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Conversion from magnitudes to flux, or vice-versa

Please enter the magnitude or flux value below along with a wavelength or filter name, and this form will return the conversions between these quantities.

For the mid-infrared "monochromatic" option the conversions 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%.

For the T-ReCS and Michelle filters the zero magnitude values were calculated from the alpha CMa spectral template and the filter profiles, scaling down from the assumed magnitudes of alpha CMa as given by Cohen et al. to 0.0. If the filter profiles are accurate, then the uncertainties here are again about 2%.

For optical and near-infrared filters the magnitude zero points are taken from various sources as noted below.

 


 

 


 

 

Magnitude or flux density value:

 

Wavelength:

Filter name?

Input quantity?

 


 

 


 

A magnitude value may be obtained for one of various Gemini mid-IR 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 values are given to more decimal places than the accuracy of the calibrations really affords. Round off as appropriate.


References for magnitude zero points:


This page was last modified on October 2, 2008.


Kevin Volk (kvolk@gemini.edu)