Standard Photometric Systems

Standard star photometry dominated the second part of the th century reaching its zenith in the s It was introduced to take advantage of the high sensitivity and large dynamic range of photomultiplier tubes compared to photographic plates As the quantum e ciency of photodetectors improved and the wavelength range extended further to the red standard systems were modi ed and re ned and deviations from the original systems proliferated The revolutionary shift to area detectors for all optical and IR observations forced further changes to standard systems and the precision and accuracy of much broad and inter mediate band photometry su ered until more suitable observational techniques and standard reduction procedures were adopted But the biggest revolution occurred with the production of all sky photometric surveys Hipparcos Tycho was space based but most like MASS were ground based dedicated survey telescopes It is very likely that in the future rather than mak ing a measurement of an object in some standard photometric system one will simply look up the magnitudes and colors of most objects in catalogs accessed from the Virtual Observatory In this review the history of standard star photometry will be outlined and the calibration and realization of standard systems will be examined Finally model atmosphere uxes are now very realistic and synthetic photometry o ers the best prospects for calibrating all photomet ric systems Synthetic photometry from observed spectrophotometry should also be used as a matter of course to provide colors within standard systems and to gain insights into the spectra and colors of unusual stars star clusters and distant galaxies