  We report on imaging and spectroscopic observations of comet C/2016~R2
(Pan-STARRS) obtained with the 0.8\,m and 2.7\,m telescopes of McDonald
Observatory in November and December 2017 respectively. The comet was at a
heliocentric distance greater than 3\,{\sc au} during both sets of
observations. The images showed a well-developed tail with properties that
suggested it was an ion tail. The spectra confirmed that we were observing
well-developed bands of CO$^+$ and N$_2^+$. The N$_2^+$ detection was
unequivocally cometary and was one of the strongest bands of N$_2^+$ detected
in a comet spectrum. We derived the ratio of these two ions and from that we
were able to derive that N$_2$/CO = 0.15. This is the highest such ratio
reported for a comet.
  Note erratum - N2/CO=0.06 not 0.15.
