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Sources

In the Field Setup page, the user is asked for source parameters, the most important of which are the name, coordinates and velocity. The source names should only contain the following characters: a-z, A-Z, 0-9, -,+, _ or a . (full stop). Characters other than these will be removed or replaced when the project is validated.

The source coordinates must be entered in the FK5 system i.e. the reference frame based on the Fifth Fundamental (star) Catalogue with equinox and epoch both equal to J2000.0 (this system is often simply referred to as ``J2000''). This dynamical reference system has been superseded by the International Celestial Reference System (ICRS), a frame based on the positions of extragalactic radio sources and which, therefore, is fixed in space (does not rotate with time). This, however, is not yet supported by the telescope control system. The two components of the source proper motion, Dec. and R.A. (cross declination), are both true angles on the sky and are used to convert the given source position to an observable one assuming that it refers to epoch J2000.0.

The ALMA telescope control system recognises a limited number of solar system objects that can be selected from a drop-down list. This same list also allows the use of an ephemeris for any object for which the coordinates as a function of time can be provided. This is loaded as a text file from the local disk and must be in the JPL HORIZONS format. For details on how to generate HORIZONS ephemeris files, please check Appendix A. When using an ephemeris at Phase 1, it is recommended that only a skeletal example be attached to the Science Goal, using a time interval of one day to cover the period over which the observations are likely to take place.

The velocity can be specified using the optical, radio, or relativistic convention. A number of different frames are available and the must useful of these are heliocentric, barycentric, topocentric and lsrk. The velocity is used to convert the rest frequencies of the spectral windows to the observed equivalent and for barycentric and heliocentric, this is done in the same frame i.e. the ``observed'' velocity will remain in that frame. In the case of lsrk though, the OT performs an additional conversion to the barycentric frame. When changing the source velocity reference system between the available options of heliocentric, barycentric, topocentric, lsr and lsrk, the OT is not able to convert the velocity and you will be reminded of this via a pop-up dialogue.

The velocities must be sufficiently similar such that all the sources can be observed in a single band. In addition, a maximum of five tunings per Science Goal are allowed and thus the sources must be grouped in velocity space in such a way that this limit is not exceeded. The OT will issue an error if this is the case. This issue is explained more in Section 5.3.3.8.


next up previous contents
Next: Resolving source information Up: Field Setup Previous: Field Setup   Contents
The ALMA OT Team, 2015 Sep 03