Finally, Bortle 2 skies February 9, 2008Posted by dorigo in astronomy, personal, science.
It was sky transparency. We had suspected that Casera Razzo, the site I occasionally visit with a few amateur astronomer buddies in our deep-sky observing sessions, had the potential to offer a very dark, almost perfect sky; but only once in our dozen visits we had experienced it, while in all other cases the sky had been dark but had left something wanting.
We suspected that the factor that had been adverse in most cases was the transparency of the sky, but we needed some confirmation – after all, there are indeed many possible causes for a light-polluted atmosphere, some of them due to human activity and some others due to atmospheric conditions. Yesterday we reached the same level of quality of our formerly best night at the site – April 14th, 2007-, and we convinced ourselves that light scattered by particles and humidity in the atmosphere is a major factor affecting the darkness of the night in the eastern Alps. Light from towns 50 km away is masked well by the mountains surrounding the site, but the atmosphere needs to be transparent above your head if you want to avoid the photons from those far-away sodium lamps to bounce back there and hit you.
Mauro’s sky quality meter -a calibrated exposimeter yielding a reading of the sky’s visual magnitude per squared arcsecond – started off at 21.42 at 10.50PM, and as lights in nearby towns were turned off it consistently grew to reach a 21.58 reading at 2AM yesterday night. The latter reading corresponds to a “Bortle-2” sky – just one notch below the best possible sky, which corresponds to readings of 21.9 or 22.0. Of course, those 0.4 mags of difference mean a whole lot when one observes faint galaxies visually, but 21.6 is probably the best one can hope for on italian territory. Better values can probably be found only far, far away from northern Italy; even professional sites do not often go above 21.6: for instance, it is a typical reading at sites such as the Roque de los Muchachos, at La Palma – where several observatories are located. See for instance the following plot, taken from the site of the observatorio:
In the plot you see that the V-band magnitude (y axis) reaches 21.5 at zenith on a typical night. The x axis shows the azimuthal direction where the measurement is made, and the different curves refer to different altitudes in degrees.
So, what did we see yesterday night ? Well, a lot indeed. The temperature went from minus 5 to minus 11 degrees during the four hours of observation, and the wind was almost absent -I had feared it a lot before arriving there. However, the fact that the temperature did not stay constant prevented the mirror of the telescope to reach thermal equilibrium, and this affected the resolution quite a bit, and with it our possibility to push the magnification. For that reason, we mainly observed extended objects at 120x or 200x. Indeed, we were naturally led to spending most of our time on the real showpieces of the winter sky, which -when observed under truly dark skies- show picture-like detail with a 16″ scope. So, little time was spent on the faintest objects, which are usually small and require magnifications in excess of 400x.
A list of observed objects is a rather dry way for a description of the night. Rather, I only mention what really impressed me. Messier 51, the whirlpool galaxy, was one object on which we spent several minutes. It showed filamentary detail inside the main spiral arms, and was a really glorious sight – you felt you could pick it up by one arm, peeling it off the eyepiece lens as you would remove a dead insect from your windshield. And Messier 101, another face-on spiral galaxy in Ursa Major, did not pale in comparison: it showed several H-II regions in the arms, and the details in its structure were the best I ever saw on this object.
M101 is an extended object – covering almost a fourth of a squared degree of sky – and light pollution can make it utterly invisible even with large instruments! In fact, one often sees threads on popular amateur astronomy forums where it is discussed whether M101 is visually observable at all… Quite ironic: under dark skies this galaxy is a true beauty. The picture below (taken by italian amateurs from Verona) represents a good approximation of what was visible through the eyepiece. It is sights like these that keep me wanting for more nights out, hands and feet freezing and nothing else around but snow and silence.