All of the Zodiacal Light

All of the Zodiacal Light - Annotated with planets, ZL parts and observation
All of the Zodiacal Light

All of the Zodiacal Light

This of the entire Zodiacal Light from dusk to dawn was taken from the platforms of Gemini South in Chile (left) and Gemini North in Hawai‘i (right), the two telescopes comprising the International Gemini Observatory, a Program of NSF’s NOIRLab. Since both platforms provide unique observational conditions in high altitudes (Gemini South at 2722 m, Gemini North at 4214 m), it is possible to see and capture even the faint structures of the Zodiacal Light. This bright white band is created by sunlight scattering off interplanetary dust dispersed throughout the Solar System. This light appears along the pathway of the Sun and planets across the sky, known as the ecliptic (where the constellations of the Zodiac appear as well, thus the name), and it’s so bright that it can easily be mistaken for light pollution on a dark night. The bright patch of light at the center of the image is directly opposite the Sun in the sky. This spot is the Gegenschein. On the left side of the image, you can zoom in to see a rising Jupiter along with Venus (the brightest spot on the left), Mars, and Saturn. The dust that produces the Zodiacal Light and Gegenschein comes from a variety of sources, including dusty comet tails and asteroid collisions. Interestingly, Mars may actually produce a large portion of this dust as it sheds its atmosphere, based on recent studies. Taken from both hemispheres to avoid the slope of the ecliptic, this could be the most complete Zodiacal Light panorama.

All of the Zodiacal Light - Annotated with constellations

All of the Zodiacal Light – Annotated with zodiacal constellations

The image was taken from both hemispheres at the beginning of April 2022, which required perfect timing of the Moon phase but also quick traveling. The photography was very difficult (and the resulting image is not perfect so far), especially due to the very strong wind at Mauna Kea, while the southern part was taken during incredibly strong airglow, and for this image, I had to create very specific false flatfield to subtract the structures and colors of the airglow and volcanic dawn. While the northern (right) part was mostly taken on April 2nd evening, southern (left) on April 6th morning… So 3,5 days are different, which is almost nothing considering the movement of the Gegenschein in the sky. This is, however, the reason why it was so crucial to have good weather last night at Mauna Kea and the first night at Cerro Pachón. Crucial was also the Moon phase, which was favorable for the project just around these days (no Moon in the evening from Mauna Kea and then no Moon in the morning from Cerro Pachón).

All of the Zodiacal Light - Annotated with planets, ZL parts and observation

All of the Zodiacal Light – Annotated with planets, ZL parts and observation

Full credit of the image: NOIRLab/NSF/AURA/ Petr Horálek (Institute of Physics in Opava), Tomáš Slovinský.

NOIRLab page of the image: https://www.noirlab.edu/public/images/iotw2225a/

Brief post-process steps

  • The image is a panorama of 21 segments, each segment is a stacked image of 60×30 seconds frames captured with Canon Ra and Samyang 24 mm, f set to 2.2, ISO 4000, used Vixen Polarie U.
  • Calibration was made via dark frames (163 dark frames with temperature +- 3 deg from the temperature of light images). First color unification of fragments before stitching, then stitched.
  • Gradient and color of foreground reduced by Gaussian filter (colors of the foreground), inversed gaussian filter in grayscale (gradient), and color filter of airglow colors. Thus especially the green and light-orange colors are not visible in the foreground (while the same colors were preserved for stars and Milky Way, using color removal adjustment depending on the brightness of objects).
  • For the image saved in three copies, each was then processed individually by different values of gamma (from 0,9 to 1), contrast (from 0 to 25), noise reduction level (1-50), and curves (using the full range of histogram). Images were then stitched in a similar way to HDR, which better showed the ZL band, avoided overexposure parts of the Milky Way and enhanced “natural” contrast between the ZL band and the starry foreground.
  • Using dark contrast and star diameter reduction (high pass filter) helped with better enhancement of the ZL itself, including its structures
  • The image was color calibrated for as neutral as possible foreground on three different monitors to get enough color-faithful results (with help of Mahdi Zamani from NOIRLab)
  • Finally, the H-Alpha regions were enhanced by adding a layer of the narrowband images of the regions taken with an Astronomink 12mm filter (with Canon 6D, Sigma 28mm; author Tomáš Slovinský).

A window to history

Additionally, 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 is scattered in the direction of Earth from particles placed along the ecliptic plane. Infrared observations from the IRAS satellite and COBE satellite revealed emissions from those small grains composed of dust and ice surrounding the Sun. Most of the observed particles have sizes in the range of 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 the 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.

Recent studies based on data from Juno spacecraft show the origin of the dust in the Zodiacal cloud could, actually, come even from planet Mars. However, how the dust from storms in Mars’ atmosphere gets into the ecliptic plane, is now a matter of further research.