docs: def offset(self, index=0):
          """Offset the camera pointing to be centred on a particular CCD."""
          eta = self._geometry[self.camera][index]["ra"]
          xi = self._geometry[self.camera][index]["dec"]
          ra = self.origin.ra - (eta/math.cos(self.dec.radian))*units.degree
          dec = self.origin.dec - xi * units.degree + 45 * units.arcsec
          self._coordinate = SkyCoord(ra, dec)