"What are the conditions needed to have the darkest sky possible?"
Obviously there should be no Moon light. The Milky Way and Zodiacal Light should not be visible, or kept to a minimum. Last, there should be no light pollution. If these are correct criteria, there is a limited window of opportunity for the darkest sky experience and a certain geographic location is needed.
The picture was taken by Stéphane Guisard, at new Moon (No Moon light), middle of the night (minimum Zodiacal Light effect), in Atacama Desert close to Paranal Observatory (no light pollution). The chosen place for this picture has the particularity to be located at a latitude whose value is close to the declination of the South Galactic Pole (SGP). The picture, therefore, has to be taken when the galactic pole passes at zenith, ensuring that there is no Milky Way up in the sky. When all these conditions are met, the very faint Gegenschein becomes visible. SGP lies in the constellation of Sculptor, at about 00:51 Right Ascension and -27d Declination. The image was taken just after local midnight on 10th October 2010, when SGP was lying just a couple degrees away from the local zenith of Paranal Observatory (24d 38'S, 70d 24'W). The natural by-product of this image is that the Milky Way plane lies all around the horizon as visible in the enclosed 360 degrees panoramic view. It is the ultimate look away from the Milky Way to the galactic southern hemisphere. The faint oval glow of Gegenschein can be seen in Pisces. It was 3 weeks after Jupiter's opposition, so the bright planet lies just to the west of Gegenschein.
Guisard's all-sky image was first published in October 2010:
http://www.astrosurf.com/sguisard/Pagim ... t_sky.html
Upon this publication, Tunç Tezel started seeking opportunities to create the northern counterpart image. He was then working in Libya, not far away from the optimum geographic latitude to have the North Galactic Pole passing overhead. NGP lies in the constellation of Coma Berenices, at about 12:51 Right Ascension and +27d Declination.
But the events of the last few months in Libya prevented the execution at the original chosen location, an oasis in Sahara Desert. So, Canary Islands emerged as the most reasonable location. The left-hand side image was taken from at Roque de los Muchachos (28d 45'N, 17d 53'W), in La Palma, Canary Islands, Spain. As the image was taken on 3rd May 2011, the oval glow of Gegenschein is lying in Libra. The only planet visible in the sky is Saturn in Virgo. It was about a month past Saturn's opposition, so the planet lies again to the west of Gegenschein. Just like Jupiter in the southern image. Roque de los Muchachos is a quality dark sky location where many astronomical observatories reside, but there are still some sources of light pollution. So, this look away from the Milky Way to the galactic northern hemisphere is not perfectly dark. But still, the Milky Way is nearly lost, as it is again lying around the horizon.
So, the initial quest of having the darkest possible sky have delivered us these two all-sky images of the two galactic hemispheres as seen from Planet Earth. An annotated version of the double picture is available in TWAN Gallery page:
http://www.twanight.org/newTWAN/photos.asp?ID=3003290 Both all-sky images are also presented as Virtual Reality videos, again TWAN Gallery page:
http://www.twanight.org/newTWAN/photos.asp?ID=3003324
http://www.twanight.org/newTWAN/photos.asp?ID=3002841
Last, the two all-sky were arranged to show the whole visible sky in 360x180-degree equirectangular projection, with the galactic equator crossing the image through the middle and the Galactic Center in the direction of Sagittarius located in the center of the image. The main difference of this picture from other whole sky pictures is the presence of a mountainous Planet Earth squeezed in between:
& Tunç Tezel (TWAN)
*Los Cielos de America
http://www.astrosurf.com/sguisard/
**TWAN
http://www.twanight.org/newTWAN/index.asp