Calibration of Photometric System 

In differential multi-color photometre it is recomended to calibrate the photo-metric system - e.g. Johnson-Cousins, Stroemgen e.g. For the determinatoin of the transformation coefficients, it is required making a set of images from calibrated reference stars.

Possible reference standards are:

  • Landolt-Sequence, 1973 updated 
  • Int. Pol-Sequence (Hnatek, 1917)
  • Selection of white dwarfs (ESO 2010)
Reference System Advantage Disadvantage
Landolt fields approved reference system needs air mass correction, 
small amount of stars within
fiels, costly
Int. Polsequence wide range of brightness wide field, needs air mass
correction, costly, outdated
White dwarfs (e.g. ESO) stabil source (also known variable white dwarfs) only two stars in Nothern
hemisphere, only one spectral
typ, not suitable for color
Open clusters many stars with wide
range of color index, usually cluster fits
to FOV even for longer
focal length
quality of catalog datas
varies, these are secondary
reference sources, determination of extinction is possible

For declination -30 < +90 degrees WEBDA offers more than 100 open clusters, for which are Cousins Rc magnitudes available, but only two clusters with Johnson R CCD magnitudes.