steppar: generate the statistic “surface” for 1 or more parameters
Perform a fit while stepping the value of a parameter through a given range. Useful for determining confidence ranges in situations where greater control is needed than given with the error command.
Syntax: steppar <step spec> [ <step spec>...]
where
<step spec> ::= [<log | nolog>] [<current | best>]
<[modelName:]param index> <low value> <high value> <# steps>
The parameter is stepped from <low value> to <high value> in <# steps> plus one trials. The stepping is either linear or log. Initially, the stepping is linear but it can be changed by the optional string log before the parameter index. nolog will force the stepping to be returned to the linear form. If more than one parameter is entered, then <# steps> must be entered for each one except the last.
The number of steps is set initially to 10. At each value, the parameter is frozen, a fit performed, and the resulting value of chi-squared given. If best is given as an argument then the non-stepped parameters are reset to the best-fit values at each grid point. Alternatively, if current is given as an argument then the non-stepped parameters are started at their values after the last grid point (the default).
If multiple <step spec> are given for different parameters, then a raster scan of the parameter ranges is performed. At the end of the set, the parameters and chi-squared are restored to the values they had initially.
Assume that the current model has four parameters:
XSPEC12> steppar 3 1.5 2.5
//Step parameter 3 from 1.5 to 2.5 in steps of .1.
XSPEC12> steppar log
//Repeat the above, only use multiplicative steps of 1.0524.
XSPEC12> step nolog 2 -.2 .2 20
//Step parameter 2 linearly from -.2 to .2 in steps of 0.02.