North polar sequence

The International Polsequenz ( IPS) or North Polar sequence is a series of precisely calibrated stars near the north celestial pole. This finely graduated scale on the brightness of standard stars used for the photometric calibration of instruments and calibration of measured brightnesses of stars.

The location of this gauge field at the celestial pole has the advantage of almost constant elevation angle. Thus, the influence of atmospheric extinction varies little and is easy to determine.

The Polsequenz includes 96 constant luminous stars that the brightness range from 2 to 17 might like cover and are located at a distance of up to 2 ° around the North Star (Polaris ). The magnitudes were determined photographically and visually photo. The sequence of the calibration stars was introduced in 1922, as it turned out, that until then may as photometric reference star with 2.08 serving Polaris has low brightness fluctuations.

In the 1960s, the Polsequenz included not only Polaris 61 stars from 4.4 to 16.9 mag and was extended to 96 stars by the redefinition of the magnitude system. Today it has lost its importance as now a few hundred distributed over the whole sky standard stars of different color ranges are available, the data are more accurate by photoelectric measurement methods.