def add_channel_cb(self, viewer, channel):
        """Called when a channel is added from the main interface.
        Parameter is channel (a bunch)."""
        fitsimage = channel.fitsimage
        fitssettings = fitsimage.get_settings()
        for name in ['cuts']:
            fitssettings.get_setting(name).add_callback(
                'set', self.cutset_cb, fitsimage)
        fitsimage.add_callback('transform', self.transform_cb)

        rgbmap = fitsimage.get_rgbmap()
        rgbmap.add_callback('changed', self.rgbmap_cb, fitsimage)

        # add old highlight set to channel external data
        channel.extdata.setdefault('thumbs_old_highlight', set([]))
Called when a channel is added from the main interface.
        Parameter is channel (a bunch).