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Convert between GALEX Count Rates, Fluxes, and AB Magnitudes
Enter a number in one of the text fields below, and then hit either Tab or Return.
To convert from GALEX counts per second (CPS) to flux:
To convert from GALEX counts per second (CPS) to magnitudes in the AB
system (Oke 1990):
For this purpose, we have taken the relative response of all locations on the detector as 1. The current estimates are that the zero-points defined here are accurate to within ± 10% (1 sigma).
To convert from flux to AB magnitudes:
Note that one GALEX count corresponds to one detected "average" photon for the (respective) bandpass. Since detector background is very small (less than 1%), GALEX counts may be used in Poisson statistics to compute S/N for sources or sky background.
To determine the detector background:
Typical GALEX background in the FUV is 2000 cps for the whole field, which corresponds to 3 x 10-4 cps/pixel (where a pipeline pixel is 1.5 arcsec). This is the typical total signal, and about 1/2 of it is detector background, so take the FUV detector background to be ~10-4 cps/pixel for simplicity. NUV detector background is 10 times higher, or ~10-3 cps/pixel. if a typical FUV source (19 mag, or 1 cps) covers 9 pixels (5" FWHM), then the detector background is about 0.001 cps/source, compared to 1 cps of signal. Thus, the background would be ~0.1% of the typical souce for the the FUV. In the NUV, for a 20 mag source (1 cps), the detector background would be 10-3 cps/pixel, or 0.01 cps/source for a 5" FWHM, and, in this case, the NUV background is ~1% of the source flux.
To estimate the photometric repeatability vs. magnitudes based on
For unsaturated bright stars the GALEX photometric precision is limited by the flat field to delta magnitudes of +/- 0.050 and +/- 0.027 for FUV and NUV respectively. (See figure 5 Morrissey et al. 2007.) Fainter sources are limited by Poisson noise. The photometric repeatability as a function of magnitude is shown as 3-sigma in figures 6 and 7 in Morrissey et al. 2007.