[madbadgalaxyman (Robert)]

[note : the first sentence was not clear, before my edit ]

Schlegel et al. extinction values work very well , when we compare them to the
' baseline truth' which is the numerical value of extinction that is derived individually for each individual field;
it is a long time since I looked into exactly how this is done, but you derive the reddening value & the extinction value for a specific field of interest by comparing the 'intrinsic'(real)(unreddened) B-V or U-B colour of a star of a specific spectral type (which is derived from measuring the colors of several stars of the same type which are not very reddened and extincted) with the actual observed B-V or U-B colour of a star in the desired field of the same spectral type.

While extinction is not much of a concern for visual observers , except near to the 'great circle' of the Milky Way in the sky, the numerical value of the foreground extinction in front of a galaxy is certainly something you always need to plug into the magnitude equation, so as to figure out more accurate distances and luminosities for external galaxies.

From practical experience, I do find that the observed surface brightnesses of many individual galaxies are noticeably reduced by extinction, even at 10 degrees from the Galactic Plane; a good example is NGC 4945.

Fortunately for us, the Sun is not situated in a very dusty part of the Milky Way Galaxy (just by chance, the local interstellar medium is of low density), so there is very little extinction at and near the Galactic Poles (+/- 90 degrees from the observed Milky Way). There are dusty locations near to the plane of most non-dwarf Spiral Galaxies where there is 1-4 magnitudes of extinction even at right angles to the plane of a galaxy!

Robert

I shall shortly post on what observational data can be used to model the interstellar dust; I have a draft document, but need to check that it makes sense.