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(2) The spectrometer coordinate system is defined as a right-handed system fixedon the φ rotation stage at the sample position such the coordinate system isaligned with the laboratory coordinate system when all four spectrometerangles are zero. This definition determines the zero of φ.(3) The third coordinate system is aligned with specific directions in the sample.Acommon and useful example are coordinates defined as the lattice vectors ofa crystalline sample. When placing a sample in the spectrometer, it isunlikely that its axes will line up with the spectrometer axes. Nevertheless,fourc allows the sample orientation to be fully specified by finding the anglesat which two Bragg peaks are detected and giving the corresponding reciprocallattice indices. This process is described fully in the section on the OrientationMatrix.To orient a sample so as to measure the intensity at a particular reciprocal latticeposition requires that the reciprocal lattice vector of interest is aligned with the scatteringvector of the spectrometer. Since any rotation about the scattering vector doesnot change the diffraction condition, there is a high degree of degeneracy that mustbe resolved in order for fourc to determine unique angle settings. How the degeneracyis lifted described in the section on Four-Circle Modes.Diffractometer AlignmentThis section presents a guide on how to set up a four-circle spectrometer. This summaryapplies whether the scattering plane is horizontal or vertical.The first step, which should need only be done the first time fourc is used with thediffractometer, istoensure each diffractometer motor is set up with the correct name,mnemonic, rotation sense, steps-per-degree, etc. The config macro is generally usedfor this purpose. For stepping motors, the rotation sense of an axis depends on thedetails of the motor controller and cable connections. If the rotation sense isn’t asdescribed in the Introduction, change the sign_of_user_×_dial parameter in the motorconfiguration file.For each motor, fourc keeps track of a both a dial and a user position. The dial positionis meant to agree with the readout of the physical dial on the spectrometer. Thevalue and the sign of the steps_per_unit parameter should be chosen so that the dialposition and its direction in the computer agree with the physical dial reading. Usethe set_dial macro to set the dial positions. The user positions should correspond tothe underlying “true” orientation angles of the spectrometer that satisfy the constraintsgiven above. Use the set macro to set the user positions.166 FOUR-CIRCLE REFERENCE INTERIM WORK-IN-PROGRESS (8/16/01) NOT FOR GENERAL DISTRIBUTION