spec - Local Sector 7 web page

Four-CircleFilesMost questions regarding the behavior of spec when used with a four-circle diffractometercan ultimately be resolved by consulting the appropriate source code. Thefile geo_fourc.c in the standard distribution of spec contains the code for all the fourcirclecalculations. The file u_hook.c contains a few lines of code that connect thecode in geo_fourc.c with the rest of the program. Finally, the files macros/fourc.srcand macros/ub.mac in the standard distribution contains the definitions for all thefour-circle and orientation matrix macros.Four-CircleVar iablesThe four-circle coordinate variables (H, K and L, inparticular) are stored in a built-inarray named Q[] . The four-circle geometry calculations either use the motor positionscontained in the A[] array to calculate values for the Q[] parameters or placemotor positions in A[] based on the current values in Q[] . Each four-circle variablehas a descriptive macro definition as an alias, such as def OMEGA ’Q[6]’ .Variable Alias Description¢ Q[0] H x component of the scattering vector.Q[1] K y component of the scattering vector.Q[2] L z component of the scattering vector.Q[3] LAMBDA Incident X-ray wavelength λ.Q[4] ALPHA Incident angle α.Q[5] BETA Exiting angle β.Q[6] OMEGA ω = θ 2θ)/2.Q[7] AZIMUTH Azimuthal angle.Q[8] F_ALPHA Frozen value of α for alpha-fixed mode.Q[9] F_BETA Frozen value of β for beta-fixed mode.Q[10] F_OMEGA Frozen value of ω for omega-fixed mode.Q[11] F_AZIMUTH Frozen value of ψ for azimuth-fixed mode.Q[12] F_PHI Frozen value of φ for phi-fixed mode.Q[13] F_CHI_Z Frozen value of χ for zone mode.Q[14] F_PHI_Z Frozen value of φ for zone mode.INTERIM WORK-IN-PROGRESS (8/16/01) NOT FOR GENERAL DISTRIBUTION FOUR-CIRCLE REFERENCE 175