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The demerit maps were constructed by searching the SUMSS catalog south of δ = −35° and the NVSS catalog from δ ≥ −35° to δ = +10° over a fine grid of potential pointings in the ~24000 degree area defined by δ < 10°. At each grid position we computed D from flux densities shifted to 1.28 GHz assuming S ∝ ν−0.7 and with parameters conservatively appropriate to MeerKAT: Primary Beam FWHM θb = 68' (L-band), θb = 107' (UHF), rms pointing error σp = 30'', and rms receiver gain fluctuation σg = 0.01. At each position in the grid we included SUMSS and NVSS sources out to radius ρ = 3° in L-band and ρ = 5° in UHF-band, extending beyond the second sidelobe of the MeerKAT primary beam. Figures 4-5 plot the cumulative percentage of the sky area with demerit scores < D. If the demerit score at the proposed position is in the top 80-100% of sky shown in Figures 4 or 5 , it is recommended that the proposer simulate their potential pointing using sources from SUMSS/NVSS to determine what impact direction dependent errors will have.
The ‘critical’ (i.e. 80%) cumulative Demerit score for each bandsis:
UHF: 17.2 mJy/beam
L: 9.8 mJy/beam
S: 4.4 mJy/beam
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The demerit maps are in FITS format with file size of ~ 800 MB. |
Mitigation
The effect of such bright sources can be modelled using CASA and MeqTrees. A quickstart guide is available here. In some cases, the worst effects of a bright source could be mitigated by adjusting the pointing centre in such a way as to move the bright source away from regions of rapidly changing beam response. See the page on the primary beam for more information.
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