fitting, step

  Hi,

I'm having some trouble getting isis to fit data in a certain case, and 
I can't quite figure out what the problem is.

I am importing a histogram I made in another program. I use 
"define_counts" to make it into a spectrum I can fit with isis, and I 
assume a perfect response, fitting individual lines as Gaussians. I can 
define a Gaussian model with "fit_fun", preset parameter values to be 
close to the best fit values using "set_par", and use "eval_counts" or 
"renorm_counts" to make sure that the function evaluates, and 
renormalize the line strength before fitting the centroid and width. So 
far so good.

However, when I do "fit_counts", it doesn't fit the data, or it barely 
changes the fit parameters. The one clue that something may be wrong is 
that the "step" field is set to zero for all model parameters I want to 
fit, even if I explicitly set it using "set_par". I have checked this 
with XMM and Chandra data, and there, when I load data, define a model, 
and fit, the fitting algorithm proceeds normally, and the "step" field 
of each thawed parameter is non-zero. So somehow this seems to be a 
problem with this data format.

Maybe this is a problem with "step", or maybe that's a red herring, but 
either way, I can't change it from zero, and I can't fit the data. Any 
suggestions?

Thanks,

Maurice
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