Fitting Gaussians

Hi

I'm trying to model emission lines in a CCD spectrum by adding
Gaussians to the continuum model. The problem I have is that when I
try to fit the data with the line energy a free parameter I get the
following error message:

Current parameter values and their derivatives yielded a singular
matrix
Chi-square minimization failed

Does anyone have any idea at to what may be causing this and how to
get around it? A similar procedure works OK in XSPEC (but that can't
take into account the pile-up in our data)

Cheers
Andy
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