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 Polsequence	wide range of brightness	wide field, needs air mass correction, costly, outdated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 calibration
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 R_c magnitudes available, but only two clusters with Johnson R CCD magnitudes.