GNU Astronomy Utilities - Bugs: bug #62702, Clumps on strong gradients of...
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bug #62702: Clumps on strong gradients of larger galaxies become too large
Submitter: | Mohammad Akhlaghi <makhlaghi> | ||
Submitted: | Sun 03 Jul 2022 11:29:31 PM UTC | ||
Category: | Segment | Severity: | 3 - Normal |
Item Group: | Output not reasonable | Status: | In Progress |
Privacy: | Public | Assigned to: | makhlaghi |
Open/Closed: | Open |
Mon 16 Jan 2023 11:29:38 AM UTC, comment #2: |
Mohammad Akhlaghi <makhlaghi>![]() ![]() |
Sun 03 Jul 2022 11:35:44 PM UTC, comment #1: The image below is the output of the script in the original post:
On the left, you see the image before adding noise. In the middle, the image with noise is shown. On the right, you can see the pixels associated with the two clumps as the white and yellow regions. The blue pixels are the undetected sky regions, and the black pixels are the detected, but diffuse regions that couldn't be associated with any individual clump.
As you see above, a small star has been placed very close to the larger galaxy's center.
We expect the "clump" of the star to only be as large as the black circle that the two white arrows point to. However, because it has the gradient of the background galaxy under it, Segment's current algorithm will associate the full right-side of the larger galaxy to also be part of this clump.
We need to define a way to stop the growth of the smaller clump in such cases and let the galaxy take the pixels that belong to it. |
Mohammad Akhlaghi <makhlaghi>![]() ![]() |
Sun 03 Jul 2022 11:29:31 PM UTC, original submission:
When a clump falls on the gradient of a large object, in the current implementation of Segment, it will steal a large fraction of flux from the large object. The code below in the P.S can be put in a simple shell script to reproduce the problem.
P.S. Code to reproduce the problem.
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Mohammad Akhlaghi <makhlaghi>![]() ![]() |
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Until now, I had been investigating ways to step back to brighter pixels for finding a good stage to stop the growth of the clump. However, a few days ago, while investigating another issue with Sepideh Eskandarlou, we discovered a very interesting potential solution:
We can derive the azimuthal angle of a pixel in a profile by looking at its four-connected neighbors: If we define 'dx' as the difference of value between the next and previous pixels in the horizontal (X) direction, and 'dy' similarly for the Y direction, we can obtain the azimuthal angle of the pixel by taking the 'atan2' of 'dy/dx'.
In the left image below, you can see a small point-source on the wing of a bright elliptical Sersic profile. and on the right, you can see how the output of the analysis above (done through simple kernels with 'arithmetic').
As you see from this image, as long as we are primarily influenced by the point source, we fully cover the range of azimuthal degrees (0 to 360). But once we reach the part where the main galaxy dominates, the range of azimuthal angles significantly decreases to one small interval.
By checking the interval of the azimuthal angles as layers are added, we should be able to constrain the border of the small object without eating too much of the wing of the bright one.
We can use bits in one integer (maybe 16bit) to keep track of the azimuthal coverage as we expand the clump. We should explore this...
Thanks a lot to Sepideh for the very productive discussion that lead to this.