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In this section, the OT first presents a number of sensitivity-related parameters based on user input entered elsewhere in the Science Goal. These are designed to help users with the textual discussion of the sensitivity goals to be entered in the free-format text box below. Note that for multiple sources the most restrictive values of the Expected Source Properties are used i.e. the flux of the weakest source and the narrowest line width is assumed. If the OT detects potentially problematic issues, a message will appear in blue text and special attention should be paid to addressing these in the justification text.
The parameters reported by the OT vary depending on the contents of the Science Goal. The full list is:
- The signal to noise ratio (SNR) over the user-requested bandwidth for sensitivity: This calculation divides either the continuum or line flux by the requested rms. The continuum flux is used for single continuum setups (or where the bandwidth for sensitivity is more than twice the line width) and the line flux in the remaining situations. The user is notified if this or any of the other SNR values in the TJ node are less than three.
- The SNR over the aggregate, non-overlapping bandwidth: For spectral line (not single continuum) setups, this calculates the rms that would be achieved over the full, non-overlapping bandwidth (continuum rms) and subsequently the continuum SNR (continuum flux / continuum rms). This assumes that the entire bandwidth is free of spectral lines.
- The SNR over one third of the line width: For any Science Goal that includes line parameters in Expected Source Properties, the OT will calculate the rms over one third of the line width and calculate the corresponding line rms. In order to properly sample a spectral line profile, it is common to have approximately three spectral bins over the spectral line width.
- The ratio of the line width to the bandwidth for sensitivity: Again, if line parameters have been entered in Expected Source Properties, this calculation will be presented. Three spectral points are often placed across each line, although higher values are often used if the shape of the line profile is to be measured. The user is notified if this value is less than three.
- Spectral dynamic range: If both spectral line properties and a continuum flux have been entered, the ratio of the continuum and the line rms are reported as the ``spectral dynamic range''. The line rms is taken as the requested sensitivity, unless the bandwidth for sensitivity is greater than twice the line width, in which case the rms is that which would be achieved over one third of the line width. The spectral dynamic range is relevant as detecting weak lines against a strong continuum is difficult and requires good bandpass calibration. The OT will issue a warning message if the band-dependent limit is exceeded.
When calculating many of these parameters, the OT will use the time that will actually be observed, rather than the time implied by the sensitivity alone. For example, a minimum time per source is enforced and when calculating the rms over one third of the line width, the OT would use this time if the sensitivity request meant that the limit had not been reached.
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The ALMA OT Team, 2017 Dec 18