instrument file

hi... i,ve jus started learning the rietveld method.. and am using gsas... i wanted to know how to prepare an instrument file

RE: error in peak positions that is linear in theta

Kurt, It looks like incorrect step size (difference between actual step size and one that is written in the file) or incorrect distance to the detector. The later may incorrect step size if detector is PSD. I have similar situation with much smaller difference in peak position but the shift (diff