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Table 1: Expected thermal noise for continuum observations in the L-band. We assume 58 antennas and consider only robust -0.5, which is a good default for continuum imaging, with no confusion noise estimates or Gaussian tapering. Confusion is not reflected in this table since it is dependent on declination and imaging parameters. Please use the continuum sensitivity tool for a more realistic calculation.
Integration time | Thermal noise at robust=-0.5 (uJy/beam) excluding persistent RFI channels | |||
|---|---|---|---|---|
L-band | UHF band | S-band: S1 | S-band: S4 | |
12 minutes | 20.4 | 26.6 | 15.4 | 15.8 |
1 hour | 9.1 | 11.9 | 6.9 | 7.1 |
8 hours | 3.2 | 4.2 | 2.5 | 2.5 |
| Note |
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The calculators give the recommended time on target only. Calibration and slewing overheads need to be added to your time request. For longer (> 5 hours) single target observations following the standard calibration scheme an overhead of 25% can be assumed. Shorter observations, or slews to multiple targets, may incur higher overheads. Please refer to our page on average overheads for further details. Your observation can be simulated in the Observation Planning Tool (OPT). |
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The top left area consists of a series of drop-down boxes for the observation and imaging parameters, and the planned integration time. Note that the tapering field can be left blank (see empty option in the dropdown menu) and only Briggs weighting will be applied. Outputs are in the To only apply Briggs weighting, select ‘Untapered’ in the tapering drop-down box.
Outputs are in the top right - details of calculations are described below.
The plot can be used to determine the optimal robust weighting for the proposed observations. Extraneous curves can be turned off by clicking on the legends:
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Assumptions
L- and UHF bands
A system-equivalent flux density (SEFD) of 425 Jy in L-band and 550 Jy in UHF band, per antenna, is assumed. Note that the SEFDs do have a slope as a function of frequency - full plots can be found here.
The observatory minimum requirement for a science array is 58 antennas, though more are generally available. The calculator assumes 58.
While careful flagging can, in some instances, yield some useful data in bands dominated by GNSS and GSM signals, these bands are generally discarded by continuum observers. This gives worst case effective bandwidths as follows:
L-band | UHF band | |
Total bandwidth | 856 MHz | 544 MHz |
RFI loss | 45% (mainly GNSS / GSM downlink) | 10% (mainly GSM downlink) |
Rolloff low | 44 MHz | 36 MHz |
Rolloff high | 42 MHz | 73 |
Effective available bandwidth
385 MHz
MHz | ||
Effective available bandwidth | 385 MHz | 380 MHz |
S-band
Due to currently logistal issues, the sensitivity calculator assumes 54 antennas in operation at S-band. The sensitivity calculations are based on a limited set of measurements, and may be updated in future.
Sub-band | Effective frequency range excluding band roll-off (GHz) | Flag fraction (including band roll-off) (% ) | Mean SEFD (Jy) |
|---|---|---|---|
S0 | 1.8 - 2.55 | 40 | 365 |
S1* | - | 30 | 364 |
S2 | 2.3 - 2.95 | 30 | 365 |
S3 | 2.52 - 3.18 | 30 | 366 |
S4 | 2.75 - 3.38 | 30 | 369 |
*No separate measurements were made in S1. The SEFD is inferred from the overlapping sub-bands.
Details of the calculations
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The table below provides the channel widths for each mode in kHz, and and indication of the velocity width (which is dependent on the frequency of interest). Note that narrowband modes are currently not available for the UHF bandor S bands.
Mode | Channels | L-band channel width | UHF-band channel width | S-band channel width | |||
|---|---|---|---|---|---|---|---|
(kHz) | velocity width at 1420 MHz (km/s) | (kHz) | velocity width at 816 MHz (km/s) | (kHz) | velocity width at 2600 MHz (km/s) | ||
Wideband coarse | 4096 (4K) | 208.984 kHz | 44.12 | 132.812 kHz | 36.41 | 213.623 | 17.4 |
Wideband fine | 32768 (32K) | 26.123 kHz | 5.52 | 16.602 kHz4.55 | 4.55 | 26.703 | 2.18 |
Narrowband (107 MHz bandwidth) | 32768 (32K) | 3.3 kHz | 0.70 | - | - | ||
Narrowband (54 MHz bandwidth) | 32768 (32K) | 1.633 kHz | 0.35 | - | - | ||
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This calculator does not calculate the rms noise of a single pointing. This needs to first be calculated by using the continuum or spectral line calculators discussed above and entered into the appropriate field of the calculator. The mosaic calculator will take into account beam overlap to produce a plot of the combined sensitivity across the region of interest.
Recommended workflow
Use either the continuum or spectral line sensitivity calculator to calculate the rms noise for a single pointing, according to your planned observation and imaging parameters.
Determine your pointing grid, either from your own calculations or using the ‘Targets’ tab on the mosaic calculator.
Determine the resulting rms noise distribution across the map area.
Using the calculator
If you are not certain of the exact pointing centres to be used, but you have a boundary of the area that you wish to cover, the first tab of the calculator can be used to generate an optimal set of pointings. If you are uncertain of what the separation should be, it can be calculated for you based on the frequency to be optimised for. Note that MeerKAT’s large bandwidth implies almost a factor of 2 difference between the highest and lowest frequencies. General practice is to space the pointings by FWHM/sqrt(2), however, you may choose your own separation or pointing centres to concentrate on areas of particular interest.
| Note |
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Due to performance issues on the server in processing a large number of data points, we have set a minimum separation of 0.4 deg (which is somewhat smaller than the standard recommendation for the top of L-band). Please contact us if you have reason to use a smaller separation. |
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Calculate pointing centres
Decide on the area that you wish to image. Use your favorite image viewer or published image to calculate the nodes of a polygon to encapsulate the area of interest.
Decide on the separation that you wish to use. If uncertain the tool can compute it for you, based on the frequency of interest. For spectral line work, this would be at the observed line frequency. For continuum observations, you may wish to optimise at the highest observed frequency to ensure optimal signal-to-noise across the entire band.
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3. Enter or upload polygon nodes. An example csv file is shown below:
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