INPUT PARAMETERS allows an initial fit model to be defined. The fit model can be defined GRAPHICALLY or from a FILE, which has previously been saved using OUTPUT PARAMETERS. Selecting INPUT PARAMETERS re-initialises the fit model so any previously defined fit model parameters are lost (prior to this operation values may be saved with OUTPUT PARAMETERS).
This is the method to define an initial fit model. First any masked-off
elements are defined (See Section 17.8,
Page ).
The fit model can be made up of a general polynomial background plus one
or more 2-D peaks. The order of the polynomial in the X and Y-directions
is entered by the keyboard, followed by the a number of points used to
define initial values for the polynomial. The initial polynomial may be
defined from a lower order, which is more stable. The higher coefficients
of the polynomial are then initialised to zero. To define the initial
coefficients it is necessary to click on a given number of data points e.g.
where
and
are the orders of the initialisation
polynomial. It is recommended to use only lower order polynomials, since
higher order polynomials are not very stable. The user should click on the
given number of data points spread throughout the data region, in an
approximate grid. (If the points are all co-linear it will not be possible
to set initial coefficient values.)
A choice of three peak types is available. A menu appears presenting the three types. the user should click on the required peak type, or on exit to finish this stage of the model definition. The three peak types are:
For each peak a number of peak parameters need to be defined. These are all entered graphically by clicking on the centre of the peak (position and maximum intensity), and two half height positions. Care should be taken with the initialisation or greater danger of falling into local minima will be present.
Finally the user may specify a ``row-line'' as part of the model. A ``row-line'' is a line of polar Gaussians which all share the same radial and angular standard deviations. The peaks are placed at equal angles along a line at an angle from the vector from the symmetry centre to the ``row-line'' centre. The ``row-line'' is reflected in the vector from the symmetry centre to the row-line centre, but with a change in intensity. All the parameters which define a ``row-line'' are entered with the keyboard. (It is planned to define a tri-clinic unit cell diffraction pattern peak model to replace the ``row-line'' model which is an approximation.)