HA coverage for the different pointings
is more or less similar. Figure shows the uv coverage for the field R01D41
(Pointing centre: ra=00h00m00.0s; dec=11d39'12s).
The 150 MHz band with an IF bandwidth of 16 MHz is used.
This results in the observing band of 140 to 156 MHz. Both the IF and baseband bandwidths
are selected to be
16 MHz. The correlator is used in the USBpolar mode.
The hardware correlator is configured to result in 128 channels
and the software correlator to give 256 spectral channels
across the 16 MHz bandwidth. This results in a channel width of 125 and 65 kHz respectively.
The ALCs (automatic level controls) are switched on.
We use a pre-integration time of 2s which leads to a large increase in the data volumes
(about 6 GB per hour of observing and about 120 GB for about a 13h observing run). Using
small integration times
has proved to be effective in identifying and editing radio frequency interference.
We follow Becker et al. (ApJ, 1995, 450, 559) in placement of the
field centers of the pointings. Click here
to see an example of the pointing centres.
The spacing between the two pointing centers is equal to the half power
width of the 150 MHz primary beam ie about 3 degrees and each pointing
covers roughly about 7 sq deg.
Results from test data of 30 hours acquired in 2007 which helped
us arrive at the observing strategy for the 150 MHz survey can be found
Comparing angular resolution of TGSS with other surveys.
The sensitivity of TGSS quantified as 5sigma detection limit of 40 mJy is compared
with other surveys.
The TGSS pointings are shown in the figure.
We follow Becker et al. (ApJ, 1995, 450, 559)
in placement of the pointing centers of the fields.
Aitoff projection of Galactic coordinates is shown below.
The green line is the Galactic plane and each point represents
a TGSS pointing centre. The unfilled region is the sky south of declination -35 degrees.
UV coverage for a pointing near declination 11 deg.