Using the Pegase data set of synthetic UV stellar spectra (see synth-stars ) and the current effective area curves, we can compute the grism to direct ratio for dwarf stars.
Here is a plot of the flux ratio using synthetic stars vs. the nuv_mag - fuv_mag color: syn_ratio.ps .
On the same scale, here is the ratio computed from GALEX NUV grism data from the four deepest grism surveys: nuv_ratio.ps . The same plot for FUV data: fuv_ratio.ps . Only sources with both FUV and NUV spectra were used, so the sample is biased towards fainter, bluer sources, including many galaxies which do not line up with the dwarf star models.
The ratio is typically about 2% and is less than 5% for almost all sources.
Here is the ratio versus NUV magnitude: nuv_mag_ratio.ps .