Note the mesh number and the name of the spec data file.  Choose the ranges of th (Theta) and chi (Chi) to include all of each Theta rocking curve above the half maximum, to resolve at least five points above the half maximum in each Theta rocking curve, to include at least five values of Chi, and to keep the diffracted beam on the detector.  Invoke chiopt as a unix program:

chiopt -i specdatafile -l logfile -o outfile meshnumber

and then either cat or edit outfile to find the optimum value for Chi.

The program chiopt reads the data recorded by spec and determines the full width at half maximum of the Theta rocking curve for each value of Chi in the mesh, and then fits the FWHM as an hyperbolic function of Chi to determine the value of Chi at which the FWHM should be at a minimum.  The output file also includes a table of the FWHM, the fit, and the error in the fit as a function of Chi, which can be graphed to evaluate the acceptability of the fit.  The program has some logic to help reject data which are flawed because the diffracted beam partially or completely missed the detector.

If the program is not on your station computer, the source files can be downloaded via ftp from b1.chess.cornell.edu, where they are located in the directory me/jlg31/chiopt.  The command to port the program is: