Galaxy Image Denoising

Code author: Samuel Farrens <samuel.farrens@cea.fr>

In this tutorial we will remove the noise from an example galaxy image using the PySAP-Astro plug-in.

Import dependencies

Import functions from PySAP and ModOpt.

import numpy as np
import matplotlib.pyplot as plt
from pysap.data import get_sample_data
from astro.denoising.denoise import denoise
from modopt.signal.noise import add_noise

Clean image

First, we load the image of galaxy NGC2997 from the PySAP sample data sets.

galaxy = get_sample_data('astro-ngc2997')

Then we can show the clean galaxy image that we will attempt to recover.

plt.imshow(galaxy)
plt.show()

Out:

/volatile/Chaithya/actions-runner/_work/pysap/pysap/examples/pysap-astro/plot_galaxy_denoising.py:36: UserWarning: FigureCanvasAgg is non-interactive, and thus cannot be shown
  plt.show()

Generate a noisy observation

To simulate an observation of NGC2997 we add random Gaussian noise with standard deviation 100 using the ModOpt add_noise function.

obs_data = add_noise(galaxy.data, sigma=100)

Now we can show the noisy galaxy image.

plt.imshow(obs_data)
plt.show()

Out:

/volatile/Chaithya/actions-runner/_work/pysap/pysap/examples/pysap-astro/plot_galaxy_denoising.py:52: UserWarning: FigureCanvasAgg is non-interactive, and thus cannot be shown
  plt.show()

Note

\(\sigma=100\) is quite excesive, but we can more easily visualise the noise added to the image in this example.

Denoise

We will use the denoise function from PySAP-Astro to denoise the noisy image. We set the number of wavelet scales to 4.

denoised_data = denoise(obs_data, n_scales=4)

We can show the denoised galaxy image.

plt.imshow(denoised_data)
plt.show()

Out:

/volatile/Chaithya/actions-runner/_work/pysap/pysap/examples/pysap-astro/plot_galaxy_denoising.py:74: UserWarning: FigureCanvasAgg is non-interactive, and thus cannot be shown
  plt.show()

Residual

Next, we calculate the residual of our denoised image to get a measure of how well the denoising process performed.

residual = np.abs(galaxy.data - denoised_data)

Finally, we can show the residual.

plt.imshow(residual)
plt.show()

Out:

/volatile/Chaithya/actions-runner/_work/pysap/pysap/examples/pysap-astro/plot_galaxy_denoising.py:89: UserWarning: FigureCanvasAgg is non-interactive, and thus cannot be shown
  plt.show()

Tip

Typically for a denoising problem we are aiming for a residual without any structure, i.e. just noise.