The column density in our Galaxy

Occasionally it is necessary to determine the Galactic hydrogen column density prior to spectral fitting. The program nh calculates the Galactic hydrogen column density in a specified direction of the sky. This is calculated using an interpolation of data taken from Marshall & Clark (1984; this data is stored in the file $XACAL/nh2x2.dat). This covers the entire sky and uses various 21-cm surveys for its source (Stark et al. 1984; Clearly, Heiles & Haslam 1979; Heiles & Clearly 1979). The following is an example of its use:

qcl>nh

** NH Version 1.5 Tue Aug 19 18:03:13 1997 **
 
GNG_RA of target in degrees = 0.0 =193.0
GNG_DEC of target in degrees = 0.0 =-60.0
 RA : 193.00  Dec : -60.00
 l : 303.25  b :   2.60
GNG_SNH Input Nh file name = $XACAL/nh2x2.dat =
 Nearest points to the data : 
  l      b    nH/10**22      # scans  dist(deg)     weight
 302.64  2.62   0.1580          4.     0.609         9.0825
 304.50  2.62   0.1674          3.     1.249         3.3245
 302.68  0.87   0.1498          4.     1.818         3.0438
 distance*scan no. weighted nH =    0.1584 * 10.**22
qcl>