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December 8: A Double Eclipse

  December 8: A Double Eclipse  
  PROBA2 Eclipse Observations  


Click on the above image for a movie of the eclipse



Once or twice a year PROBA2 witnesses a solar eclipse, as the Moon passes between the satellite and the Sun. Since the spacecraft travels rapidly in its orbit — more than 25,000 km/hr! — the eclipse unfolds extremely rapidly compared to an eclipse viewed from a fixed location on the Earth's surface.

The movie above shows an especially unique eclipse. First the moon passes between the spacecraft and the sun, then the Earth passes in front of both Sun and Moon. So this is particular event is a double eclipse! In fact, during the winter, PROBA2 experiences Earth eclipses regularly. Its special orbit gives it a good view of the sun nearly all the time, but in order for the orbit to properly precess to account for the Sun's changing position in the sky over the course of the year, the spacecraft's orbit has to be inclined in such a way that PROBA2 drops into the Earth's shadow near the north pole briefly during every wintertime orbit.

Eclipses are provide useful opportunities to do all kinds of science, of course, but they also provide excellent opportunities to study the performance of the SWAP telescope. SWAP has a complicated set of internal baffles to help block unwanted stray light from the images, but some unwanted light always gets through. This light arises either from reflections off of polished surfaces inisde the telescope or from microscopic imperfections in the mirrors that generate the image SWAP observes. Most of the time we cannot measure this light because it cannot be easily separated from the bright sun in each image.

However, when the Moon passes in front of the Sun, we see the dark disk of the Moon, which produces no Extreme-Ultraviolet light of the variety SWAP observes. As a result, we know that any brightness we observe on the Moon must arise from stray light in the instrument. So we use eclipses to measure this stray light and, once measured, we can develop tools to remove it and improve the image contrast. You can read about how we measured stray light using an eclipse and, in turn, used the measurements to make some really special high-quality images in this recent paper.