From Gegenschein to False dawn
Zodiacal light is one of the most prominent “unsolicited” lights in the sky, if speaking about deep sky observation at professional astronomical sites such as ESO’s La Silla Observatory in Chile. But the light in the night sky is caused by one of the structures of the Solar System that is curently being studied a lot, and it is definitely a big challenge for ground photographic projects. It had been a while since I’d tried to capture an all-night zodiacal light panorama from Mangaia in the Cook Islands and I was really encouraged by prof. Miloslav Druckmüller’s and prof. Shadia Habbal’s result from Mauna Kea to start over with brand new project which could reveal more structures along the band of the faint light, coming from the ecliptic plane. And I finally succeeded on 14th April 2016 in the second half of the night at La Silla, when the Moon set over the horizon and the sky turned dark.
But first, let me introduce the zodiacal light a bit deeper. As recently known, the origin of the zodiacal light is to be found in the inner Solar System as the sunlight that is forward scattered in the direction of Earth from particles placed along the ecliptic plane. Infrared observations from the IRAS satellite and COBE satellite revealed emission from those small grains composed of dust and ice surrounding the Sun. Most of the observed particles have sizes in the range 1 to 100 µm. The Poynting–Robertson effect forces particles inward through absorption of solar radiation and isotropic emission, reducing their angular momentum, while the dominant force for µm-sized particles is the solar radiation pressure that accelerates them away from the Sun. The zodiacal dust hence needs to be constantly replenished. This replenishment is primarily served by crumbling icy comets, but also by colliding asteroids and possibly interstellar dust. Recent studies also show that over 85 percent of the cometary material in the zodiacal cloud comes from dust of Jupiter-family comets.
When viewed from Earth, this disc of particles – distributed in the plane of the ecliptic – appears as a band across the sky passing through the constellations of the zodiac, hence the name zodiacal light. As the scattering of sunlight is most effective at smaller angular distances from the Sun, the band of light along the ecliptic gets fainter and narrower further away from the Sun. Along the ecliptic, at the point in the sky opposite the Sun (the antisolar point), coherent backscattering from dust particles further out in the Solar System beyond the Earth’s orbit leads to the oval patch of light known as the Gegenschein. The name was given by the German explorer Alexander von Humboldt.
Observations of zodiacal light in more recent times have mostly focused on particular structures of the zodiacal cloud. Interesting zodiacal dust bands were discovered by the IRAS mission in 1984. They are produced by collisions of asteroids in the Main Belt between Mars and Jupiter within the last few million years. Hints of those dust bands are noticeable even on photographs captured on DSLR cameras and mathematically post-processed from the ground observations. And this is also the case with my image. Using very similar methods as prof. Druckmüller, I was able to reveal the structures you can see in the image. Unfortunately, I was limited by the moonlight and small slope of the evening column of the light, so I couldn’t map it from the dusk to the dawn. So I focused at least for the almost 270 degrees of the band, showing the light poetically from the Gegenschein to the false dawn. Following shows the post-process steps to get the resultant image.
- Image is panorama of 7 segments, each segment is stacked image of 47×30 seconds frames captured with Canon 6D Baader IR Modified (technical info + sensitivity characteristic) and Samyang 24 mm, f set to 3.5, used Vixen Polarie
- Calibration was made via darkframes (137 darkframes with temperature +- 3 deg from temperature of light images). First color unification of fragments before stitching, then stitched.
- Gradient and color of foreground reduced by gaussian filter (colors of foreground), inversed gaussian filter in gray scale (gradient) and color filter of airglow colors. Thus especially the green and light-orange colors are not visible, the image doesn’t show colors of all stars in same level.
- For the image saved three copies, each was then processed individually by different values of gamma (from 0,9 to 1), contrast (from 0 to 25) and curves (using full range of histogram). Images were then stitched in similar way like HDR, which better showed the ZL band, avoided overexposured parts of Milky way and enhanced “natural” contrast between ZL band and the starry foreground.
- Foreground reduced by color-noise filter to avoid prominent color differences after curves and contrast actions were applied before, which enhanced the noise by the artificial HDR (three same images had the same noise-mask, of course)
- The image was color calibrated for as neutral as possible foreground on three different monitors (with deeply valuable help of prof. Druckmüller). Due to really prominent airglow it was not possible to get enough color-faithful result.
I would like to acknowledge support from the European Southern Observatory, the VIXEN company for providing the Vixen Polarie mount and prof. Miloslav Druckmüller of Brno University of Technology (Faculty of Mechanical Engeneering, Istitute of Mathematics), without whom this image would not have been made possible. The image in high resolution (3500x900px, 300dpi, PNG file) can be found here. If you are more interested on the zodiacal light, just check the following video produced by ESO.
Check out more in video: ESOcast about Zodiacal light
(produced by European Southern Observatory)
