Specutils Gaussian1D fitter and estimators

Question

I am running the "simple example" from specutils to do some line-fitting.

The demo code is here:

https://specutils.readthedocs.io/en/stable/fitting.html

This runs and does what it says.

However I want to use parameter estimation to get an approximation to the initial parameters in the Gaussian, because I am interested in looking at a number of long spectra with a lot of lines in and want to attempt initial fits automatically.

So I modify the specutils.fitting import to

    from specutils.fitting import fit_lines, estimate_line_parameters

and add in

    e1 = estimate_line_parameters(spectrum, models.Gaussian1D())
    a = round(e1.amplitude.value,2)
    b = round(e1.fwhm.value,2)
    c = round(e1.stddev.value,2)

This gives meaningful values and I replace the call to the Gaussian by:

    g_init = models.Gaussian1D(amplitude=a*u.Jy, mean=b*u.um, stddev=c*u.um)

then

    g_fit = fit_lines(spectrum, g_init)
    y_fit = g_fit(x*u.um)

The initial values I get from the estimator are:

initial amplitude= 3.35 initial fwhm = 2.41 initial stddev= 1.02

But on looking at the output parameters using g_fit.amplitude.value etc, I get:

final amplitude= -0.24 Jy final fwhm = 0.0 um final stddev= 0.0 um

for the output values!! If the estimators are correct then the output values should be very close.

Is there an issue using the estimators like this?

FWIW I am running matplotlib 3.2.2, specutils 1.0, numpy 1.19, python 3.6 over Ubuntu 19.10

Answer 1

The answer turned out to be that I was pushing the models package too far with the data that I was giving it. More careful background subtraction and much more localised fitting helped greatly.