Sidgwick method

To determine the magnitude of comet by using Sidgwick method,  you should observe comet in a focal plane and remember its magnitude and angular size. After it, you should defocus comparison stars up to the such size, which comet coma has. After it, observer should compare the magnitude of comet, he remembered before, and the magnitude of defocused comparison stars.

Morris's method

Reference information --- comets and their observations

Comparison star sources