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Sensitivity considerations

The time required to achieve the requested sensitivity depends crucially on the spectral setup, particularly the Representative Frequency (this sets, amongst other things, the atmospheric opacity) and the spectral resolution. If the sensitivity is requested in kelvins, the angular resolution will also have an effect. The Representative Frequency should have been chosen to reflect the most important spectral feature that is to be imaged, although in parts of the spectrum where the opacity is well behaved, which spectral window is chosen will actually make very little difference to the time estimate.

How the entered sensitivity is interpreted also depends on whether a rectangular field definition has been selected. If so, the sensitivity entered should be that required in the final mosaiced map, not that corresponding to a single pointing. As the pointings in a mosaic usually significantly overlap, the combined sensitivity is higher and thus requires less time per pointing. For Nyquist sampling, and assuming a Gaussian beam shape, the sensitivity of a mosaic is 68 per cent better than that of a single pointing and the time required per pointing is reduced by a factor 2.8. The OT will calculate this factor for the selected spacing and use it when estimating the time for the project. In contrast, the senstivitity entered for a custom mosaic is always for a single pointing.

It is also necessary to define the bandwidth over which the sensitivity should be calculated. This will depend on the scientific goals, but usually refers to the full usable bandwidth for continuum observations, or to some fraction of the line width otherwise. The OT provides a number of shortcuts for completing this parameter and ``AggregateBandwidth'' (the sum of the non-overlapping bandwidth of all spectral windows) and ``RepresentativeWindowResolution'' (the resolution of the representative spectral window, including Hanning smoothing and any spectral averaging) are the defaults for Single Continuum and Spectral Line setups, respectively. Finally, a user-defined value is possible, this usually being set to a width in velocity units.

At this point, enough information has been entered for a time estimate to be calculated.


next up previous contents
Next: Time Estimation Up: Control and Performance Previous: Imaging considerations: arrays and   Contents
The ALMA OT Team, 2018 Sep 25