# Fitting Models to Data¶

This module provides wrappers, called Fitters, around some Numpy and Scipy
fitting functions. All Fitters can be called as functions. They take an
instance of `FittableModel`

as input and modify its
`parameters`

attribute. The idea is to make this extensible and allow
users to easily add other fitters.

Linear fitting is done using Numpy’s `numpy.linalg.lstsq`

function. There are
currently two non-linear fitters which use `scipy.optimize.leastsq`

and
`scipy.optimize.fmin_slsqp`

.

The rules for passing input to fitters are:

- Non-linear fitters currently work only with single models (not model sets).
- The linear fitter can fit a single input to multiple model sets creating
multiple fitted models. This may require specifying the
`model_set_axis`

argument just as used when evaluating models; this may be required for the fitter to know how to broadcast the input data. - The
`LinearLSQFitter`

currently works only with simple (not compound) models. - The current fitters work only with models that have a single output
(including bivariate functions such as
`Chebyshev2D`

but not compound models that map`x, y -> x', y'`

).

## Simple 1-D model fitting¶

In this section, we look at a simple example of fitting a Gaussian to a simulated dataset. We use the

`Gaussian1D`

and`Trapezoid1D`

models and the`LevMarLSQFitter`

fitter to fit the data:import numpy as np import matplotlib.pyplot as plt from astropy.modeling import models, fitting # Generate fake data np.random.seed(0) x = np.linspace(-5., 5., 200) y = 3 * np.exp(-0.5 * (x - 1.3)**2 / 0.8**2) y += np.random.normal(0., 0.2, x.shape) # Fit the data using a box model. # Bounds are not really needed but included here to demonstrate usage. t_init = models.Trapezoid1D(amplitude=1., x_0=0., width=1., slope=0.5, bounds={"x_0": (-5., 5.)}) fit_t = fitting.LevMarLSQFitter() t = fit_t(t_init, x, y) # Fit the data using a Gaussian g_init = models.Gaussian1D(amplitude=1., mean=0, stddev=1.) fit_g = fitting.LevMarLSQFitter() g = fit_g(g_init, x, y) # Plot the data with the best-fit model plt.figure(figsize=(8,5)) plt.plot(x, y, 'ko') plt.plot(x, t(x), label='Trapezoid') plt.plot(x, g(x), label='Gaussian') plt.xlabel('Position') plt.ylabel('Flux') plt.legend(loc=2)As shown above, once instantiated, the fitter class can be used as a function that takes the initial model (

`t_init`

or`g_init`

) and the data values (`x`

and`y`

), and returns a fitted model (`t`

or`g`

).

## Simple 2-D model fitting¶

Similarly to the 1-D example, we can create a simulated 2-D data dataset, and fit a polynomial model to it. This could be used for example to fit the background in an image.

import warnings import numpy as np import matplotlib.pyplot as plt from astropy.modeling import models, fitting # Generate fake data np.random.seed(0) y, x = np.mgrid[:128, :128] z = 2. * x ** 2 - 0.5 * x ** 2 + 1.5 * x * y - 1. z += np.random.normal(0., 0.1, z.shape) * 50000. # Fit the data using astropy.modeling p_init = models.Polynomial2D(degree=2) fit_p = fitting.LevMarLSQFitter() with warnings.catch_warnings(): # Ignore model linearity warning from the fitter warnings.simplefilter('ignore') p = fit_p(p_init, x, y, z) # Plot the data with the best-fit model plt.figure(figsize=(8, 2.5)) plt.subplot(1, 3, 1) plt.imshow(z, origin='lower', interpolation='nearest', vmin=-1e4, vmax=5e4) plt.title("Data") plt.subplot(1, 3, 2) plt.imshow(p(x, y), origin='lower', interpolation='nearest', vmin=-1e4, vmax=5e4) plt.title("Model") plt.subplot(1, 3, 3) plt.imshow(z - p(x, y), origin='lower', interpolation='nearest', vmin=-1e4, vmax=5e4) plt.title("Residual")