Photovisual observation of twilight

In the first approximation, the brightness of twilight sky is a function of the coordinates of a sky point and the position of the Sun at a particular instant. The objective of this work is to determine how does the sky brightness depend on these parameters from the photographic observations in V system. Besides that, the relation of the sky brightness upon limiting magnitude of visible stars is investigated. The knowledge of these dependencies helps a lot in preparing observing programmes to be done at twilight time.