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Shape

The shape frame displays information about the spatial structure of the component. It always shows the reference direction of the component, the reference frame and the shape type. If the shape type in not a point there will be additional fields containing the other parameters relevant to specific shape.

The reference direction is the for all currently available component shapes the direction of the centre of the component. It can be displayed in a variety of units, formats and precisions using the pull-down menu to the right of the displayed values. Because the direction values can be entered in a variety of different formats entering these strings is more prone to error. Hence I recommend you always press return after entering a value to see how the editor has interpreted your string. The previously mentioned note about loss of precision when in edit mode is particularly relevant in the direction fields. If sub-arcsecond precision is important to you set the RA units to 'H:M:S.s' and the declination units to 'D.M.S.s' prior to entering edit mode.

The reference frames allowed for the direction are currently limited to those that do not require additional information about when are where the observation took place. Currently this is 'B1950', 'J2000' & 'Galactic' frames.

Three component shapes are currently supported are 'point', 'Gaussian' & 'disk'. For the Gaussian and disk shapes there are three additional parameters, namely the major axis, minor axis and position angle required. Fields displaying these parameters will be displayed if the shape is not a point shape.

The major axis is always larger than (or equal to) the minor axis and the position angle is the angle between the major axis and the North-South axis. The north-south axis is defined using the current reference frame and hence a component with a position angle of zero degrees in the galactic reference frame will have a very different position angle if the component is converted to the J2000 frame. As with other fields in the componenteditor the units are specified using pull-down menus to the right of the displayed values.