Keogram - aaronwmorris/indi-allsky GitHub Wiki

Keograms

A keogram compresses an entire night of sky conditions into a single composite image. It is constructed by extracting a vertical strip of pixels along the central meridian from each sequential capture frame and appending them chronologically from left (sunset/dusk) to right (sunrise/dawn).

The horizontal axis represents time; the vertical axis represents the sky from horizon to zenith to opposing horizon. Keograms provide an immediate diagnostic summary of cloud cover, auroral activity, light pollution shifts, and clear-sky intervals across the night.

1. Meridian Alignment & Rotation

For celestial and meteorological features to track consistently, the vertical slice must align with the local celestial meridian (the North-South celestial circle passing through the zenith).

In Configuration → Image Processing → Keogram:

  • KEOGRAM_ROTATION: Rotates the source frame before extracting the central pixel column.
    • 0: No rotation applied.
    • Positive values (1 to 180): Rotates the image counter-clockwise (CCW).
    • Negative values (-1 to -180): Rotates the image clockwise (CW).

Determining the Rotation Angle

  1. Identify true North and South in a calibrated night image (e.g. using Polaris in the northern hemisphere or Crux in the southern hemisphere, or reference the VirtualSky overlay).
  2. Measure the angular deviation between your camera sensor vertical axis and the true North-South meridian line.
  3. Apply the corresponding degree adjustment to KEOGRAM_ROTATION so North appears at the top and South at the bottom of the extracted column.

2. Generation Modes

Automatic Nightly Compilation

The background capture daemon compiles the final nightly keogram automatically at dawn once the Sun rises above the configured daylight horizon threshold. Assets are recorded in IndiAllSkyDbKeogramTable and stored in /var/www/html/allsky/images/.

Realtime Incremental Keograms

indi-allsky can generate an incremental keogram that updates dynamically after every individual capture throughout the night:

  • Enable Configuration → Image Processing → Realtime Keogram (REALTIME_KEOGRAM__ENABLE).
  • View the live progress in the web interface at: https:///indi-allsky/realtime_keogram

Historical Keogram Regeneration

If a historical date requires regeneration:

  1. Open the web interface and navigate to Images for the target date (https://<allsky-host>/indi-allsky/images?dayDate=YYYYMMDD).
  2. Select Actions → Generate Video/Keogram/Star Trails.
  3. The web application dispatches a generateKeogramStarTrails task into the daemon's internal VIDEO task queue (TaskQueueQueue.VIDEO). The capture daemon processes the task asynchronously in the background.

3. Related Documentation