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Imaging considerations: arrays and configurations

For the imaging goals of a project to be met, a suitable range of antenna baselines must be observed such that the requested angular resolution is achieved (longest baselines) and the largest angular structure in the source reliably imaged (short baselines). It will often be the case that the required range of baselines cannot be achieved with a single array configuration and that additional ones, including the ACA Total Power Array, might be needed. Which arrays are required will be decided by the OT automatically based on the user input.

To guide a user in entering parameters on the Control & Performance node, the OT displays various information regarding the configurations that are available in the advertised cycle. As it is not possible to show all the configurations, information is only displayed for the most compact and extended 12-m configurations and the ACA 7-m Array. This includes the angular resolution ( $\lambda / L_{\mathrm{max}}$) and Maximum Recoverable Angular Scale (MRAS - $0.6 \, \lambda / L_{\mathrm{min}}$) which have been calculated using simulations. The displayed values are declination-dependent and assume a robustness of 0.5 during imaging (the ALMA Pipeline default). Also shown is the beamsize of both the 12-m and 7-m antennas ( $1.13\,\lambda / D$) the size of which will determine whether multiple pointings are required to image the requested area. The Representative Frequency that was entered on the Spectral Setup page is used to calculate $\lambda$.

The requested angular resolution can lie anywhere between that achievable with the most extended and compact 12-m configurations, but the value corresponding to the ACA 7-m Array can also be entered - this triggers a ``stand-alone ACA'' mode i.e. no 12-m configurations will be scheduled. Any value can be entered for the Largest Angular Scale (LAS) and this should reflect the best estimate of the largest structure in the source that needs to be imaged. If the source is much smaller than the requested angular resolution, then entering a value of zero is acceptable, but otherwise the user should try and enter a realistic, non-zero value.

Once the angular resolution and LAS have been entered, the OT can calculate which configurations are necessary. If the OT has split the sources into multiple clusters (Section 5.3.2.2) the algorithm is applied separately to each Pseudo-Science Goal. The algorithm can be summarised as follows:


next up previous contents
Next: Sensitivity considerations Up: Control and Performance Previous: Control and Performance   Contents
The ALMA OT Team, 2016 Aug 23