A year-long keogram
This image is a year-long keogram, showing the sky above the village of Dwingeloo in the Netherlands. The keogram is constructed from about 2.1 million individual exposures taken with an all-sky camera during 2021.
The all-sky camera uses an ZWO ASI178MC color camera and a 2.5mm F/1.2 fisheye lens. The camera was operated from a Raspberry Pi 3B+ using software based on https://github.com/thomasjacquin/allsky. The exposures were obtained using automatic exposure time and gain settings based on the brightness of the previous image, where the exposure time could vary between 32 microseconds and 15 seconds, and the gain could vary between 0 and 200 (in camera units). Daytime observations were obtained with gain 0, and once the exposure time had increased to 15 seconds during evening twilight, the gain was allowed to increase to 200. During morning twilight, the gain first decreased to 0 before the exposure time was allowed to decrease. This approach maximized the total exposure on the sky.
For each day a keogram was constructed by taking the central column of pixels of each individual exposure taken that day, ordering by time. This central column of pixels is approximately aligned with the local meridian running from South to North through Zenith. Using the timestamps of the exposures taken approximately 15 seconds apart, the keogram was interpolated in time to obtain the pixel values at minute intervals, creating a 1440 by 900 pixel keogram for each day, showing the movement of the Sun, Moon, stars and clouds through the sky. The keogram of each day was then appended in the vertical direction to create this 1440 by 329400 pixel image. A total of 366 days are appended in this keogram, from January 1, 2021 at 00:00UTC up to January 2, 2022 at 00:00UTC. Hence, the horizontal scale runs from 00:00UTC to 23:59UTC daily (left to right), while the vertical scale runs from January 1, 2021 to January 2, 2022 (top to bottom, 900 pixels per day).