.. _datasets:

Datasets and Simulation (`photutils.datasets`)
==============================================

Introduction
------------

`photutils.datasets` provides tools for loading datasets or making
simulated data. These tools mostly involve astronomical images, but they
also include PSF models and source catalogs.

These datasets are useful for the Photutils documentation examples,
tests, and benchmarks. However, they can also be used for general data
analysis or for users that would like to try out or implement new
methods for Photutils.

Functions that start with ``make_*`` generate simulated data.
Typically one would need to use a combination of these functions
to create a simulated image. For example, one might use
:func:`~photutils.datasets.make_model_params` to create a table of
source parameters, then use :func:`~photutils.datasets.make_model_image`
to create an image of the sources, add noise using
:func:`~photutils.datasets.make_noise_image`, and finally create a world
coordinate system (WCS) using :func:`~photutils.datasets.make_wcs`. An
example of this process is shown below.

Functions that start with ``load_*`` load datasets, either from within
the Photutils package or remotely from a GitHub repository. Very
small data files are bundled with Photutils and are guaranteed to be
available. Larger datasets are available from the `astropy-data`_
repository. On first load, these larger datasets will be downloaded and
placed into the Astropy cache on the user's machine.


Simulating Images
-----------------

For this example, let's simulate an image of 2D Gaussian sources on a
constant background with Gaussian noise.

First, we'll create a table of 2D Gaussian source
parameters with random positions, fluxes, and shapes using
:func:`~photutils.datasets.make_model_params`::

    >>> from photutils.datasets import make_model_params
    >>> from photutils.psf import GaussianPSF
    >>> model = GaussianPSF()
    >>> shape = (500, 500)
    >>> n_sources = 500
    >>> params = make_model_params(shape, n_sources, x_name='x_0',
    ...                            y_name='y_0', min_separation=5,
    ...                            flux=(100, 500), x_fwhm=(1, 3),
    ...                            y_fwhm=(1, 3), theta=(0, 90), seed=123)

Next, we'll create a simulated image of the sources using the table of
model parameters using :func:`~photutils.datasets.make_model_image`::

    >>> from photutils.datasets import make_model_image
    >>> model_shape = (25, 25)
    >>> data = make_model_image(shape, model, params, model_shape=model_shape,
    ...                         x_name='x_0', y_name='y_0')

Next, let's add a constant background (``mean = 5``) and Gaussian noise
(``stddev = 2``) to the image::

    >>> from photutils.datasets import make_noise_image
    >>> noise = make_noise_image(shape, distribution='gaussian', mean=5,
    ...                          stddev=2, seed=123)
    >>> data += noise

Finally, let's plot the simulated image:

.. plot::
    :include-source:

    import matplotlib.pyplot as plt
    from astropy.visualization import simple_norm
    from photutils.datasets import (make_model_image, make_model_params,
                                    make_noise_image)
    from photutils.psf import GaussianPSF

    model = GaussianPSF()
    shape = (500, 500)
    n_sources = 500
    params = make_model_params(shape, n_sources, x_name='x_0',
                               y_name='y_0', min_separation=5,
                               flux=(100, 500), x_fwhm=(1, 3),
                               y_fwhm=(1, 3), theta=(0, 90), seed=123)
    model_shape = (25, 25)
    data = make_model_image(shape, model, params, model_shape=model_shape,
                            x_name='x_0', y_name='y_0')

    noise = make_noise_image(shape, distribution='gaussian', mean=5,
                             stddev=2, seed=123)
    data += noise

    fig, ax = plt.subplots()
    norm = simple_norm(data, 'sqrt', percent=99)
    ax.imshow(data, norm=norm, origin='lower')
    ax.set_title('Simulated image')

We can also create a simulated world coordinate system (WCS) for the
image using :func:`~photutils.datasets.make_wcs`::

    >>> from photutils.datasets import make_wcs
    >>> wcs = make_wcs(shape)
    >>> wcs.pixel_to_world(0, 0)
    <SkyCoord (ICRS): (ra, dec) in deg
    (197.8899676, -1.3750039)>

or a generalized WCS using :func:`~photutils.datasets.make_gwcs`:

.. doctest-requires:: gwcs

    >>> from photutils.datasets import make_gwcs
    >>> gwcs = make_gwcs(shape)
    >>> gwcs.pixel_to_world(0, 0)
    <SkyCoord (ICRS): (ra, dec) in deg
    (197.8899676, -1.3750039)>


API Reference
-------------

:doc:`../reference/datasets_api`


.. _astropy-data: https://github.com/astropy/astropy-data/
.. _skymaker: https://github.com/astromatic/skymaker
