Arbitrary coordinate systems

Coordinate type

In addition to standard longitude/latitude coordinates, APLpy supports any valid quantities for coordinates (e.g. velocity, frequency, …) as long as the WCS header information is valid. To differentiate between longitudes, latitudes, and arbitrary scalar values, APLpy keeps track of the axis coordinate ‘type’ for each axis, and will try and guess these based on the header. However, it is possible to explicitly specify the coordinate type with the set_xaxis_coord_type() and set_yaxis_coord_type() methods in the FITSFigure object:

f = FITSFigure('2MASS_k.fits')

Valid coordinate types are longitude, latitude, and scalar. Longitudes are forced to be in the 0 to 360 degree range, latitudes are forced to be in the -90 to 90 degree range, and scalars are not constrained.

Label formatting

How label formats are specified depends on the coordinate type. If the coordinate is a longitude or latitude, then the label format is specified using a special syntax which describes whether the label should be decimal or sexagesimal, in hours or degrees, and indicates the number of decimal places. For example:

  • ddd.ddddd means decimal degrees, where the number of decimal places can be varied
  • hh or dd means hours (or degrees)
  • hh:mm or dd:mm means hours and minutes (or degrees and arcminutes)
  • hh:mm:ss or dd:mm:ss means hours, minutes, and seconds (or degrees, arcminutes, and arcseconds)
  • or means hours, minutes, and seconds (or degrees, arcminutes, and arcseconds), where the number of decimal places can be varied.

If the coordinate type is scalar, then the format should be specified as a valid Python format. For example, %g is the default Python format, %10.3f means decimal notation with three decimal places, etc. For more information, see String Formatting Operations.

In both cases, the default label format can be overridden:


Aspect ratio

When plotting images in sky coordinates, APLpy makes pixel square by default, but it is possible to change this, which can be useful for non-sky coordinates. When calling show_grayscale() or show_colorscale(), simply add aspect='auto' which will override the aspect='equal' default.