When using this method, the observer should first select several comparison stars of neighboring comets (some are brighter than the comet, and some are darker than the comet). Then follow the steps below:
(A) Adjust the focal length of the telescope to make the star and comet have similar apparent size (that is, the star is not on the focal plane of the telescope, forming an external image, called defocus).
(B) Adjust the focal length back and forth, and interpolate comets between a pair of bright and dark stars (see Morris method for interpolation method).
(C) Repeat the second step between several pairs of comparison stars.
(D) Take the average value measured in the second and third steps and record it to 0.1 magnitude. [1]
