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| AB magnitude system | |
|---|---|
| Name | AB magnitude system |
| Type | Photometric magnitude system |
| Introduced | 1983 |
| Creators | Oke and Gunn |
| Base unit | Spectral flux density |
| Zero point | 3631 Jy |
| Used in | Astronomical photometry, Survey calibration, Spectrophotometry |
AB magnitude system
The AB magnitude system is a flux-based photometric scale used in observational astronomy for expressing spectral flux density and comparing photometric measurements across instruments and wavelengths. It defines magnitudes by reference to a constant spectral flux density per unit frequency, allowing transformations between photometry and spectrophotometry for instruments, surveys, and observatories. The system underpins modern projects in wide-field imaging, spectral energy distribution fitting, and cross-calibration among facilities.
The AB magnitude is defined by a logarithmic relation linking observed spectral flux density to a fixed zero point. In its canonical form, m_AB = -2.5 log10(f_ν) - 48.60, where f_ν is the spectral flux density in erg s^-1 cm^-2 Hz^-1; equivalently the zero point corresponds to 3631 Jansky. This formulation connects to standard units used at European Southern Observatory, National Radio Astronomy Observatory, Space Telescope Science Institute, and survey teams such as Sloan Digital Sky Survey and Pan-STARRS. The expression assumes a monochromatic or narrowly band-integrated flux; practical usage integrates f_ν over an instrument transmission function, requiring convolution with filter throughput curves maintained by observatories like Mauna Kea Observatories and facilities such as Subaru Telescope and Keck Observatory. Conversion between frequency and wavelength representations invokes the relation f_λ = (c/λ^2) f_ν, linking the AB definition to spectrophotometric products from missions like Hubble Space Telescope and James Webb Space Telescope.
The AB system was formulated to address inconsistencies among earlier magnitude scales and to provide a spectrally flat reference useful for modern detectors. It grew from discussions in the late 20th century involving researchers at institutions including California Institute of Technology, University of Arizona, Harvard University, and instrumentation groups supporting projects such as Palomar Observatory Sky Survey and UKIRT. The practical adoption accelerated with large digital surveys—most notably by the Sloan Digital Sky Survey—and with calibration needs for space missions from European Space Agency and National Aeronautics and Space Administration. Key contributors worked in the context of standards established at conferences organized by bodies like the International Astronomical Union and committees within the International Organization for Standardization affect measurement metadata conventions used in archives at NASA/IPAC and European Space Agency Science Archives.
The AB magnitude system contrasts with the Vega-based system and the STMAG system. Vega magnitudes anchor zero colors to the spectrum of the star Vega, historically tied to calibrations at Mount Wilson Observatory and catalog work by teams at Royal Greenwich Observatory. STMAG uses flux per unit wavelength and a different zero point, relevant for instruments on Hubble Space Telescope instruments developed by teams at Ball Aerospace and Goddard Space Flight Center. Transformation between AB and Vega systems requires synthetic photometry using stellar spectral libraries from projects at European Southern Observatory and empirical standards observed by Cerro Tololo Inter-American Observatory and La Silla Observatory. Surveys like Two Micron All Sky Survey and instruments such as Wide-field Infrared Survey Explorer often report magnitudes in Vega or AB after applying color terms derived from spectrophotometric calibrators including CALSPEC standards.
Implementing AB magnitudes requires precise knowledge of instrument throughput, detector response, atmospheric transmission, and filter bandpasses. Calibration pipelines at facilities such as Subaru Telescope, Gemini Observatory, Large Synoptic Survey Telescope teams, and space missions adopt standard-star networks and synthetic photometry against spectrophotometric standards maintained by Space Telescope Science Institute and national metrology institutes like National Institute of Standards and Technology. Flat-fielding, bias subtraction, and aperture corrections are performed in data reduction systems developed by groups at European Southern Observatory and software packages like those from Astropy Project collaborators. Absolute calibration to the AB zero point commonly uses observations of white dwarf standards or composite spectra cross-referenced to radio calibrators from Very Large Array programs and millimeter facilities like Atacama Large Millimeter/submillimeter Array.
The AB system is widely used in photometric redshift estimation, spectral energy distribution fitting, galaxy evolution studies, and multi-wavelength survey science. Projects such as Sloan Digital Sky Survey, Pan-STARRS, Dark Energy Survey, and planned surveys for Vera C. Rubin Observatory rely on AB-calibrated catalogs for cosmology, stellar population analysis, and transient detection in programs coordinated with European Southern Observatory and space missions like Hubble Space Telescope and James Webb Space Telescope. Cross-matching catalogs from radio arrays like Karl G. Jansky Very Large Array and X-ray observatories such as Chandra X-ray Observatory benefits from AB-consistent photometry when constructing broadband spectral energy distributions for active galaxies studied by teams at Max Planck Institute for Astrophysics and Harvard–Smithsonian Center for Astrophysics.
Systematic errors in AB magnitudes arise from imperfect knowledge of instrument bandpasses, atmospheric variability at sites like Mauna Kea and Paranal, detector nonlinearity for instruments at Keck Observatory and Gemini Observatory, and uncertainties in spectrophotometric standard spectra including those maintained by Space Telescope Science Institute. Color terms, filter aging, and mismatches between synthetic and observed photometry introduce biases that affect precision cosmology programs led by collaborations at Lawrence Berkeley National Laboratory and Fermi National Accelerator Laboratory. Mitigation involves regular re-calibration, cross-survey comparisons, and propagation of covariance matrices in data releases by consortia such as Sloan Digital Sky Survey and Dark Energy Survey.
Category:Photometry