As I expected, the blue channel produced the best grayscale image (the comet was mostly blue and thus little of it was recorded by the red and green sensitive layers of the film). The three large stars above the tail are on the handle of the Big Dipper.

Inverting the image showed the extent of the tail more clearly.

Next, I applied Photoshop's threshold command with the threshold set at 80, converting the grayscale image to black-and-white (bilevel). This further enhanced the tail. The profusion of white spots around the comet are image processing artifacts.

Finally, I inverted the bilevel image.