Conversion from mid-infrared magnitudes to flux, or vice-versaPlease enter the magnitude or flux value below along with either a wavelength -- for the "monochromatic" case -- or a filter name, and this form will return the conversions between these quantities. The "monochromatic" conversions apply for wavelengths between 1.0 and 35.0 microns. These "monochromatic" values are based upon the Cohen et al. Spectral Irradiance Calibration in the Infrared series of papers. The "system" is described in the paper Cohen, Walker, Barlow, and Deacon, 1992, Astronomical Journal, vol. 104, 1650-1657. Zero magnitude flux values have been derived from the Cohen spectral template for alpha CMa and the listed magnitude values from this paper. The formal uncertainties in the values are about 2%. Note that the MSX calibration work implies that the alpha CMa Cohen template values are low by 1%. For wavelengths outside this range conversions from wavelength to frequency flux density are still calculated, but no magnitude values can be calculated. I have also included absolute flux conversions for a variety of other filters, mostly for optical and near-infrared filters but including some filters on mid-infrared instruments/satellites as well; the references for these are given below. This is a intrinsically tricky business. For the wider filters "colour corrections" are an issue when converting from magnitude to flux. Even for the narrow-band filters there are other issues in transformation and standardization. The paper M. S. Bessel (2005; Annual Reviews of Astronomy and Astrophysics, volume 43, page 293) discusses standard photometric systems and the transformation between them at some length...but it gives no information about conversion from magnitudes to fluxes. A magnitude value may be obtained for one of various optical through mid-infrared filters, which has to be chosen above. The alternative is use the "monochromatic" value which is found directly from the alpha CMa spectral energy distribution template given by Martin Cohen. In that case the zero magnitude value is interpolated to the specified wavelength. Where one of the Gemini mid-infrared filters is specified, the zero magnitude flux density value is found by integrating the Cohen spectral template over the assumed filter profile, weighted by photon number since the normal mid-infrared detectors are photon-counting devices. There is some uncertainty in this process because the filter response curves were measured at liquid N temperature, 77 K, but they are used at liquid He temperatures, around 8 K. This is a potentially serious problem. Where possible the effects of the instrument window and any blocking elements have been included. Any wavelength dependent reflectivity in the telescope mirrors or other wavelength dependent effects in the system, including the atmospheric transmission, have not been taken into account in these calculations. If you specify a T-ReCS or Michelle filter the effective wavelength (for a hot star as in the case of alpha CMa) will be returned, but there still has to be some value in the "wavelength" field above even though it will not be used. The same is true for the non-Gemini filters. The values are given to more decimal places than the accuracy of the calibrations really affords. Round off as appropriate.References for non-Gemini magnitude zero points:
The Gemini filter values were calculated by Kevin Volk. These are not published except in the Gemini instrument pages. This page was last modified on September 21, 2011. |