The ACE external package is used to catalog objects in images and manipulate the catalogs.

Version : 0.2
Author(s) : Frank Valdes (
License : AURA
Website :

Installs from Open Source Astronomy for Linux cd 2
Disk space required for installation is 3.30 Mb

This package requires prior installation of iraf


The detection of objects in an image is conceptually quite simple. Each pixel is compared against the expected sky at that point and if it is more that a specified number of sky sigma above the sky it is a candidate object pixels. Candidate object pixels are grouped into objects on the basis of being connected along the eight neighboring directions. The candidate object is then accepted if it satisfies the criteria of a minimum number of pixels, a sufficiently significant maximum pixel, and a sufficiently significant flux above sky.

To detect faint objects where individual pixels are not significantly above the sky but all pixels taken together are significant a detection filter is applied. This consists of applying a convolution function to the image and performing the detection described in the previous paragraph on the convolved pixels with the sky sigma suitable adjusted for the convolution. The convolution acts as an optimizing filter for objects with shapes corresponding to the convolution weights. The remaining discussion is in terms of the convolved pixel values. The case of no convo­ lution can be thought of as a convolution with a delta function though the implementation is not done as a convolution for efficiency.

Two other options to the detection are to also find pixels that are significantly below sky (using an independent threshold to that used for detecting pixels above sky) and form them into "dark" objects and to take the remaining pixels that are not significantly above or below the sky and use them to define a sky sample for output or for updating the initial sky.