Fitting Data#

A Tracer object represents a strong lens system and allows us to create images of the lens galaxy and lensed source galaxy.

Loading Data#

We are now going to use a Tracer to fit imaging data of a strong lens, which we begin by loading from .fits files as an Imaging object:

dataset_path = "/path/to/dataset/folder"

imaging = al.Imaging.from_fits(
    data_path=path.join(dataset_path, "data.fits"),
    psf_path=path.join(dataset_path, "psf.fits"),
    noise_map_path=path.join(dataset_path, "noise_map.fits"),

imaging_plotter = aplt.ImagingPlotter(imaging=imaging)
imaging_plotter.figures_2d(image=True, noise_map=True, psf=True)

Here’s what our image, noise_map and psf (point-spread function) look like:

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We now need to mask the data, so that regions where there is no signal (e.g. the edges) are omitted from the fit. To do this we can use a Mask2D object, which for this example we’ll create as a 3.0” circle.

mask = al.Mask2D.circular(
    shape_native=imaging.shape_native, pixel_scales=imaging.pixel_scales, sub_size=1, radius=3.0

imaging = imaging.apply_mask(mask=mask_2d)

imaging_plotter = aplt.ImagingPlotter(imaging=imaging)

Here is what our image looks like with the mask applied, where PyAutoLens has automatically zoomed around the Mask2D to make the lensed source appear bigger:

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Following the lensing API guide, we can make a Tracer from a collection of LightProfile, MassProfile and Galaxy objects. This Tracer then allows us to create an image of the strong lens system.

By passing a Tracer and Imaging object to a FitImaging object, we create a model-image from the Tracer. The model-image is the image of the Tracer blurred with the Imaging dataset’s PSF, ensuring our fit to the data provides a like-with-like comparison.

fit = al.FitImaging(dataset=masked_imaging, tracer=tracer)

fit_imaging_plotter = aplt.FitImagingPlotter(fit=fit)

Here is how the Tracer’s image and the FitImaging’s model-image look, note how the model-image has been blurred with the PSF of our dataset:

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The FitImaging object does a lot more than just create the model-image, it also subtracts this image from the data to produce a residual-map and combines this with the noise-map to compute a chi-squared-map, both of which we can plot:

fit_imaging_plotter = aplt.FitImagingPlotter(fit=fit)


For a good lens model where the Tracer’s model image is representative of the strong lens system the residuals and chi-squared values minimized:

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Most importantly, the FitImaging object also provides us with a log likelihood, a single value measure of how good our Tracer fitted the dataset:


Bad Fit#

In contrast, a bad lens model will show features in the residual-map and chi-squareds:

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If we can find a Tracer that produces a high log likelihood, we’ll have a model which is representative of our strong lens data! This task, called lens modeling, is covered in the next API overview.

Wrap Up#

If you are unfamilar with data and model fitting, and unsure what terms like ‘residuals’, ‘chi-sqaured’ or ‘ likelihood’ mean, we’ll explain all in chapter 1 of the HowToLens lecture series. Checkout the tutorials section of the readthedocs!