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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 12-m array configuration alone and that additional 12-m array configurations and perhaps the ACA 7-m and Total Power arrays are needed. Based on the user input, the OT will automatically calculate how many 12-m configurations are required and will give a suggestion (which should usually be followed) as to whether the ACA should be requested.

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. This includes their maximum and minimum baseline lengths, as well as the angular resolution ( $\lambda / L_{\mathrm{max}}$) and Maximum Recoverable Angular Scale (MRAS - $0.6 \, \lambda / L_{\mathrm{min}}$). 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 angular resolution can lie anywhere between that corresponding to the most extended configuration, and twice that of the most compact configuration. The first limit simply states that it is impossible to achieve a higher angular resolution than that provided by the largest configuration, whilst the latter reflects the possibility that an image may ultimately be smoothed. 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 strive to enter a realistic, non-zero value.

The OT's algorithm for selecting configurations can be summarised as follows:

The configurations chosen will be used to set the time required for the Science Goal and a breakdown in the times is given in the Time Estimate dialogue. Note though that the actual 12-m array configurations can in practice differ from those chosen by the OT, partly because ALMA will be being constantly reconfigured and thus never conform exactly to one of the model array configurations, but also because the ultimate choice of what configuration will be used is ultimately a matter for the scheduling team. Therefore, the OT does not specify which configurations it assumed in its calculations.


next up previous contents
Next: Sensitivity considerations Up: Control and Performance Previous: Control and Performance   Contents
The ALMA OT Team, 2015 Sep 03