# 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:

- Code: https://github.com/astropy/photutils
- Issue Tracker: https://github.com/astropy/photutils/issues
- Documentation: https://photutils.readthedocs.io/

## 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.