How dark is your sky? How does it compare with other skies?
CCD images can be used to measure sky-glow intensity. A simple measurement of the image's background level is sufficient to compare skies that have been imaged with the same scope, camera, setup, filter, and exposure time. But to compare images from different setups it is necessary to mathematically normalize these elements. The most difficult elements to normalize are spectral QE and filters, but because the Kodak CCDs in the ST-7/8/9/10 “e” non-ABG cameras have very similar QE it is relatively simple to compare skies based on unfiltered images from these cameras via "Standard Flux":
e- / arcsec2 / meter2 / second
The calculator below will automatically normalize your system and compute the standard sky flux. Additionally, it will estimate the magnitude of the sky background. (A more accurate method of measuring sky background is to calibrate via a reference star of known magnitude, which is applicable to all CCDs and most filters).
For normal conditions with no moon, your standard flux should be somewhere between 50 and 500, or 21 to 17 mag/arcsec2. The finest professional observing sites have mag 21+ skies. Rural skies are usually mag 19-20. Suburban areas generally have mag 18-19 skies, while urban skies are as bright as mag 16.