Harald Lang wrote in message ...

To summarize -

Naked-eye-limiting-magnitude to background brightness
conversion table

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Backgrd
brightns Limiting magnitude
Ba Knoll/Schaefer Blackwell/Clark

18.4 4.30
19 4.77 5.80
20 5.49 5.81
21 6.12 6.56
22 6.62 7.17
23 7.02 7.59
24 7.31 7.83
25 7.52 7.95

.. . .

Given the visual limit, the apparent background brightness Ba can be
had from the inverse of the formula above:

Ba = 21.58 - 5 log(10^(1.586-lim_mag/5) - 1)

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Excerpt from Nils Olof Carlin internet page Schaefer's paper at:

http://w1.411.telia.com/~u41105032/visual/Schaefer.htm