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The Double Focusing Monochromator- front view



MACS Operations


The double focusing monochromator on MACS helps the instrument make the most of the PeeWee cold source by focusing the cold neutrons on the sample for increased flux or on the detectors for enhanced resolution.

The monochromator was built fifteen years ago and uses a collection of 32 motors to position HOPG crystals for energy focusing in the scattering plane and sagittal focusing out of that plane. When the system was originally built, each motor had an independent motor controller whose communications were bussed together using the RS485 electrical standard. That constellation of motors was managed by a large Labview application which calculated motor angles for the desired energy and focusing mode. As the motor controllers used in the apparatus are obsolete and the supervisory application has some demonstrable inefficiencies which compromise instrument performance, we elected to adapt its control electronics to use the NCNR facility standard VIPER motor control application.


  • built adaptor boxes to route motor and feedback signals to the VME based indexers and associated stepper drivers
  • replaced the legacy Labview application with the standard NCNR motor control application
  • NCNR standard motor control application was also rewritten as a compiled application using the Qt cross platform development toolkit
    • the reengineered VIPER is more responsive and stable 
    • designed for easier deployment on modern variants of Linux
    • multithreaded implementation is intended to decrease overhead in motor positioning and event counting on traditional reactor-based spectrometers
    • supports both serial and Ethernet remote interfaces
    • emits motor positions with absolute timestamps in the IEEE-1588 (Precision Time Protocol) format

The legacy ICE instrument control program, running on MACS since 2006, has been adapted to work with the new motor control software and electronics. And use of that new DFM control software is a critical prerequisite for the installation of the NICE instrument control software. The updated DFM controls have passed operational tests with both ICE and NICE software suites.

Created January 18, 2022, Updated March 8, 2022