Hot Pixel

Author: Tianjiang Says

Website: https://cislunarspace.cn

Definition

A hot pixel is an anomalous pixel in an astronomical sensor (CCD or CMOS) caused by manufacturing defects or radiation damage. These pixels continuously generate signals above normal levels (dark current) even without illumination, causing them to appear as abnormally bright isolated pixels or pixel clusters in output images.

Core Principles

Causes

Hot pixels arise primarily from:

  1. Manufacturing defects: impurities or defects in the silicon lattice during sensor manufacturing create additional carrier generation centers at specific pixel locations
  2. Radiation damage: sensors exposed to space radiation environments over time accumulate lattice defects from high-energy particle impacts, producing new hot pixels
  3. Temperature effects: the number and intensity of hot pixels are positively correlated with sensor temperature — higher temperatures mean greater dark current

Impact on Source Detection

The impact of hot pixels on astronomical image processing manifests primarily during the source detection stage. Sun et al. (2026) note that "hot pixels are common for wide-field sensors. This affects the source extraction, increasing the number of spurious detections."

Specific impacts include:

  • Spurious source detection: source detection algorithms may mistake hot pixels for real astronomical sources
  • Photometric errors: hot pixels can interfere with flux measurements of nearby real sources
  • Positional errors: hot pixels may cause systematic offsets in source positions

Special Impact on Stacking Search Algorithms

When using the stacking search algorithm (SAA) to detect moving objects, the impact of hot pixels is further amplified. Sun et al. (2026) warn: "after SAA, the number of spurious detections may increase dramatically, so it is important that the hot pixel problem is carefully addressed."

This occurs because the SAA algorithm shifts and stacks images under different velocity assumptions, and fixed-position hot pixels may coincide with the trajectories of real moving targets in different stacking combinations, producing large numbers of false detections.

Removal Methods

3×3 Median Filter

Sun et al. (2026) employ a 3×3 median filter to remove hot pixels. The principle is:

  1. For each pixel, replace it with the median value of its 3×3 neighborhood
  2. Hot pixels, due to their abnormally high values, are replaced by the normal background level
  3. This operation preserves most of the signal from real astronomical sources (since sources typically span multiple pixels)

Double-Interpolation Artifact Risk

A subtle artifact risk exists during hot pixel removal. Sun et al. (2026) point out that "the hot pixel and its interpolated counterpart may occupy areas that are too small to initially exceed the minimum detection area threshold, but after two consecutive interpolations, the artifact may be extracted as a distinct object."

This means that while the median filter can eliminate individual hot pixels, it may create a minor brightness anomaly at the hot pixel location that could evolve into a detectable spurious source after multiple image processing steps. Careful balancing between filter parameters and detection thresholds is therefore required.

Applications in Cislunar Observation

In optical surveys of cislunar moving objects, hot pixel processing is an indispensable step in the entire image processing pipeline. For wide-field survey telescopes, the number of hot pixels can be quite substantial; if not properly handled, they will severely degrade the survey's false alarm rate and detection efficiency.

Sun et al. (2026) schedule hot pixel removal before the stacking search algorithm, ensuring that images fed into the SAA are cleared of hot pixel interference, thereby effectively controlling the false alarm rate.

References

  • Sun, R., Zhang, Q., Yu, S., et al. Optical Survey for Cislunar Moving Objects Using Image Stacking. AJ, 2026.