Stacking, frame by frame

Explainer · 0:39 · silent, with on-screen words

Watch noise melt away as 1, 4, 16 and 64 frames of the Southern Cross are stacked, and see the signal-to-noise ratio grow as the square root of the number of frames.

The video file (MP4, 2347 kB)

Shows a simulated 32° by 18° field around the Southern Cross: real star places and magnitudes (Yale Bright Star Catalogue), seeded random noise in every frame.

Chapters

  1. 0:03 One frame
  2. 0:10 Four frames
  3. 0:16 Sixteen frames
  4. 0:22 Sixty-four frames
  5. 0:28 The square-root law

From the lesson Your First Astrophoto Session.

What the video shows, in words

A silent explainer with a simulated image, said so on screen. It shows a 32° by 18° patch of sky around the Southern Cross with 67 stars where the Yale Bright Star Catalogue puts them and as bright as it says, with fresh random noise in every frame, and averages 1, 2, 4 ... 64 frames.

The signal-to-noise ratio of a magnitude 5.0 star, measured on the stacked pixels (its signal over the measured noise of the background): 1 frame 1.5, 2 frames 2.1, 4 frames 3.0, 8 frames 4.3, 16 frames 6.0, 32 frames 8.4, 64 frames 12.0. It grows as the square root of the number of frames: 4 frames double it, 16 frames quadruple it.

  1. 0:00 Why do astrophotographers take dozens of the same shot? Stack them and watch.
  2. 0:03 One frame: the bright stars of the Cross are clear, but the faint star in the ring (magnitude 5.0) is lost in the noise. Measured signal-to-noise: 1.5.
  3. 0:10 Average 4 frames: the star stays the same, the random noise partly cancels. Signal-to-noise 3.0, 1.99 times one frame.
  4. 0:16 16 frames: signal-to-noise 6.0, 4.02 times one frame. Fainter stars come through.
  5. 0:22 64 frames: 12.0, 8.00 times one frame. The sky is smooth and every catalogue star shows.
  6. 0:28 Four times the frames, twice the signal-to-noise: it grows as the square root of the number of frames, so doubling it takes four times as many.
  7. 0:35 Shoot more frames, not just longer ones: stacking is free signal-to-noise.

Tools · Learn