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Field definitions
The observing patterns that are used to observe a source fall into two categories:
- Rectangular area - a uniformly-spaced pointing pattern that covers the rectangle is calculated by the OT, for both the 12- and 7-m Arrays. The TP Array will observe a larger version of the user-defined rectangle and this is displayed in light blue in the visual editor. If one of the sources is defined using a rectangle, all the sources in the SG must be.
- Multiple individual pointings which are to be mosaiced together - if the source extent is not easily approximated with a rectangle, then a custom mosaic can be used to cover the source more efficiently. All the pointings must overlap and the Pipeline will produce a single image. If the ACA is required, the OT will automatically calculate the smallest rectangle that encompasses the 12-m pointings. This will then be tiled with 7-m Array pointings using Nyquist sampling and redundant pointings removed. The TP Array will raster scan the rectangle.
In both cases all the pointings must overlap such that there are no gaps in the coverage i.e. one contiguous source is being mapped. If it is desired to map multiple separated areas of emission, each must be entered as a different source.
For rectangles, the 12- and 7-m Array pointings that are calculated by the OT are overlapped so that the final map will have approximately the same sensitivity at each point - this is necessary as the sensitivity of a radio telescope drops away from the pointing centre. The spacing parameter defaults to the Nyquist value,
, this being appropriate for the OT's hexagonal/triangular mosaic pattern. As the antenna beam size is
, the actual value reported in beam units is
i.e. 0.51. Particularly for projects that are mapping large structures, this should not be changed, but projects that aim to cover a large area of relatively small sources can use a larger value. The value of
is set from the Representative Frequency (Section 5.3.3.6).
Both rectangles and individual pointings can be defined using the Visual Spatial Editor (Chapter 8) if desired. This shows the user-defined rectangle, the 12- and 7-m antenna pointing positions, as well as a larger rectangle that corresponds to the area that will be covered by the Total Power Array. Individual pointings can additionally be read into the OT from an ASCII file, whilst all pointings can be written to file, including those derived by the OT from a rectangle.
Next: Import/Export of Source Information
Up: Field Setup
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The ALMA OT Team, 2017 Dec 18