Short Observation & High Declination Observations
This short note on resolution expands upon the assumptions made in both the continuum and line sensitivity calculators, and is of particular importance to observers requiring high resolution for:
Short observations (including multi-target and mosaicing observations), and/or
Observations at high declinations above the equator (20 degrees declination and higher)
Both the continuum and line sensitivity calculators state resolution for transitory tracks at a given declination. This page includes a discussion on low elevation observation.
The resolution and shape of the Point Spread Function (PSF) of an interferometer depend on the weighted uv-coverage, which in turn is hour angle and declination dependent. In particular, short observations have poor uv coverage, which is exacerbated at hour angles far away from transit. Generally, this results in elongated beams and increased sidelobes which, when coupled with residual calibration error, is dynamic range limiting.
Planning multi-pointing short / mosaic observations
MeerKAT antennas have an elevation limit of 15 degrees that observation planners need to take into consideration when planning observations requiring a limited number of short scans on target fields intended for imaging. Large starting LST spans may result in low elevation observations, resulting in poorer than expected resolution.
The following viewgraph shows the effect of starting elevation on the achievable resolution of a short observation covering 10 degrees of elevation.
It is recommended that, if the observation length permits, users interleave multiple scans at different hour angles on each target, for targets with less than 3 hours of observation time to improve uv-coverage on individual fields.
The effect of track length on the resolution
The observing track length has a significant effect on the resolution attainable by, especially, the long baseline coverage. The following plot shows, as a function of continuous observation duration, the effect on achievable resolution for uniform-weighted uv coverage.
The strong resolution dependence on track duration mainly affects the uv sampling coverage greater than 1km. For most declinations, excluding polar declinations and declinations above the equator, there is little to no change in achievable resolution for core-tapered naturally weighted maps.
The effect of declination on the structure of the PSF
At declinations above +20 degrees the MeerKAT PSF elongates substantially. Below we summarize profiles of the PSF for a range of declinations. Observers wishing to propose observations that include targets above +20 degrees declination should take into consideration possible dynamic range limitations which, depending on the quality of transfer calibration and target field structure, may not be improved self-calibration. See Imaging simulations for more details on how to perform detailed simulations in determining final pointing coordinates before submitting schedule blocks via the OPT.
Declination (deg) | Maximum non-redundant uv track length (hrs) | Peak elevation (deg) (limit 15 deg) | UV coverage | Highest MFS resolution (asec) (BMAJ, uniform), 1.28GHz | Idealized PSF (uniform), 1.28 GHz |
|---|---|---|---|---|---|
-85 | 12 |
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-70 | 12 |
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-60 | 12 |
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-50 | 12 |
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-40 | 12 |
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-30 | 11.9 |
|  |
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-20 | 11.1 |
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-10 | 10.4 |
|  |
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Equatorial | 9.6 |
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10 | 8.7 |
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20 | 7.6 |
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30 | 6.2 |
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40 |
| 19.32 |  | 8.5 |  |
44 | 1 |
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