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Observation times

The gegenschein is one of the hardest to observe phenomena in the sky. It is similar to the dry-heiligenschein effect observed on Earth (the bright spot around the shadow of an airplane, for example). But the gegenschein is very faint, a few magnitudes fainter than the Milky Way. If you live anywhere near a city or other light-polluted area, you will have no chance to observe it.

The gegenschein is one of the faintest phenomena to observe. It requires long time exposures on fast film.

The gegenschein is best visible during September-October or February-March, while the antisolar point or opposition point (the point opposite the sun) moves along the ecliptic in an area free of the Milky Way. Observation times are best around local midnight, when the sun is deepest below the horizon and thus the antisolar point with the gegenschein around it, highest in the sky. Before you go out to observe, look up the location of the antisolar point in a sky chart program, or try to figure it out for yourself based on the inclination and position of the ecliptic. This can be tricky for beginner observers, however, since the ecliptic makes an angle with the equator. But it helps to know that in winter, at midnight, the antisolar point is where the sun would be at noon in summer.

Go out to a dark location, and let your eyes adapt to the dark at least 20 minutes. The sky must be free of dust, clouds, and water vapor, and ofcourse the night should be moonless. Bright planets such as Jupiter and Saturn should be at least 90 degrees away from the point as well.1 After scanning the approximate location of the antisolar point, you will see a very faint glowing patch of light, approximately 10 x 20 degrees wide. You can see it best when not looking straight to it, but look a few degrees away from it, as your eyes are more sensitive for peripheral viewing.

Film & exposure settings

Needless to say, photography of this phenomenon requires a very fast film, or very long exposure times. If the sky is not dark, photography of the gegenschein is next to impossible. Use 1600 ISO film or faster, f/2.8 or wider aperture, and exposure times of at least 30 minutes. Use a sturdy tripod and cable release for the camera.

Lens

If you do not have a star-guiding setup, the stars will produce trails on your photo, and after 30 minutes the gegenschein will start to blur out as well, as it is (almost) fixed with the position of the stars for any given night. Hence, use a wide-angle lens such as a 20mm or wider. If you do have a guide system, you can best use 28mm or 35mm.

Note that the later you go out to observe, the darker and more clear the sky generally gets, due to the cooling down of the atmosphere and the reduction of the airglow. But also, the gegenschein will be at a lower elevation. I think the best time to photograph this phenomenon is between 00h and 02h local time.


1. However, on February 21st, 2003 in New Mexico I was still able to see the gegenschein with effort, while Jupiter, being almost in opposition, was just 10 degrees away from the opposition point. I had to shade Jupiter with my hand. Also, the gegenschein was at that moment just about 40 degrees high in the sky, towards the ESE with the light arch from Socorro, NM nearby. So, the requirement that bright planets are far away from the point and the opposition point be high in the sky is apparently not so strict. However, it should be added that I was observing at an altitude of almost 6000 ft AMSL (almost 2 km) on the day after a front moved through to clean the air.