![]() ![]() The WFC and HRC flux calibrations in terms of new PHOTFLAM values are updated in ACS ISR 2020-08. Use the ACS Zeropoints Calculator or to calculate them yourself, follow the instructions on the ACS webpage on zeropoints. The "Vega magnitude" of a star with flux F is The spectrum of Vega used to define this system is a composite spectrum of empirical and synthetic spectra ( Bohlin & Gilliland 2004). The VEGAMAG system uses Vega (α Lyr) as the standard star. Any new determination of ACS's absolute efficiency will result in revised magnitudes for these three photometric systems that are based on absolute physical flux. The choice between observational and flux-based systems is mostly a matter of personal preference. ![]() 1986) are directly related to physical units. The commonly used photometric systems ABMAG ( Oke, J. VEGAMAG is a widely used standard star system, defined as relative photometry to the actual star Vega Vega magnitudes can be converted to absolute effective fluxes using the flux distribution of this HSTsecondary standard star 1 in the CALSPEC database. The absolute effective flux for any ACS filter and for any Spectral Energy Distribution (SED) can be computed from the throughput for the entire system (OTA + ACS CAMERA + FILTER + DETECTOR) according to Equation 3 of Bohlin et al. (2005) but such transformations have limited precision and strongly depend on the color range, surface gravity, and metallicity of the stars.įor ACS filters, three magnitude systems are commonly used: VEGAMAG, STMAG, and ABMAG all of which are based on absolute flux. Transformations to other photometric systems are possible (see ACS ISR 2019-10 and Sirianni et al. It is strongly recommend that, whenever practical, ACS photometric results be referred to a system based on its own filters. 5.1.1 Photometric Systems, Units, and Zeropoints ![]()
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