The observing patterns that are used to observe a source fall into three categories:
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 the further away a source is 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 5.3.3.5).
Although the TP array is most commonly required for sources that are mosaiced using a rectangular definition, it can also be used with field patterns created using individual pointings e.g. custom mosaics or even single pointings. As no rectangle is entered by the user in this case, the OT calculates the area required for the ACA using an algorithm that calculates the smallest rectangle that encompasses all the pointings.
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.