Photometric system

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In astronomy, a photometric system is used to determine the apparent brightness of stars in different spectral ranges of light .

In order to be able to describe the spectroscopic properties of the stars more precisely, a set of filters , also called bands, are used for different wavelength ranges . Such a filter set together with information about the transmission properties of the filters forms a photometric system.

An important property of the filter is the isophotic wavelength , i.e. the wavelength of the center of gravity of the sensitivity function.

A photometric system is calibrated with its own list of standard stars .

Depending on the size of the spectral range that the filters cover on average, one differentiates:

  • broadband systems ( broad-band )
  • narrowband systems ( narrow-band )
  • an intermediate band .

If the properties of a celestial body are viewed and compared in different bands, one speaks of multiband photometry . The color index can be defined to compare the brightnesses in different filters .

Historical development

With the introduction of photographic astronomy towards the end of the 19th century, it turned out that the photographic brightness of the stars did not match the visual brightness of the star catalogs . The reason for this is that the spectral sensitivity of the photographic plates used does not match the sensitivity of the eye. Brightness information therefore requires information about the spectral sensitivity of the detector used and the absorption properties of the filters used.

Examples

  • The simplest system is the photovisual system, which determines the visual brightnesses with an isophotic wavelength around 510 nm with photographically determined brightnesses and those with orthochromatic emulsions . The isophotic wavelength of an orthochromatic emulsion is further in the blue region of the spectrum around 450 nm.
  • The most widely used photometric system, which indicates luminosity in the near ultraviolet , blue and visual range, is the broadband UBV system , originally defined by Johnson and Morgan in 1953. The most common modification today is the Bessell system from 1990, which is mainly based on the versions of Johnson and Cousins ​​and is therefore often referred to as the Johnson-Cousins ​​system . There are also extensions to the infrared range, see UBV system #Extensions .
  • Another successful photometric system is the Strömgren-Crawford system with the uvbyβ filters (β refers to the H-beta line from the Balmer series ). In this system, astrophysical parameters such as temperature or the Balmer jump can be measured precisely using the color indices . In particular, interstellar extinction and redness can be estimated using the system.
  • The other two common intermediate systems from Geneva, the Geneva (UBB 1 B 2 VV 1 G) system , and from Vilnius, the Vilnius (UPXYZVS) system also have a similar advantage . In the latter system, the color indices for de-reddened O stars form the zero point by definition, so the color indices of all normal stars are positive.
  • Various large telescopes and space probes use their own filter systems. This includes the u'g'r'i'z 'system of the Sloan Digital Sky Survey . The Gaia space probe also uses its own, newly developed system, see G-band magnitude .

Web links

Individual evidence

  1. ↑ Krautter, Joachim et al., Meyers Handbuch Weltall , 7th edition 1994, ISBN 3-411-07757-3 , p. 232
  2. ^ A b Michael S. Bessell: Standard Photometric Systems . In: Annual Review of Astronomy & Astrophysics . tape 43 , no. 1 , September 2005, p. 293–336 , bibcode : 2005ARA & A..43..293B .
  3. ↑ Krautter, Joachim et al., Meyers Handbuch Weltall , 7th edition 1994, ISBN 3-411-07757-3 , p. 231 ff
  4. ↑ Kitchin, CR, Astrophysical Techniques , Third Edition 1998, ISBN 0-7503-0498-7 , p. 263
  5. ^ Bessell, MS: UBVRI passbands . In: Publications of the Astronomical Society of the Pacific . tape 102 , no. 1181 , October 1990, p. 1181-1199 , bibcode : 1990PASP..102.1181B .
  6. ^ E. Paunzen: A new catalog of Strömgren-Crawford uvbyβ photometry . In: Astronomy & Astrophysics . tape 580 , A23, August 2015, p. 1–3 , bibcode : 2015A & A ... 580A..23P .