Pole sequence
The International Pole Sequence (IPS) or North Polar Sequence is a series of precisely measured stars near the celestial north pole . This scale of standard stars, finely graduated according to brightness , has been used since 1922 for the calibration of photometric instruments and for the calibration of measured star brightness .
The location of this calibration field at the celestial pole has the advantage of an almost constant elevation angle. As a result, the influence of the atmospheric extinction varies little and is easy to determine.
The pole sequence comprises 96 constantly shining stars, which cover the brightness range from +2 mag to +17 mag and are located at a distance of up to 2 ° around the pole star (Polaris). The brightnesses were determined photographically and photo-visual. The sequence of calibration stars was introduced in 1922, when it turned out that the polaris, which had previously served as a photometric reference star with +2.08 mag, had slight fluctuations in brightness.
Test cards for amateur astronomers
For amateur astronomy, Simon Newcomb designed three round, overlapping star maps up to mag 14 in 1921 , which are well suited for testing telescopes. They have the radii 5 ° (overview map I), 2 ° (map II) and ½ ° (map III) with correspondingly decreasing brightness limits . Reddish stars are marked with r .
In maps I and II, the polar star (α UMi) with 2.08 mag bears the number 1s, followed by stars 1 ( δ UMi , 4.37 m ) and 2 (5.28 m ). Map I extends up to 9 mag (star 5r = 8.63 m and 9 = 8.83 m ), map II up to 12.5 m (star 20) and map III up to 13.7 m (star 26).
Extensions of the pole sequence
In the 1960s, the pole sequence included Polaris and 61 stars from +4.4 to +16.9 mag with calibrated photo-visual magnitudes. After the redefinition of the magnitude system, it was expanded to 96 stars, but has lost its importance today.
In the meantime there are several hundred standard stars in different color ranges distributed all over the sky , the data of which can be precisely determined by electrical photo sensors . All in all, relatively precise brightnesses of over 500,000 stars up to the 19th magnitude are known.
History
The first steps in creating comparative stars for photometry go back to John Herschel (1792–1871) and his first development of a photometer . However, it was only with the invention of the prism photometer by Carl August von Steinheil (1801–1870) that brightness measurement became more precise. Ludwig v. In 1860 Seidel published a list of 208 standard stars that he had measured with the Steinheil photometer from 1852 onwards. It became the basis of scientific photometry for a few decades, especially since Norman Pogson defined the logarithmic magnitude scale in 1850 .
Further developments such as the Pickering and the Zöllner photometer enabled the exact measurement of brightness differences . They were mostly related to the Pole Star , which was set at 2.00 mag. The accuracy, which soon increased to at least 0.1 mag, made a correspondingly precise list of comparison stars necessary.
Numerous observatories and institutes began to work on such brightness catalogs, in Europe especially in Göttingen, Hamburg, Leipzig, Leyden, Vienna and Lemberg. The most important of these catalogs were the Göttingen actinometry , the Harvard photovisual photometry and the Yerkes actinometry , from which the North Pole Sequence (NPS) and the Harvard Pole Sequence arose around 1900 . Around 1920 they were transferred to the international pole sequence together with other brightness catalogs.
See also
literature
- A. Hnatek: Adjustment of the international pole sequence up to the 9th magnitude and brightness of some additional stars . In: Astronomical News . tape 204 , 1917, pp. 5 , bibcode : 1917AN .... 204 .... 5H .
- Rudolf Brandt : The telescope of the star friend (p. 34–36 "How do you test the performance of a telescope"). Kosmos-Verlag, Stuttgart 1958
- S. v. Hoerner, K. Schaifers: Meyers Handbuch über das Weltall , 2nd edition (370 p. + Map section), Bibliographic Institute, Mannheim 1960
- H. Zimmermann, A. Weigert: Lexicon of Astronomy , Spectrum Akadem.Verlag, Heidelberg-Berlin