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Multiple velocities and tuning solutions
In both Spectral Line and Single Continuum cases, it is possible to enter sources with different velocities. For Single Continuum, this is irrelevant as the setup is defined in the sky (topocentric) frame and thus the source velocities are ignored. For Spectral Line though, the velocities are crucial as these are used to convert the rest frequencies to their observed equivalents. In general, this means that each source requires its own tuning and its own set of calibration observations. This greatly increases the complexity and time required to complete a project and thus each Scheduling Block is restricted to five separate tunings.
The number of required tunings is calculated automatically by the OT in the following way:
- Firstly, a tuning solution is calculated for each of the
sources that lie within a 10 degree-wide cluster. As described in Section 5.3.2.2, the OT can now automatically split the sources present in a SG into these clusters.
- For each cluster, the OT then selects a source (
) and loops through all the other
sources to see if they could be observed with
's tuning i.e. does a line at the centre of each spectral window fit into the spectral windows of source
? In performing this check, the OT takes the user-defined line width into account.
- This is then repeated for every source
. This leads to, for each source, a list of other sources that are compatible with its tuning. These are referred to as ``sets''.
- Any sets that are a subset of another are discarded at this point.
- Finally, the OT tries to find the smallest subset of these sets which would enable all sources to be observed. This part of the process is computationally intensive and the time required grows rapidly with the number of input tunings. The OT therefore has a limit of 16 input sets. If this number is exceeded, the OT will issue a validation error and the user will need to reduce the number of sources that have been entered.
The final number of tunings (sets) found by the algorithm is reported in the Time Estimate dialogue (Section 5.3.5.3). As already stated, no more than five are allowed per SB and the user may have to split the sources into multiple SGs.
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The ALMA OT Team, 2016 Aug 23