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| Kron-Cousins | |
|---|---|
| Name | Kron-Cousins |
| Other names | RC, R_C, Cousins R, Kron R |
| Introduced | 1970s |
| Developers | E. Krisciunas; A. W. Cousins; G. Kron |
| Bandpasses | R_C, I_C |
| Wavelength range | optical (red to near-infrared) |
| Typical use | photometry of stars, variables, supernovae, galaxies, clusters |
| Related | Johnson–Morgan photometric system, UBVRI system, Sloan Digital Sky Survey photometric system |
Kron-Cousins is a photometric system widely used in optical astronomy for measuring red and near-infrared fluxes with standardized R and I passbands. It combines filter designs and calibration practices that trace back to work by G. Kron and A. W. Cousins and has been adopted in studies of stars, supernovae, novae, variable stars, open clusters, globular clusters, galaxies, and surveys such as the Sloan Digital Sky Survey and various ground-based observatories.
The Kron-Cousins system defines red and near-infrared photometric bands, typically labeled R_C and I_C, intended to improve on earlier Johnson–Morgan photometric system filters by offering better match to detector sensitivities and atmospheric transmission. It is extensively used alongside the UBV bands and integrated into the modern UBVRI system. Major observatories and instruments, including the European Southern Observatory, Cerro Tololo Inter-American Observatory, Kitt Peak National Observatory, Palomar Observatory, and the Anglo-Australian Observatory, have applied Kron-Cousins standards in calibration of imaging data.
Kron-Cousins provides a two-band extension that complements the UBV system to form a five-band sequence often used for color indices, spectral classification, and photometric parallaxes. The system emphasizes empirical standard stars established by A. W. Cousins and subsequent networks expanded by researchers at South African Astronomical Observatory, Mount Stromlo Observatory, and other institutions. Calibration stars include lists overlapping with catalogs from Hipparcos, Tycho Catalogue, and targeted sequences used by the Hubble Space Telescope and ground-based follow-up programs for Type Ia supernovae.
The R_C and I_C bandpasses were defined to account for typical photocathode, photomultiplier tube, and later CCD response curves, alongside atmospheric OH and H2O features that affect long-wavelength transmission at observatories like Mauna Kea and La Silla Observatory. Typical effective wavelengths place R_C near 640 nm and I_C near 790–800 nm, though exact values depend on detector and telescope optics such as those at Subaru Telescope, Very Large Telescope, and Keck Observatory. Filter manufacturers and instrument teams at Bessell, Schott glass suppliers, and detector groups often publish transmission curves for instruments like the Mosaic camera, ANDICAM, and CCD arrays used on CTIO 4m and WIYN Observatory.
Standardization relies on photometric zero points determined from observations of standard stars by A. W. Cousins, later expanded by networks including those compiled by Landolt and others at South African Astronomical Observatory. Transformation equations relate instrumental magnitudes from CCDs and photomultipliers to the Kron-Cousins system using color terms tied to indices like (V–R)_C and (R–I)_C, often referencing color standards from Johnson, Morgan, and Schaefer. Observers correct for atmospheric extinction measured at sites such as La Palma and Cerro Tololo, and tie flux scales to spectrophotometric standards maintained by institutions like the National Optical Astronomy Observatory and calibration programs for Gaia cross-matches.
Kron-Cousins photometry underpins many observational programs: light curves and color evolution of Type Ia supernovae used in cosmological distance work; monitoring of Cepheid variables and RR Lyrae for distance ladders; color–magnitude diagrams of open clusters and globular clusters for stellar evolution studies; galaxy color studies in surveys such as COMBO-17 and follow-up of Sloan Digital Sky Survey objects; and calibration of photometric redshifts for deep fields observed by Hubble Space Telescope and ground-based facilities. It is also used in characterization of brown dwarfs and late-type M dwarfs, where the I_C band helps separate molecular band effects.
Compared with the original Johnson–Morgan photometric system, Kron-Cousins filters provide improved match to modern detectors and reduced sensitivity to atmospheric features. The Sloan system (u'g'r'i'z' and ugriz) used by SDSS employs different filter shapes and absolute calibration tied to AB magnitudes, whereas Kron-Cousins often uses Vega-based zero points as in Landolt standards. Instruments on Hubble Space Telescope employ distinct filter sets (e.g., WFPC2, ACS, WFC3) with transformations derived to Kron-Cousins for ground–space comparisons. Other systems like the Strömgren photometric system and 2MASS near-infrared bands cover complementary wavelength ranges and diagnostic power for stellar parameters.
The system evolved from photometric work by G. Kron and the precise standardization by A. W. Cousins in the mid-20th century, with significant expansion by observers at South African Astronomical Observatory, Mount Stromlo Observatory, and catalog compilers like Arne B. Landolt. Instrumental adoption involved teams at Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and major surveys such as SDSS that required cross-calibration. Modern refinement continues through contributions from observatory calibration groups, filter manufacturers, and projects like Gaia and Pan-STARRS that link wide-area photometry to established Kron-Cousins standards.
Category:Photometric systems