Galactic absorption : ag

The galactic absorption is computed from Schlegel's maps
"Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds", Schlegel et al 1998
And can be downloaded from:
For Galactic objects, ag is also provided, although it does not correspond to the real extinction toward a given object (because it integrates the extinction through the whole depth of the Galaxy; for nearby objects it over-estimates the extinction).
The E(B-V) color excess read on the map is converted to Ab (Johnson) with: Ab= 4.33 E(B-V), corresponding to with Rv=Av/E(B-V)=3.33 (we use 4.33 and not 4.1 to agree with Cardelli et al. 1989).

Warning: for some objects, (in particular near big ones where the absorption is strongly over-estimated by this routine), the value from Schlegel has been corrected as follows:

Note: It has been reported ( Chen et al 1999.) that the present map overestimates visual absorption (Av) by a factor 1.16.