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Time Estimation

When a Science Goal time estimate is requested, a dialogue is displayed (Figure 5.9) that attempts to indicate how the total time, including calibration and overheads, was arrived at.

Broadly speaking, the OT time estimates are arrived at by (silently and invisibly) converting the information in the Science Goal into the Scheduling Blocks that will be executed at the telescope. It does this by taking the required on-source times, the default times for the various calibrations (frequency and array dependent), the best estimate of the current overheads and latencies, as well as a standard time for how much on-source time an SB will typically contain (50 minutes). The requested sensitivity can imply a very short amount of observing time and therefore the OT enforces a minimum amount of time that can be spent observing a single pointing (10 seconds). In addition, the total time for all sources combined cannot be less than 2 minutes.

Every observation will include an amplitude, bandpass and phase calibrator, and each of these has an associated pointing calibrator. The time spent observing a calibrator at each visit, and the time between visits (cycle time) is a function of band, the integration times being longer and the cycle times shorter at higher frequencies. Observations that require either long baselines or high frequencies also have a ``control'' source included. This is similar to a phase calibrator, but has a longer cycle time and can be used to check that the phase calibration has worked. This is listed in the dialogue as a delay calibrator. Full polarization observations also include a dedicated polarization calibrator.

In deriving its time estimates, the OT usually assumes that the observations in a Science Goal will go into a single SB (that might be executed more than once). The exception is for SGs that include sources whose velocities are different enough that they cannot be observed using the same spectral setup (Section 5.3.3.4). The OT assumes that these will be split into separate SBs and therefore that these incur additional overheads e.g. due to the time required to start up and stop each SB.

Other interesting cases include:

The above somewhat rigorous derivations are only carried out for the most-extended 12-m configuration. If any other configurations are required (a more compact 12-m configuration and perhaps the ACA 7-m and TP arrays) then their times are calculated as a multiple of the the total time (including calibration and overheads) required for the single 12-m configuration. The multiplicative factors were arrived at through extensive simulations and have been set as follows:

It is assumed that the ACA 7-m and TP arrays will be able to observe in parallel and therefore the multiplicative factor used for both ACA arrays in total will actually be equal to that for the TP array alone, as that is larger. As the compact configuration multiplier is less than 1, a minimum time for the resulting SB is enforced that is equal to the time required to perform the various calibrations.

Figure 5.9: Example of the Time Estimate dialogue. This is for a mosaic project and the time per pointing has been calculated assuming antenna beam overlap. Two 12-m configurations are required as is the ACA. Their times are calculated as multiplication factors of the largest 12-m configuration time, but note that the total time is equal to the total 12-m time plus the TP array time only - the ACA arrays will run in parallel and thus only the TP array time counts in forming the total time estimate.
Image TimeEstimate


next up previous contents
Next: More time required due Up: Control and Performance Previous: Sensitivity considerations   Contents
The ALMA OT Team, 2014 May 21