The Twilight unit was recently used in the verification of Alshaahed Hilal Visibility Law (HVL). The unit used at Alshaahed was developed with special-purpose multiple filters & Temperature compensation, providing brightness in magnitudes per square arc second at regular intervals. The system is capable of readings from a remarkable 2 mpsas all the way to 26 mpsas!

 

Figure: Background sky brightness measured while tracking  a twilight summer sky with Alshaahed Setup

Verification of the HVL, which is based on contrast measurements between the new crescent moon brightness  and the background sky in the visible wave lengths. The setup tracks the motion of the background sky as the new moon descends towards the horizon during Twilight.

The photo below depicts the setup in the outskirts of Boulder, Colorado ( at exactly 40 deg latitude).