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Convert between GALEX Count Rates, Fluxes, and AB Magnitudes
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To convert from GALEX counts per second (CPS) to flux:
FUV: Flux [erg sec-1 cm-2 Å-1] = 1.40 x 10-15 x CPS
NUV: Flux [erg sec-1 cm-2 Å-1] = 2.06 x 10-16 x CPS
To convert from GALEX counts per second (CPS) to magnitudes in the AB
system (Oke 1990):
FUV: mAB = -2.5 x log10(CPS) + 18.82
NUV: mAB = -2.5 x log10(CPS) + 20.08
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:
FUV: mAB = -2.5 x log10(FluxFUV / 1.40 x 10-15 erg sec-1 cm-2 Å-1) + 18.82
NUV: mAB = -2.5 x log10(FluxNUV / 2.06 x 10-16 erg sec-1 cm-2 Å-1) + 20.08
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:
GALEX "counts" are detected photons, as present in the raw data (photon list)
from the detector. This photon list is assembled by the pipeline into the
following images:
- A "dose" image, which is a picture of what the detector saw during the eclipse (i.e., little donuts of dithered source images). "Wiggle" and "walk" corrections have been applied to these images, which move counts around but do not scale them. The "dose" image has 3-arcsec pixels.
- A "cnt" image, in which the stars looks like stars, but which still has no scaling. This map is still in detector count units. The "cnt" image has 1.5-arcsec pixels.
- An "int" image, which has had the relative response correction folded in; thus, the summed signal for a given source in this image is the proper quantity to use in determining a GALEX magnitude (i.e. take the log and scale). The "int" image has 1.5-arcsec pixels.
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
GALEX counts:
FUV: delta_mAB = -2.5 x (log10(CPS) -
log10(CPS +
(CPS x Exp_time +
(0.050 x CPS x Exp_time )2)1/2 / Exp_time))
NUV: delta_mAB = -2.5 x (log10(CPS) -
log10(CPS +
(CPS x Exp_time +
(0.027 x CPS x Exp_time )2)1/2 / Exp_time))
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.
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