This close-up of the northwestern section of Messier 67 is based on a total exposure time of 70 minutes. Careful image processing was then used to reveal the dimmest possible stars.
An article by Bradley Schaefer in Sky and Telescope (March, 1989, p. 332) gives the magnitudes of selected stars in this open cluster ranging from about magnitude 10 to 21. The stars with their magnitudes listed in this image show that 18th magnitude stars are easily seen, 19th magnitude stars are quite dim, and a star of magnitude 20.1 is just barely visible.