Sky background brightness is 21.5 for a dark site, 18.5 for a city site.
The chart plots the visibility of extended objects like nebulae and galaxies. The object is visible when the eye's perceived contrast is greater than zero. However, because of the impreciseness of object magnitudes and object sizes, it is better to divide the log contrast into zones. Log contrasts greater than 0.5 are easy, contrasts down to 0.25 are visible, contrasts between -.25 and 0.25 are difficult and log contrasts under -0.25 are not visible. For comparison, half the aperture and double the aperture are also plotted. The plot stops if the object is too big to fit into a 100 degree apparent field of view eyepiece.
The visibility is plotted logarithmically to match the eye's performance. The contrast is the object's brightness + the sky background brightness divided by the sky background brightness, mapped to the eye's response at different sky background brightnesses, object brightness and apparent object size. To be clear, the object's contrast never changes. But the eye's ability to detect the object depends on three factors: the sky background brightness (light pollution and exit pupil), object brightness and apparent size of the object.
The chart is plotted for a range of exit pupils. The exit pupil is calculated by dividing the eyepiece's focal length in mm by the telescope's focal ratio. For instance, a 30mm eyepiece with a f/5 telescope produces a (30/5) 6mm exit pupil. A 10mm eyepiece with the same telescope yields a 2mm exit pupil. A 6-7mm exit pupil is typically the widest field lowest practical magnification and a 1/2 to 1mm exit pupil the narrowest field highest practical magnification. The apparent sky background brightness at the eyepiece is solely mediated by the exit pupil. In other words, at a given location, the sky background brightness will be identical regardless of eyepiece and telescope when the exit pupils are the same.