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Segmentation map missing object - roygal - 06-02-2017 22:08

I am using SExtractor 2.19.5 and have come across a bug that seems to have been reported back in 2008. In rare instances, we are seeing that a detected object is not present in the segmentation map, and its pixels seem to be assigned to a different, adjacent object. It looks like it is a result of depending, but only in rare instances.

We are using the pixels from the segmentation map to select pixel values from a corresponding monte carlo variance map whose values we use for an error computation, so we do need to know specifically which pixels are assigned to each object.

We also see that there are cases of individual pixels that belong to one object that are entirely embedded within another object, which I am not sure is correct either.

An example cutout from a segmentation map and the correspond aperture checkimage is shown. You can see the two apertures that are close together in the aperture image (objects 9 & 2) are a single object (9) in the segmentation map

Ap vs. seg image

RE: Segmentation map missing object - roygal - 06-03-2017 10:00

A little more checking
We became worried that the object pixels were not being accurately shown in the segmentation map. We computed the isophotal flux and area of each object in the segmentation map directly from our data frames, and the values are in agreement with those from the SExtractor catalog, EXCEPT in the cases where an object is missing in the segmentation map. As suspected, the missing object is getting merged into a neighboring object. For example, in one .cat file, objects 2 and 9 have areas of 99 and 332 respectively, but object 2 is not seen in the segmentation map. When we measure the area of object 9 manually, Iit has an area of 431 - the sum of objects 2 and 9 from the catalog.