Standard magnitude systems
 
In order to allow direct comparisons between observing stations, internationally accepted detector systems and magnitude scales are used. An early system was obviously one related to the sensitivity of the eye and is known as the visual system (mv). It has now been virtually replaced by the photovisual system (mpv). By using orthochromatic plates which are sensitive up to about 5900 ˚A and by using a yellow filter to cut out the response below 5000 A˚ , the spectral response roughly matches that of the eye and the two magnitude scales correspond closely.
The photographic system (mpg) is obtained by using ordinary plates whose spectral sensitivities have not been deliberately extended and cut off at about 5000 A˚ . The sensitivity is, therefore, limited to the blue part of the spectrum down to the ultraviolet, where a cut-off is provided by the Earth’s atmosphere or by the telescope optics. The zero point of the photographic scale is chosen so that stars of a certain temperature and colour, corresponding to certain types of star (spectral type A5), have identical magnitudes on the visual and photographic scales when the visual magnitude is equal to 6·0. Stars chosen as standards are found in a catalogue known as the North Polar Sequence (NPS).
More modern magnitude systems depend on the combination of the spectral sensitivity of the cathodes of photoelectric multipliers and specially chosen filters. In the U, B, V system developed by Johnson and Morgan, three bands are used with equivalentwavelengths of 3650 ˚A (U), 4400 ^˚A (B) and 5500 ^˚A (V), corresponding to the ultraviolet, blue and yellow regions of the spectrum (see figure 1). Magnitudes measured in the V system correspond closely to those measured on the photovisual scale. Similar passbands to UBV have been established using CCDs and selected colour filters. Many other

Figure 1. The international UBV system.
Table 1. Standard photometric passbands.

systems are in use, including extensions to six or even more passbands using detectors with sensitivity in the infrared. Some of the more regular systems are given in table 1.