It consists of an LN2 cooled Tektronix 1k detector, a standard MPIA readout electronic and a photographic widefield lens of 28mm focal length, equipped with a standard Johnson V filter. It is controlled by a SUN workstation. The instrument has a field of view of 55 degrees and is oriented towards the north pole covering an airmass range of approx. 1.1 to 2.4.

The monitor is housed in a hut which can be removed completely. The hut opens and closes automatically at twilight. It is equipped with rain sensors and connected to the Calar Alto weather station in order to close or beeing kept closed under unfavourable weather conditions.

The CCD camera takes permanently images with 20s integration time. These images are analysed by a MIDAS programme. Each image contains some 15 - 20 selected stars brighter than mag 6.0 which are detected by an algorithm that calculates their position on the detector depending on the sidereal time. Their intensity is measured and an extinction curve is calculated. One cycle needs about 1 min. The error of a single measurement is 0.02 mag/airmass, so a floating mean of 5 cycles is taken, reducing the error to about 0.015 mag/airmass under photometric conditions. This means that after approx. 5 min the first extinction coefficient is available. From then it is updated every minute.

Additionally a reference star near the pole is monitored in order to have a rough estimate about the sky quality in case of non photometric conditions.