The BEAM CENTRE Command

The BEAM CENTRE Command allows the beam centre to be defined or re-defined without changing any other experimental geometric parameters. This may be useful when the detector tilt angles are accurately determined using a high quality calibrant measurement, but data images suffer some non-reproducibility in positions. The BEAM CENTRE MENU is shown in figure 37.

Figure 37: The BEAM CENTRE MENU

The following commands are available:

?: List of commands with short description

HELP: More detailed help on available commands

2-D GAUSSIAN FIT: Automatic fitting of a 2-D Gaussian function on the direct beam mark. The user should click on the beam mark to start the fitting.

AVERAGED GRAPHICAL: Set beam centre from the average of entered coordinates. This will work well if there are clearly defined symmetric peaks.

CIRCLE COORDINATES: Set the beam centre from the least squares fit of a circle to 3 or more entered coordinates. This is useful if a well defined powder ring is present.

ELLIPSE COORDINATES: Set the beam centre from the least squares fit of an ellipse to 5 or more entered coordinates. This is useful if a well defined powder ring is present.

GRAPHICAL COORDINATE: Single graphical coordinate click to define the beam centre. This is clearly rough and ready, but may be sufficient for many purposes, in particular an initial beam centre for refinement.

FIT 1-D PROJECTION: This option is designed for grazing incidence SAXS data. A rectangular region is specified in the same manner as for the PROJECTION command. Two coordinates define the projection line, and other two coordinates define the width of the 2-D projection region. The 1-D projection is displayed and a menu allowing masking and un-masking of data points. The symmetry point of the 1-D data is refined. For this to work optimally the intensities must be symmetric e.g. there must not be differing absorption effects. The best symmetry point on the 1-D projection line is used to calculate the 2-D coordinate. This is presented to the user as the default X/Y coordinate for the beam centre.

KEYBOARD: Enter beam centre coordinate from input values from the keyboard.

NO CHANGE: Use the existing position.