bugGNU 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

Sun 03 Jul 2022 11:35:44 PM UTC, comment #1: 

The image below is the output of the script in the original post:

large clump

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>
Project Administrator
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.

# Parameters.
oversample=5
zeropoint=22
background=20

# Abort in case of an error.
set -e

# Make a build directory if it doesn't already exist.
if ! [ -d build ]; then mkdir build; fi

# Generate the catalog
cat=build/cat.txt
echo "1 0   0   2 3  2.8 0 1   5  5"  > $cat # PSF
echo "1 100 100 1 15 1   0 0.4 9  5" >> $cat # Galaxy
echo "1 110 100 4 0  0   0 0   13 0" >> $cat # Star

# Build an image and a convolution kernel.
psf=build/psf.fits
raw=build/raw.fits
astmkprof $cat --mergedsize=200,200 \
          --zeropoint=$zeropoint -o$raw \
          --oversample=$oversample
mv build/0_raw.fits $psf

# Convolve the image with the kernel.
conv=build/conv.fits
astconvolve $raw --kernel=$psf -o$conv

# Undersample the convolved image.
nonoise=build/no-noise.fits
astwarp $conv --centeroncorner --scale=1/$oversample \
        --output=$nonoise

# Add noise
noised=build/noised.fits
export GSL_RNG_SEED=1657794821
astarithmetic $nonoise $background $zeropoint \
              mag-to-counts mknoise-poisson \
              -o$noised --envseed

# Detect the signal
det=build/detected.fits
astnoisechisel $noised --tilesize=10,10 -o$det

# Segment the signal
seg=build/segment.fits
astsegment $det -o$seg

Mohammad Akhlaghi <makhlaghi>
Project Administrator

 

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Attached Files
file #53390:  large-clump.png added by makhlaghi (62KiB - image/png)

 

Depends on the following items: None found

Items that depend on this one: None found

 

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    Date Changed by Updated Field Previous Value => Replaced by
    2022-07-03 makhlaghi Attached File- Added large-clump.png, #53390
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