The ISM attenuation due to neutral H, neutral He and once ionized He. This is a modified version of the Rumph, et al. 1994 model, using the ismatten program by Pat Jelinsky, and allows the user to set the model redshift.
par1 nH: H column density (1022 atoms/cm2)
par2 nHeI: HeI column density (1022 atoms/cm2)
par3 nHeII: HeII column density (1022 atoms/cm2)
par4 z: Redshift