2 The visual photometer

While photoelectric photometers and CCDs are readily available at most observatories to monitor night sky brightness, these instruments are rarely made for field use, since it is usually impractical to transport the associated electronics and computers. Nevertheless, small, portable photoelectric photometers have been especially constructed to measure night sky brightness (Treanor & Salpeter 1972; Walker 1973).

Figure 1: Schematic view of the Visual Photometer (VP). a) Cut away of the device. b) Details of the adjustable, square diaphragm

On the other hand, portable, visual photometers have been successfully used to measure night sky brightness with a precision of 10% (0.1 mag), (Berry 1976; Pike & Berry 1978). The visual photometer (VP) used in the observing campaign reported here is very simple. It consists of two plastic tubes, about 25 cm long and fully blackened inside, coupled together. One of them is closed; the other has a circular aperture at the end facing the sky and an adjustable, square diaphragm at the other end, just in front of the observer's eye (Fig. 1). This diaphragm opens continuously, from totally closed to about one square millimeter, by means of a calibrated precision screw. In this way, the open area of the diaphragm can be easily determined. When the observer looks through this device with the diaphragm totally open, a circular patch of sky of about 20 square degrees can be seen in one eye. The other eye is not used but can be left open since the corresponding tube is closed. Also, stray light is kept at a minimum by using rubber eyeguards around the tubes. Curiously, mild, refractive visual defects, such as myopia or hypermetropia, do not hamper measurements, since the small opening of the diaphragm in front of the observer's eye greatly increases its focus depth. To carry out a measurement the observer, with dark adapted eyes and using averted vision, closes the adjustable diaphragm until the patch of the sky seen through the VP is barely visible. The brighter the sky, the lesser is the diaphragm opening to reach this condition. At this point, the sky has been dimmed to the threshold of scotopic (dark adapted) vision for a uniform continuous source, which corresponds for the normal eye to about 27.0 mag/arcsec² (Allen 1973). This dimming can be expressed in magnitudes:

$$A = -2.5~{\rm log}~(D/P)$$ (1)

where A is the magnitude attenuation, D is the area of the diaphragm and P is the area of the pupil of the dark adapted eye. P is arbitrarily set to 38.48 mm², corresponding to a normal pupil diameter of 7 mm. Subtracting A from the scotopic threshold, we obtain the sky brightness in units of mag/arcsec². Of course, the scotopic threshold differs from observer to observer and it could also depend on the observer's age.