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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 :

Disk space required for installation is 3.30 Mb


The detection of objects in an image is conceptually quite sim­
ple. 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 suffi­
ciently significant flux above sky.

To detect faint objects where individual pixels are not signifi­
cantly above the sky but all pixels taken together are signifi­
cant 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 convo­
lution acts as an optimizing filter for objects with shapes cor­
responding 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 effi­

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 sig­
nificantly above or below the sky and use them to define a sky
sample for output or for updating the initial sky.

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