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Counter-glow in Capricornus

leyenda si/no

Fecha:15./21.08.04 Hora:22:13 / 0:04 UT Exposición:77 min / 45 min
Campo visual:44o x 30o Emulsion: Fuji Provia 400F / Kodak E200 Filtro:none
Instrumento:f=35mm 1/4.0
(reduced from 1/2.0)
Lugar:Hakos, Namibia Observador:Till Credner

What is the gegenschein?
The gegenschein, the german word for counter glow, is a faint and diffuse but extended glow in the night sky, hardly visible to the naked eye only at very dark sites. It is best seen at local midnight, since the position is always the antisolar point, the direction just 180 degrees opposite of the sun in the sky (anthelion). Dust in the plane of our planetary system is scattering sunlight. This makes the diffuse glow along the ecliptic of the zodiacal light close to dusk and dawn, but also the lightbridge and the gegenschein. Since dust scatters favorable into backward direction there appears a brighter patch, the gegenschein, in the anthelion.

The two images above, taken about five days apart in August 2004, show the diffuse night sky brightness in the constellation of Capricornus. On the right hand side are the eastern outskirts of the milky way and just left of it some diffuse background structures probably because of remaining integrated star flux. These background structures are more or less identical on both images whereas the diffuse patch close to anthelion shifts about five degrees, just the amount the sun, and so the anthelion, shifts along the ecliptic in five days. It is not quite clear if the diffuse component between Milkyway and Gegenschein might belong to the Light-bridge. The plane of the ecliptic might favour this, however, the patterns seem to be fixed in the five day intervall. It is probably both: Light-bridge and integrated star flux superposed.

A more close inspection of the gegenschein's central position shows a considerable deviation of about 1 or 1.5 degrees from the anthelion to the south. The longitude along the ecliptic matches quite well with the anthelion (within an accuracy of roughly 0.5 degrees).

An offset in ecliptical latitude was also reported by other observers and is commonly explained by an inclination of the circumsolar dust cloud with respect to the ecliptic (see discussion for example by James et al. 1997). The inclination is about 2 degrees but there is many uncertainty in this number and still the question of an existance of a stable symmetry at all. Asteroidal dust bands can influence the observed position of the gegenschein considerably as shown by Tadashi et al. 2003.

Some words for doing proper imaging of the gegenschein and the following image processing:

Image aquisition

Image processing
The above image is heavily processed and does not resemble the view by naked eye. The background intensity (but not the stars!) was smoothed and contrast enhanced to bring out the elusive gegenschein well. The filter radius for smoothing was about 2 degrees, so all possible structures with sizes of 2 degrees or smaller are gone. (Nevertheless, the accuracy for the centroid position of the gegenschein is better than 2 degrees, roughly about 0.5 degrees)
This is a single original, but constrast enhanced, exposure showing stars and gegenschein with the proper intensity relation.

Processing steps:
  • Align exposures.
  • Remove stars on each frame with a low frequency pass filter, for example a broad "dust/scratches" filter in Photoshop, this will also smooth the background heavily. Find a good compromise for the filter radius: maximise the size to remove all stars as good as possible, but minimize the size to maintain background structures.
  • Add consecutive exposures.
  • Increase contrast to show the gegenschein well.
  • Add one of the original aligned images to see the stars again.

See also:
All Sky fisheye view 22.08.2004 showing the light brigde with gegenschein
Zodiacal light, evening of 16.08.2004
Brorsen, M. Th., "Über den Gegenschein des Zodiakallichts", 1856AN.....42..219B
Tadashi et al., "The influence of the brightness of the asteroidal dust bands on the gegenschein", 2003Icar..162..337M
James et al., "The morphology and brightness of the zodiacal light and gegenschein", 1997MNRAS.288.1022J
Leinert et al., "The 1997 reference of diffuse night sky brightness", 1998A&AS..127....1L