The continuing quest for dark skies June 3, 2008Posted by dorigo in astronomy, personal, science, travel.
Tags: deep-sky, globular clusters, observing sites, sky brightness, sky quality
I reported here several times in the past on my ongoing search for good sites for deep-sky observations in north-eastern Italy. A site which I had never tested and which was high in my priority list was the delta of Po. At more than 400 miles, Po is the longest italian river. It cuts northern Italy eastward, ending in the Adriatic sea with a wide delta which spreads over an area of about 200 square miles.
There are few towns in the area, especially on the coast, so light pollution comes mainly from the cities of Ravenna (about 40 miles south) and Chioggia (about 30 miles north). What is more appealing is the total lack of any light sources for about 100 miles due south and east of the delta, toward the sea. A look at the NASA maps of city lights, a feature available with Google Earth, reveals that light pollution is indeed as low as it can be in the area (see picture, right: the site is tagged with a yellow sign), if one excludes the best mountainous sites on the alps, 150 miles north.
A site located at sea level has little chance to be competitive with a site at 5000 feet above it, and so I was not expecting the area to provide an improvement over our current best choice of Casera Razzo: the atmospheric extinction cuts by about half a magnitude the light of stars and nebulae, something that even a darker sky cannot mend. However, the typically better seeing one usually finds at sea level is certainly a plus, since the increased resolution means faint stars will concentrate their light on a smaller area. All in all, I was curious to test the delta of Po.
The drive from Venezia took about one and a half hours. I was in the company of two other crazy amateur astronomers -Mauro and Paolo- and our equipment consisted mainly in Mauro’s 16″ dobsonian telescope, plus a 10″ which was in the end kept in the trunk of the car, and a 25×100 binoculars that I used little during the evening.
So how was the sky ? Well, not impressive. Granted, the sky transparency was rather poor, but we did not need to look at Mauro’s sky quality meters (he had brought two units from the same manufacturer to test their calibration) to know the site was not excellent: a lighthouse one mile south, and lots of street lamps visible around, in the radius of a couple of miles. The SQM readings were consistently below 21.00 magnitudes per squared arcsecond – more than half point below what Casera Razzo grants on good nights, and at least 0.3-0.4 below what they would have read at the better site under similarly poor sky conditions (low transparency increases sky brightness because of increased backscattering of ground lights).
The seeing was indeed better than in the Alps though: the double double star in Lyra (epsilon Lyrae, a magnificent testing ground for telescope resolution and seeing: two pairs of stars with nearly identical brightness, with separations of 2.34″ and 2.54″, lying 208″ apart – see picture on the left) showed the four stars very neatly, and one could at times see their Airy disks. I estimated seeing at about 1″, roughly a third of what we usually get from Casera Razzo. This allowed to reach a limiting magnitude estimate of 15.7 on a star next to Messier 57, the ring nebula in Lyra. By comparison, from Casera Razzo we only reach 16.1 on the best nights.
We spent the night on targets which care less about background light and more for resolution: globular clusters and planetary nebulae. Globular clusters are, roughly speaking, failed galaxies: they easily contain tens to hundreds of thousands of tightly packed oldish stars, and they recently were hypothesized to all contain massive black holes.
M13 was a classical late-spring target, along with M67, M3, NGC5466, M56, M4, M15, M71. These globulars have very different condensations in the core, such that in a reasonably sized telescope they provide quite varied views. In all cases we could resolve them in stars, thanks to the good seeing. On the right you can see M56, a neat, smallish cluster half-way between Lyra and Cygnus.
At 2.30AM we packed and got on our way back. We will not be back to that site in the near future: from the Alps one sees more, and the drive from Venice is not much different.