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We refine a classic relation between the hydrogen column density (NH) andoptical extinction (Av) by employing 39 Galactic Supernova Remnants (SNRs) withX-rays, optical and/or infra-red data available. We find NH =(1.69+/-0.07)*10^21 Av cm^(-2) mag^(-1) . Applying this relation to threeGalactic SNRs with good historical records allows us to further constraineither their progenitor's distances or magnitudes, which is independent accessto their distances.

The Galactic NH - AV Relation and its Application to Historical Galactic SNRs