Loving ISIS - Confessions of a Former XSPEC User
Parameters
As discussed on the models page, the model
parameters can be edited via the ISIS edit_par; command
(which will pop up an emacs, vi,
jed, etc., window, depending upon
the setting of your shell's EDITOR environment variable). This
is very convenient. Likewise, there is a set_par() command
that allows parameters to be set on the command line, or in scripts:
isis> model("powerlaw(1)");
isis> list_par;
powerlaw(1)
idx param tie-to freeze value min max
1 powerlaw(1).norm 0 0 1 0 1e+10
2 powerlaw(1).PhoIndex 0 0 1 -2 9
isis> set_par(2,1.7,0,1,3); % set_par(2,1.7,1); would have frozen it
I have wrapped a function around this, newpar(), which will
also take -1 as indicating a frozen parameter. (Bowing to my
XSPEC days...)
The set_par(); function (or the newpar(); wrapper)
can take either parameter numbers or parameter names (with wild
cards!) as inputs:
isis> set_par(1,0.1);
isis> set_par("powerlaw(1).norm",0.1);
isis> set_par("pow*norm",0.1);
are all equivalent for the above defined model. (Note that the latter
function call, however, would set all powerlaw normalization to
0.1, if multiple powerlaws were defined.)
The only exceptions to being able to use a string name for a parameter
are when the function or parameter name contains parentheses, (), or
mathematical operators, e.g., +, /, -, etc.
(ISIS will try to parse those as mathematical expressions.) For
those cases, one has to use the parameter number.
Saving and Loading Parameters
To save a set of fit parameters to a file parameter_file.par, use:
isis> save_par("parameter_file.par");
To load a set of fit parameters:
isis> load_par("parameter_file.par");
Manipulating Parameters
Parameters can be tied together, and they can be arbitrary
functions of one another. This allows greater flexibility than can
be achieved with XSPEC. The ISIS intrinsic command
set_par_fun() will parse expressions either via
"model_name(id).parameter", or via parameter idx and
expressions using "_par(idx)". (Again, the latter must be used
when the function and/or parameter names contains parentheses or
mathematical operators.) The former syntax is preferred whenever
possible, as parameter ties will then properly update when redefining
an existing model definition. The
analysis script shows an example.
Here is a toy example that allows one to define a dataset dependent line offset:
public define offset ()
{
if (Isis_Active_Dataset == 2)
return 3.0;
else
return 2.5;
}
set_par_fun ("gauss(2).center", "gauss(1).center + offset()");
I.e., Isis_Active_Dataset is used to define an offset();
function which has the value 3.0 for dataset 2, but is 2.5 for all
other datasets. This function is then used to tie together two
gaussian line mid-points.
Next Up: At long last - Fitting!
