Loving ISIS - Confessions of a Former XSPEC User
Model Grids:
This is a topic that 95% of you don't have to worry about 95% of the time. If you fall into that category, you might want to skip ahead to the discussion of models.
The default for ISIS (and XSPEC) is to evaluate a model on a wavelength/energy grid for each dataset, over the channels that actually contribute to the noticed energy bins. There are several cases where you might want to override that behavior:
- You wish to evaluate, and possibly plot, the model in regions not directly related to the data fit.
- You have multiple datasets (with common normalizations), being fitted with a computationally expensive model, so you wish to evaluate the model only once on a common wavelength/energy grid.
- You are applying a convolution model to the data, and there are contributions to the data from well beyond the noticed wavelength/energy ranges.
set_eval_grid_method. The third situation
above applies to the example presented here, where the reflect
model (a convolution model) is applied to the data. This model
requires that the data be examined at high energies beyond the upper
energy boundaries of the PCA and HEXTE data. We accomplish this via
the following set of s-lang code:
define grid_hook(id,s)
{
switch(id)
{
case(1):
s.bin_lo = _A(10^[-9.:-7.:0.01]);
s.bin_hi = make_hi_grid(s.bin_lo);
}
{
s.bin_lo = _A(10^[-0.5:3:0.002]);
s.bin_hi = make_hi_grid(s.bin_lo);
}
return s;
}
set_eval_grid_method (USER_GRID, [1:3], &grid_hook);
The above is the functional equivalent of the XSPEC 11 command
extend. (Notice, however, that we are doing more than just
extending the grid - we are also redefining all the grid
points to be whatever resolution that we choose.)
I have also written a number of wrapper functions, found in my startup scripts, that make these set ups a little easier.
On the next page, we discuss models.
