The ACE external package is used to catalog objects in images and manipulate the catalogs.
Version : 0.2
Author(s) : Frank Valdes (firstname.lastname@example.org)
License : AURA
Website : http://iraf.noao.edu/
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
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.