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Field definitions

Figure 5.6: The Science Goal Field Setup form for a rectangular field definition.
Image FieldSetup

There are two kinds of field definition in a Phase-1 Science Goal, one or more individual pointings around a source position, or a single rectangular area. Depending on which of these is chosen, the contents of the third panel at the bottom of the page will change. Rectangles will ultimately either be observed as a collection of individual pointings i.e. a mosaic (12- and 7-m arrays) or scanned continuously in multiple rows (Total Power antennas). As the response of the antenna to the astronomical signal drops off with increasing distance from the pointing centre, the effective noise in a single pointing is position-dependent. This is compensated for by overlapping the pointings and the row scans. It is not currently possible to define more than one source per Science Goal if this uses a rectangular field definition.

For multiple pointings, a list of positions can be entered in a table in the Forms editor (Fig. 5.6) or can be placed onto an image using the Visual Spatial Editor (Chapter 8). As with the sources within a Science Goal, the individual pointings within a single source cannot be more than 10 degrees apart. If these pointings are ultimately to be pipeline processed as a mosaic, this should be indicated by checking the ``Custom Mosaic'' box - in this case all fields should be overlapping. Otherwise, each pointing will produce a single image.

For a rectangular field, the size and orientation can be entered manually at the bottom of the Forms editor, or it can be drawn by hand using the Visual Spatial Editor (Chapter 8). Once a rectangle has been defined, the OT will automatically calculate a mosaic pattern of evenly-spaced single pointings that will cover the defined area, both for the 12-m and 7-m arrays. The number of pointings for each array will be displayed in the Forms Editor and displayed in the Visual Spatial Editor. The latter will also display the area that will be scanned by the TP array if the ACA has been selected. If you would like to use the mosaic pointings in your own software, or as input to CASA's simdata task, their positions can be exported to file.

The spacing parameter can be specified in either angular units or as a fraction of the antenna beam (``main'' beam), but defaults to the Nyquist value in both cases. This is the value, $\lambda/(\sqrt{3}\,D)$, that is appropriate for a hexagonal/triangular mosaic pattern, the OT's default. As the antenna beam size is $1.2\,\lambda / D$, the actual value reported in beam units is $1/(1.2\,\sqrt{3})$ i.e. 0.48. Particularly for projects that are mapping large structures, this should not be changed as it ensures maximum coverage of short $u,v$ spacings. Projects which simply aim to cover a large area can use larger spacings, but non-uniform sensitivity will eventually result. Non-Nyquist spacings must be discussed in the Technical Justification node.

Regardless of the how the various pointings have been defined, it is not currently possible to submit a proposal with more than 150 12-m array pointings.


next up previous contents
Next: Spectral Setup Up: Field Setup Previous: Resolving source information   Contents
The ALMA OT Team, 2014 May 21