GALEX Exposure Time Calculator -- Notes

Input magnitudes are assumed to be in the AB system. Input fluxes/magnitudes are assumed to have no Milky Way extinction along the line of sight, i.e., input fluxes and magnitudes should be values in the absence of interstellar absorption. Thus, for extragalactic sources, the input fluxes/magnitudes should be what an observer just outside the Milky Way would observe. Users can choose to have Milky Way extinction applied to the computed GALEX fluxes/magnitudes by using the advanced interface. Note that the Milky Way extinction in the advanced interface only affects the computed GALEX fluxes/magnitudes, and does not affect input fluxes/magnitudes, as described above.

Example 1. 3C 273: (J2000) 187.278 +2.052, z=0.158, m_B=12.86

This is a QSO with an observed B apparent magnitude of 12.86 (from NED) which must be first be corrected to remove extinction along the line of sight, to determine the appropriate input magnitude.

E(B-V) = 0.021 mag
B_input = B_observed - E(B-V)*(A/E(B-V)) = 12.86 - (0.021*4.315) = 12.77

This is the input B magnitude to enter in the ETC. Note that values of extinction normalized to standard filters, A/E(B-V), are given in Table 6 of Schlegel et al. (1998), ApJ, 500, 525.

An estimate for GALEX NUV & FUV countrates can be produced by the ETC using the Milky Way extinction 'Automatically Calculate Based on Position' option. For a one orbit observation (1500s) the FUV/NUV S/N will be 482/906.

Example 2. Sample Spiral Galaxy

Q) What exposure time do I need to detect a spiral galaxy of m_B=22 at a redshift of 0.2 with S/N = 10?

A) Use the ETC with FUV S/N=10, object type = galaxy, spectral type = Sb redshift = 0.2, input filter = B, input magnitude = 22, and 'Do Not Calculate'. Because the position of the observation is not known, we assume it will be at a low extinction position so choose coordinate system = galactic, longitude = 0, latitude = 90.

The result shows that an exposure time of 8913 sec is needed. Note that the input position is important because the sky background is calculated from the position. If instead, the galactic center had been chosen, the required exposure time would be > 2.5E10 sec.