The sources of the Piave, at the foot of Mount Peralba, lugo in Italy are the darkest I could find so far. At the zenith of summer (and therefore with the constellation Cygnus above his head) I recorded SQM values \u200b\u200baround 21.5-21.7, respectively August 29, 2008, October 25, 2008 and July 25, 2009.
At these levels, the difference with a perfect site, as might be Namibia, is on the horizon. That little light that comes, and that Abbas few tenths of the zenith, the horizon comes.
Inspired by the great American parks monitoring , I thought about doing something similar. During the last exit on 25 July, at about 2 am I made a series of photographs along the southern horizon, then I made here in the panoramic photo above (click the photos to see details).
The photographs were exposed to ISO 1600, for 30 seconds at F3, 5, 18 mm focal length, with a Canon EOS 1000D. I processed the RAW image to a color temperature of 4600 ° K (10% less than the standard to play, as possibilie, la sensibilità spettrale scotopica, che ha il massimo a circa 500 nm, cioè il 10% in meno rispetto alla sensibilità fotopica che è 550 nm). Nella elaborazione dei RAW ho anche sfrtuttato la correzione della illuminazione al bordo (un flat sintetico) della fotocamera. Non ho sottratto Dark frames.
Il livello di esposizione è superiore alla sensibilità dell'occhio: per esempio solo la nuvoletta sopra Tolmezzo era vagamente percepibile, non le altre. Nel complesso si poteva notare il chiarore a Sud ma la foto da una impressione molto più enfatizzata di quanto si potesse percepire dal vivo. L'enfasi è voluta per evidenziare le sorgenti di inquinamento luminoso.
Nella foto, aiutandomi con le stelle di campo, I found the directions East, South and West and have also indicated the horizon (0 °) and a line height to 20 ° (where the effect of the IL is completely sold out). The Mountain SSO to get to 17 degrees altitude, to the south there is a fork in the direction Ampezzo, Cima Sappada, while at ESE is a fork with free horizon that points to the Friuli plain.
With the help of Google Earth I rebuilt the directions of the major population centers and I scored in the bottom of the picture. From East to West we order:
Pierabec, 5.5 kilometers (a few dozen souls, perhaps).
Avoltri Forni, 6.6 km, 700 inhabitants.
Tolmezzo, 33.5 km, 10,700 inhabitants.
Udine, 75 km, 99100 inhabitants (in the direction of Udine, there are also small towns in Pesarina Buia and NO in Udine).
Ampezzo, 23.5 km, 1100 inhabitants (in the direction of Ampezzo, there is also some small town in Pesarina).
Cima Sappada, 6 km (including residents in those Sappada).
Sappada, 6.5 km, 1300 inhabitants (including Cima Sappada).
Santo Stefano di Cadore, 14 km, 3000 inhabitants (in the direction of St. Stephen there's San Pietro di Cadore, 11 km, 1800 anime).
Comelico, 9 km, 4200 inhabitants (excluding Santo Stefano and San Pietro).
Padola, 11.5 km, 950 inhabitants.
San Candido and Dobbiaco, 35-38 km, a total of 6400 inhabitants. You can do some
interesting considerations. The mountains of Carnia are relatively low and little shield. It 's interesting to note that the worst pollution comes from Tolmezzo (33 km) and Udine (75 km), while in the direction of Comelico with nearly the same population of Tolmezzo, at a distance of only, 11 km pollution is much less . The mountains of Comelico are very steep, and the light from the valley can only go out on a lot inclianata direction, and does little damage. Instead, in the direction of the Friuli mountains are lower than the proportion of the sources of the Piave try to free up the horizon is 0 ° and you can see the clouds over the Friuli, lit from below (not seen, fortunately directly plains) . The direction
NO, ritratta nell'articolo del 25 luglio 2009 è quasi perfetta. Osservando attentamente si nota che l'orizzonte è leggermente inquinato da Dobbiaco e San Candido (se ci fosse solo airglow dovrebbe scurirsi sotto i 10° di altezza quando prevale l'estinzione). Ciò nonostante la direzione è molto molto buona e corrisponde ad una classe di Bortle fra 2 e 1 (airglw ben visibile). Da notare che le nuvole sono completamente nere.
La direzine Sud è un po' inquinata dalle luci di Sappada, Cima Sappada e oltre, ma le nuvole sono ancora scure.
La direzione SE è la peggiore e qua le nuvole cominciano a essere debolmente illuminate (a occhio nudo appaiono molto deboli) e questa direzione ha caratteristiche di Bortle 3.
I then processed the images obtained the contours of the intensity surface as the site of the American parks. The following figure shows the isophotes corresponding to 20, 20.5, 21, 21.5 mag arcsec square. It 'also see the Milky Way and the Shield. In a southerly direction on 21.5 is reached around 30 ° in height (but there is also the effect of the Milky Way. Westbound on 21.5 airglow is around 15 degrees altitude. The clouds above have Tolmezzo magnitude surface 20 . The isophotes of 21.0 is in some way limit the polluted area, which unfortunately coincides with the South, up to about 20 degrees altitude.
Val Visdende is in the westerly direction, The dairy Silvella towards NO and should be a really remarkable site.
It is also interesting to compare with American sites listed in the link above. Nearly all suffer from similar circumstances, with lights that also came from hundreds of km (the flat board does not provide defense) and the isophotes of magnitude sperficiale 21 which also extend over 30 ° high.
