Cartes du Ciel is free software released under the terms of the
GNU General Public License
These icons are shortcuts for Chart → Field of Vision → ….
Here you can set the width/height of the displayed field of vision of your active chart to a predefined value (in degrees of arc). A click on the icon sets the FOV to 360°.
You can change the default settings for the fields of Vision by Setup → Chart, coordinates → Field of Vision.
These icons are shortcuts for Chart → View Horizon → …., with the exception for the Zenith icon.
If you are interested in the objects display on the chart situated in one of the cardinal directions, simply click the appropriate icon to make your choice:
Setting the chart by these methods to one of the cardinal directions, also forces the chart to use the Altitude-Azimuth coordinate system.