those measured in “typical” comets [@AHearn1995]. 21P was found to be also depleted in NH [@Schleicher1987; @Kiselev2000] and NH$_2$ [@Konno1989; @Beaver1990; @Fink2009].

The physical properties of 21P were investigated in the previous apparitions. Its nucleus size is not well determined, the average estimate of its radius is about 1-2 km [@Tancredi2000; @Krolikowska2001; @Pittichova2008]. Its rotation period is not well constrained, a large range from 9.5 hr to 19 hr was estimated [@Leibowitz1986]. 21P activity has shown an asymmetric light curve with respect to perihelion in the previous apparitions. In 1985, the production rates pre-perihelion were two times larger than post-perihelion at heliocentric distances of 1.0-1.5 au [@Schleicher1987]. The gas and dust maximum production was observed about one month before perihelion for the previous apparitions [@Schleicher1987; @Hanner1992; @Lara2003]. Both its unusual composition and the behavior of its activity during multiple apparitions make 21P an object of great interest. In addition, as it is the parent body of the Draconids, a study of its dust properties might be valuable. The 2018 apparition was very favorable for ground-based observations since the comet was close both to the Sun and the Earth at the same time and reaching high elevation. Many observed the comet 