[bug #62702] Clumps on strong gradients of larger galaxies become too large

[Mohammad Akhlaghi]

Follow-up Comment #2, bug #62702 (project gnuastro):

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').

(file #54234)

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.


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