Why does the background noise in this image of 2020 QG look like corduroy?

Question

The news item Small asteroid becomes closest ever seen passing Earth: NASA contains a "handout image" with the caption:

This NASA/JPL/ZTF/Caltech Optical Observatories handout image obtained on August 18, 2020 shows asteroid 2020 QG (the circled streak in the center), which came closer to Earth than any other nonimpacting asteroid on record NASA/JPL-CALTECH/AFP

Question: I saw what looks like a corduroy-like pattern in the background that is likely from some processing artifact. Does anybody recognize this kind of artifact? Does it look familiar?

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above: NASA image from linked article. below: Corduroy source.

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Fourier analysis (log power) of the image of the asteroid, the two spots demonstrating the strong linear periodic modulation of the background.

Concentric rings were seen when analyzing only the red part of the initial annotated image. Now that I average all three colors they are almost gone. The inclined line in the FT is probably that of the bright, straight linear trail of the asteroid.

import numpy as np
import matplotlib.pyplot as plt

fname = '424852de3808b4d6df1101d3b4f0790afd3b9d13.png'

img = plt.imread(fname)[..., :3].sum(axis=2) # sum 3 colors makes red circle gray to minimize impact on FFT

print(img.shape)

f = np.fft.fftshift(np.fft.fft2(img))
p = np.abs(f)**2

plt.figure()
c0, c1 = [int(s*2**-1) for s in p.shape]
hw0, hw1 = [int(s*2**-3) for s in p.shape]
plt.imshow(np.log10(p[c0-hw0:c0+hw0, c1-hw1:c1+hw1]), vmin=5, vmax=8)
plt.gca().set_aspect(16/9)
plt.colorbar()
plt.title('log10(ft power)')
plt.show()

Answer 1

I agree that it’s noise in a fixed pattern, but I think it’s unlikely to be related to ADC sensitivity. Typically if you have multiple ADCs, they read out blocks of the sensor (e.g. one on each corner to read out a quadrant). And sensitivity differences across those amplifiers usually is removed pretty well by flat-fielding.

In my experience, you get things like this from time-variable electronic noise, e.g. 60 Hz noise from surrounding equipment. (Probably the electronics are well-shielded from 60 Hz because it’s so ubiquitous, but there could be noise at other frequencies.)

Because CCD pixels are read out sequentially (e.g. the classic bucket brigade analogy), then any time-variable noise source that slightly changes a pixel value will manifest itself as a spatially varying pattern, where the exact pattern depends on the relationship of the readout frequency to the noise frequency. You see the diagonal stripes because the noise signal wasn’t quite at the same phase when pixel 1 of row 2 was read out as it was when pixel 1 of row 1 was read out, and so on.

(Extra points for including an actual image of corduroy in your question, BTW. 🙂)

Answer 2

That looks like a fixed-pattern noise to me.

Fixed pattern noise is a common sensor artefact for CCD sensors, One source for this is where you have less analog - digital converters (ADC) than pixels. Say, you have N ADC, each with its own sensitivity. Then you have in the resulting image a sensitivity pattern which repeats every N pixels. Depending on whether N is identical or not with the width or height of the image, the fixed-pattern noise results in inclined or exactly vertical or horizontal patterns. You might have a somewhat rectangular repitition, if you have several set of sets of ADC which start at different rows etc... all depending on exact electronic implementation