2017t2 20200120 1843 ndj

 Observer
Nick James
Time of observation
20/01/2020 - 19:00
Object
C/2017 T2
Observing location
Chelmsford, UK
Equipment
C11 + WO 90mm refr.
Exposure
20x60s, 60x60s
This is a comparison of the total magnitude of C/2017 T2 obtained comphot with a slow (0.28m,f/10) and fast (0.09m, f/5) optical system. The total magnitude of a comet is critically dependent on how much of the faint outer coma is detected. To a lesser extent it is dependent on the colour response of the sensor. In this example of images taken at the same time the fast system gives a result which is 1 magnitude brighter than the slow system. The current COBS lightcurve gives a scatter of almost 2 mags in electronic estimates of the comet. Visual observers in dark skies are seeing around mag 9. The mean of the electronic observations is around mag 10.