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Field Source

A Field Source form looks very similar to the Field Setup form of a Science Goal (Section 5.3.2), the source position, proper motions, etc., being displayed in the first panel. However, where in a Science Goal it is only possible to define a rectangular mapping area or a set of single pointings, more options exist within an SB. Reference (offset) positions can be defined e.g. for atmospheric calibrations, and a number of field patterns can be selected.

Figure 6.3: Example of the query interface for a Field Source. This is for the phase calibrator and the source coordinates have been entered automatically by the generation. Unticking the button at the top would return this form to the fixed source format used in a Science Goal. The bar at the bottom also allows a fixed source to be chosen from the ALMA calibrator database.
Image FieldSourceQuery

An extremely important difference between the two is that within an SB it is possible to define a Field Source query (Figure 6.3). Here, instead of a specific source definition, this conducts a cone search around a position at the time of observation using the ALMA calibrator database. This is extremely useful as, at the time of SB creation, it is not normally known when the observations will be observed and therefore the positions of planets relative to the science target and the brightness of most calibrators are also highly uncertain. Any Field Source can be converted to a query by ticking the box in the Query Status (topmost) panel, and vice versa. In addition, this interface can be used to convert a dynamic calibrator (query) to a fixed calibrator by using a very similar interface to the Calibrator Selection Tool that is described in Section 7.5.

Reference positions are used during observations in order to integrate on a patch of sky that is usually devoid of significant emission. When measuring $T_{\mathrm{sys}}$, for example, a bright source will significantly add to the power detected by the antenna and increase the measured $T_{\mathrm{sys}}$. It is therefore usual to offset the antenna by a few arcmin in order to avoid the source. When observing Jovian or Saturnian satellites, the offset should be made bigger so as to ensure that the planet isn't observed by accident. Single-dish OTF mapping also requires an offset position so that the effects of changing atmospheric emission during the observations can be calibrated out. The ability to add a reference position to a Field Source is not possible for all observing intents e.g. pointing. The scanning used for the offset positions can currently only be done using equatorial i.e. J2000 and horizon coordinates. There are long-term plans to add Galactic to the list of supported systems. It does not appear that absolute offset coordinates are currently supported.

Of the various field patterns that it is possible to define, not all are currently supported and only point and rectangle are usually used. The inputs for both lie in the subsequent panel and change depending on which pattern is picked. Any observation that involves integrating when the telescope is not moving i.e. not OTF mapping, should use the point definitions. Each position in the table will be observed sequentially by the observing script, each for the subscan duration, before the total requested integration time for the source is reached. If necessary, the script will return to the first source in the list and continue. As it is usually the case that equal sensitivity is required per pointing, the total integration time should usually be set so as to ensure that each pointing is observed the same number of times. For SBs that have been generated from a Science Goal, the original rectangle definition will have been converted to a list of pointings for the ALMA 12-m and ACA 7-m antennas. similarly, it is possible to convert a rectangle definition in the SB to multiple pointings.


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The ALMA OT Team, 2017 Dec 18