# Overview¶

## Introduction¶

Photutils contains functions for:

• estimating the background and background RMS in astronomical images
• detecting sources in astronomical images
• estimating morphological parameters of those sources (e.g., centroid and shape parameters)
• performing aperture and PSF photometry

The code and the documentation are available at the following links:

## Coordinate Conventions¶

In Photutils, pixel coordinates are zero-indexed, meaning that (x, y) = (0, 0) corresponds to the center of the lowest, leftmost array element. This means that the value of data[0, 0] is taken as the value over the range -0.5 < x <= 0.5, -0.5 < y <= 0.5. Note that this differs from the SourceExtractor, IRAF, FITS, and ds9 conventions, in which the center of the lowest, leftmost array element is (1, 1).

The x (column) coordinate corresponds to the second (fast) array index and the y (row) coordinate corresponds to the first (slow) index. data[y, x] gives the value at coordinates (x, y). Along with zero-indexing, this means that an array is defined over the coordinate range -0.5 < x <= data.shape[1] - 0.5, -0.5 < y <= data.shape[0] - 0.5.

## Bundled Datasets¶

In this documentation, we use example datasets provided by calling functions such as load_star_image(). This function returns an Astropy ImageHDU object, and is equivalent to doing:

>>> from astropy.io import fits
>>> hdu = fits.open('dataset.fits')[0]


where the [0] accesses the first HDU in the FITS file.

## Contributors¶

For the complete list of contributors please see the Photutils contributors page on Github.