Standard Photometric Systems Michael S Bessel l ResearchScho ol of Astronomy and Astrophysics The Australian National UniversityWeston ACT Australia KEYWORDS metho dsdata analysis techniques photometricsp ectroscopic catalogs ABSTRACT Standard star photometry dominated the second part of the th century reaching its zenith in the s It was intro duced to take advantage of the high sensitivity and large dynamic range of photomultiplier tub es compared to photographic plates As the quantum eciency of photo detectors improved and the wavelength range extended further to the red standard systems were mo died and rened 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 suered until more suitable observational techniques and standard reduction pro cedures were adopted But the biggest revolution o ccurred with the pro duction of allsky photometric surveys HipparcosTycho w as space based but most like MASS were groundbased dedicated survey telescop es It is very likely that in the future rather than mak ing a measurement of an ob ject in some standard photometric system one will simply lo ok up the magnitudes and colors of most ob jects in catalogs accessed from the Virtual Observatory In this review the history of standard star photometry will b e outlined and the calibration and realization of standard systems will be examined Finally mo del atmosphere uxes are now very realistic and synthetic photometry oers the b est prosp ects for calibrating all photomet ric systems Synthetic photometry from observed sp ectrophotometry should also b e used as a matter of course to provide colors within standard systems and to gain insights into the sp ectra and colors of unusual stars star clusters and distant galaxies CONTENTS Intro duction Background The worldwide web resources Basic astronomical photometry assumptions The nature of standard systems Development of standard systems changes in detectors and wavelength coverage Synthetic photometry Broadband photometric systems The JohnsonCousins UBVRI system The Washington CMT T system The Sloan Digital Sky Survey ugriz system The HipparcosT ycho H B V system p T T The HST WFPC w system Intermediate band photometric systems The Stromgren color uvby system The DDO system Annu Rev Astron Astrophys The Geneva UBB B VV G system The Vilnius UPXYZVS system The Walraven WULBV system Narrowband photometric systems The Oke AB magnitude system The Wing color system Infrared photometric systems Introduction Infraredspectrophotometry The JohnsonGlass JHKLMN system The MKO JHKL M system The MASS JHK System The DENIS iJK System SpaceBased IR Photometric and Spectrophotometric Systems Observational complications with standard photometry Complications to the realisation of standard systems Standard systems may no longer represent real natural systems Concluding remarks Intro duction Background One of the ma jor achievements of th century astrophysics has b een the unravelling of the evolution of stars and the understanding of the creation of elements in stars and in sup ernov a explosions Another highlight has b een work on the formation and evolution of galaxies stimulated by cosmological nb o dy simulations and large telescop e observa tions of distant galaxies Central to all this work is the quantitative photometry and sp ectroscopy of stars and stellar systems For historical reasons these brightness mea surements are normally given in magnitudes an inverse logarithmic scale and magnitude dierences b etween dierentwavelength regions are called colors Muchofastrophysics revolves around the p osition o ccupied by stars in the luminos ityversus temp erature plane known as the HertzsprungRussell HR diagram The life history of a star is traditionally describ ed by the path it traces in the HR diagram The observational version of the HR diagram is the ColorMagnitude diagram The integrated magnitudes and colors of star clusters and galaxies is often analysed in similar diagrams for integrated systems Much eort observational and theoretical go es into converting for example the visual magnitude and color of a star into its total absolute ux b olomet ric magnitude and eective temp erature related to the surface temp erature in order to derive the mass comp osition and age of the stars This involves empirical relations derived from stars with known luminosities and measured radii together with theoretical mo del atmosphere computations The integrated magnitudes and colors of star clusters are also of great interest as they the constituent stars Similarly the can b e used to derivethe age and comp osition of integrated magnitudes and colors of galaxies can b e analysed in order to determine the nature and prop ortion of their constituent stars and gas and study galaxy evolution Such p opulation synthesis is a p owerful to ol in the study of the distant universe as well as the lo cal system for example Lancon Ro ccaVolmerange Ng Common to all these endeavours is the necessity to place the measurements onto a standard physical ux scale by removal of the absorption by the Earths atmosphere Standard Photometric Systems and the calibration of the sensitivity of the photometricsp ectroscopic equipment at dierentwavebands For mainly historical reasons but also for go o d practical reasons astronomical photometric observations are calibrated through the use of networks of constant brightness stars rather than lab oratory based calibration lamps of constant temp erature These networks of stars comprise the various standard star systems that this review will examine Three signicant pap ers on photometry have been published in this journal Oke outlined the AB absolute photometric system of pseudo mono chromatic pho tometry in which the uxes are on an absolute ux scale and the magnitude dierences colors are directly related to ux ratios This system is the basis of all sp ectrophoto metric calibrations In a very inuential and forward lo oking pap er Johnson describ ed the UB VRIJHKLMN system of broad band photometry extending from nm to that he had established This system forms the basis of all subsequent broadband systems and initiated IR astronomical research Fluxes in the Johnson system are normalized to that of Vega so conversion of Johnson magnitudes to absolute uxes require multiplication bytheuxofVega Johnson provided tables of intrinsic colors for dwarf and giant stars temp eraturecolor calibrations and b olometric corrections for stars with dierent color with whichtoderive total ux from their measured V magnitude In the same volume Stromgren outlined the intermediateband uvby system that he had devised to b etter measure the temp erature gravity and reddening of early typ e stars This system revolutionised quantitative photometry byproviding more precise estimates of temp erature gravity metallicityandinterstellar reddening for stars hotter than the sun It also stimulated the establishing of other systems b etter suited for studying co oler stars Wider discussions of photometry and photometric practice are provided in the follow ing selected b o oks Problems of Calibration of Multicolor Photometric Systems Philip High Sp eed Astronomical Photometry Warner Astronomical photometry A guide Sterken Manfroid Multicolor Stellar Photometry Straizys Handb o ok of Infrared Astronomy Glass Handb o ok of CCD Photometry Ho w ell The web also provides access to course notes and other useful photometric information some of which is describ ed b elow Since the last reviews there have b een great changes in detectors and a proliferation of photometric systems some related some dierent There are at least three outstanding photometric questions for astronomers that this review will help answer These are What are the precisions of the dierent photometric systems What are the conversions between the magnitudes and colors in the dierent photometric systems and how reliable are they How well are the passbands known and can the extant standard system be theoretically realised using mo del atmosphere uxes and observed sp ectra The worldwide web resources The advent of the worldwide web has op ened up the way for ready access to standard star and standard bandpass databases As with muchontheweb it is necessary to havea reasonable appreciation of what one is viewing in this case standard star systems their limitations and strengths b ecause