CHESS
Software Data Collection and Experimental Control

Each experimental station at CHESS is outfitted with a Unix-based computer that runs the data collection program Spec^TM.The following text summarizes the commands and procedures that might be used to in a typical experimental run.

Beginning an Experiment-
Depending on the type of diffractometer used, run the appropriate 'geometry' of Spec by typing its name (i.e. oscam if using the oscillation camera, fourc if using the four-circle diffractometer, spec if only generic motor control is needed). If a previous user has just vacated the station, consider adding the argument -f to the end of the command, as in

fourc -f

This has the effect of clearing any customizations that the previous user might have made.

Datafile-
Set the datafile by typing newfile.  If no datafile has been declared, data will be sent to the file default.dat

Motor Control-
Elementary motor motions are facilitated by any of the following commands:

• mv mot_mne position

  moves the motor given by the mnemonic mot_mne to the requested position.

• mvr mot_mne amount

  moves the motor given by mot_mne by the requested amount.

• umv, umvr

  both follow the same format as the above, but update the position while moving.

• mmv mot_mne1 position1 [mot_mne2 position2 ]
  mmvr mot_mne1 amount1 [mot_mne2 amount2 ]

  move multiple motors to absolute positions or by relative amounts, respectively.

   

Set Monochromator Energy-
Move the monochromator to the energy desired by any of the following commands:

• moveE energy

  moves the monochromator to the energy specified.

• twE amount

  tweak the monochromator energy by the given amount. A `tweak' is a small, repeated motion.

Align Experimental Apparatus-
To align the experimental apparatus with the beam, a set of macros to run the optical tables has been provided. See the graphic box or the webpage for a summary of these features.

Preparing to Take Data-
To prepare to run a scan, follow the procedure below.

1. Run counters to establish the correct counter to use as the detector DET. This is the counter that will be plotted against the y-axis if no other plot expression is defined. To define an alternate expression to be plotted, run the macro userplot.

2. Run setscans to set the sleep time between datapoints.

3. If a scan having regions with different step sizes is required, use the macro define_scan to define it. The help screen for define_scan provides details on the methodology of creating such a scan and discusses briefly the various scans which are available. To view the scan characteristics once they are in place, type show_scan.

Scanning-
At this time, it is possible to begin scanning. Below is a partial listing of the many single-region scans which may be run. If a multi-region scan has been defined, it could now be run by using the macro start.

• Escan start finish intervals count performs an energy scan from start energy to finish energy, having intervals steps and counting for count seconds. If count is negative, then count is interpreted as monitor counts. Units are keV.

• ascan mot_mne start finish intervals count similar to Escan except that it moves the motor denoted by the mnemonic mot_mne rather than energy.

• dscan mot_mne dstart dfinish intervals count identical to ascan except that the scan dstart and dfinish are take relative to the initial position. Equivalent to lup, the lineup scan.

• pklup, comlup lineup scans which drive to the peak or center-of-mass of the curve, respectively, upon completion of the scan.

• a2scan, a3scan run multi-motor scans in absolute position. See the relevant help screen for further information.

• d2scan, d3scan run multi-motor scans defined relative to initial positions of motors. See the help screen for the particular scan desired.

• mesh, emscan, mescan two-motor meshed scan and meshed energy-motor scans. Each has a corresponding help screen which may be consulted.

• hscan, kscan, lscan, hkscan, hklscan these geometry scans are only available in fourc or sixc, where scanning along a particular reciprocal space lattice vector or vectors is defined. More information is available in the help screens and in the Spec reference manual.

• xrtscan, yrtscan, zrtscan, xtscan, ytscan, ztscan optical table scans of a particular angle or along a particular axis. See the Guide to the CHESS Optical Table Macros, the station documentation, or the corresponding help screen for further information. The figure below shows the definition of the optical table geometry.

Plotting data-
At the end of the scan, it is sometimes desirable to have a plot of the data which can be retained for later reference. To do this, open a second terminal window and run bplot. bplot provides a menu-based interface for producing hardcopy of curves, and is available at all the stations.

Getting Help-
You can get detailed help on any of the above topics by typing

h [topic]

from the Spec command prompt.

Optical Table Macro Definitions-
Summary of the translational and rotational degrees of freedom of the CHESS optical tables. Note that rotations can be done about any user-defined point (usually a specimen or aperture position) located in the coordinate system of the table.

Last Update: 2006-09-14