174p 20171213 024100 ndj

Here's a comparison almost exactly a day apart taken with the same
instrument (iTel T24 at Cloudcroft). The coma diameter has gone from
23.3 arcsec to 29.6 arcsec which I think implies a physical expansion
rate at the comet of around 170m/s.

I get the following unfiltered magnitudes in a 31 arcsec diameter
aperture and using UCAC-4 R mags as reference:

20171211.94495 13.59    # C11 + FLI6303, Chelmsford
20171212.12214 13.65    # iTelescope T24 (FLI-PL09000), Cloudcroft
20171213.11196 13.59    # iTelescope T24, Cloudcroft