The imaging goals depend crucially on the baselines of the six configurations and the OT displays the maximum and minimum baseline lengths of the most compact and most extended configurations and the angular resolution (
) and maximum recoverable angular scale (
) corresponding to each. Note that the source declination is not taken into account when calculating these quantities. Also shown is the beamsize of both the 12-m and 7-m antennas (
) 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
.
Based on these displayed values, it should be possible to enter the requirements for the proposed observations. The angular resolution, for example, 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. Depending on the entered value, the OT will now choose the configuration that achieves an angular resolution at least as good as that requested. This choice is displayed in the ``Time Estimate'' dialogue (see below).
As a configuration has been chosen, the OT can now compare the largest angular scale in the source that you wish to image, with the maximum recoverable angular scale of that 12-m configuration. If it is not possible to image the requested structures, shorter baselines will be required and thus the OT will recommend that ACA observations also be scheduled. These will always include the 7-m array - the TP array will normally also be used unless continuum observations have been requested (the TP antennas do not currently have nutators). However, if the requested angular resolution is high enough that the two largest configurations are required (C32-5 and C32-6), the OT will not allow complementary ACA observations as there will not be enough overlap between the 7-m and 12-m array configurations. Point sources will, of course, never need the ACA.
Whether or not the ACA is required can be tested by clicking the ``Suggest'' button at the bottom of the page. This will show a dialogue giving the result, the reasons why the ACA is or is not needed, as well as automatically selecting or unselecting the ACA as appropriate. As long as the largest 12-m array configurations are not required, the OT's ACA choice can be overridden, but this will produce a validation warning and will need to be justified in the attached PDF.