by Bruce Jensen
Hi, all - Last night, a group of about nine amateur astronomers went to the Lone Rock Staging Area at Lake Sonoma west of Cloverdale to enjoy a *splendid* night sky and take some readings on the limiting magnitude there. The sky was totally clear of clouds except a very few wispy cirrus visible near the northern horizon until after 1 AM, when we left, and the air was fairly humid with the very light westerly wind coming in from the ocean over the hills. Moderate dew occurred after about 11 PM, but not severe. We were at perhaps 1,400 feet elevation on a ridge; lower elevations saw heavier moisture and fog. The temps were reasonably warm shortly after sunset when the inversion kicked in, but radiational cooling everywhere dropped the temperature to perhaps the low 50s by midnight.
The sky was very dark, except for a handful of small light domes from Santa Rosa/Healdsburg (SE) and another small town to the north-northeast. These improved dramatically after about midnight, when the fog layer spread over Santa Rosa and some of the dew settled out of the air near us. The seeing was about 5/10 lower to the horizon, and maybe 7.5/10 near the zenith; at times it became rock-solid overhead. Stars were fairly visible all the way to the horizon except in the Santa Rosa light some. Omega Centauri was resolvable at 1/2 degree above the horizon for most scopes present, 8" and up. For the limiting magnitude check, several of us used the triangle region bounded by Arcturus/Alpha Coronae/Gamma Bootes, as we did at Coe last weekend. Early in the evening, counts ranged from 16 to 22 for various observers, but these numbers rose a lot after midnight, when counts ranged from 25 to 30. The LM based on these values would have been as high as 6.3 or 6.4. I managed to get 30 at about 1 AM, although I believe if I had my glasses, which correct my astigmatism, I would have been able to resolve some of that lumpy darkness that seemed to permeate the area between the visible stars. I reckon a person with flawless eyes would have been able to reach a count that would indicate an LM of perhaps 6.5 or even higher.
I had my wife and her friend with me, so we concentrated on bright showpieces most of the evening. The big spring galaxies all looked tremendously beautiful, with dazzling detail in such favorites as M51, M101, M104, and even the Antenna Galaxies in Corvus, by this time getting low, and Centaurus A, just a few degrees above Omega Centauri. The big globulars M22, M13, M3 and and especially that gorgeous hunk of burning hydrogen, M5, were spellbinding, seeming to burst like a blizzard from the velvety dark. After midnight, the Milky Way features from the tail of Scorpius all the way to Cassiopeia, both naked-eye and telescopic, were distinct and detailed. M57 and M27 both displayed themselves proudly at 225 power, with details seemingly as much as the scope could deliver; althoguh I did not go to higher power, I probably should have, because for the first time I detected an occasional brightening at the center of the Ring that I usually only get beginning at about 560 power on my 18" scope - that would bode well for a glimpse of the central star. The Great Rift through Cygnus was a darker, inkier feature than the surrounding non-Milky Way regions, a characteristic that I have seen only a few places (including Lassen) and that, to me, signifies a wondrously dark and transparent sky.
The best part is that the great majority of the sky, almost horizon to horizon, was of high quality. If I waited another 2 hours until the bootes region was low to the west, I still could have hit 25 or more stars in an LM count.
Given reasonably good weather conditions, Lake Sonoma can deliver the goods, and everyone who went last night highly recommends this site, with no fee at all, plenty of room for probably more than 100 observers, a restroom (thank you wonderful park staff!) and even running water!
I strongly suggest we try a stint there at the end of August.