Castle Rock Star Party

1997 May 30

Members of the San Francisco Bay area internet-based astronomy group, "The Astronomy Connection", tried a new close-in observing site at Castle Rock Park on the evening of May 30, 1997. The location, which required a special permit, is in the hills southwest of San Jose, California, not far from the intersection of State highways 9 and 35. The site is small -- there is room for a dozen cars at most -- though Castle Rock Park has numerous other places where much larger groups could set up.

I arrived after sunset, but well before the end of twilight. Old friend Venus, returned from a long sojourn in the morning sky, was shining spectacularly bright near the western horizon. I set up my 98 mm Brandon refractor, and waited to see how good the night would be. I will summarize those results first, for Castle Rock appeared to be a promising close-in site.

As darkness gathered, it became clear -- not unexpectedly -- that the high, wide, light dome from the Bay area cities would restrict the visibility of faint fuzzies in roughly the third of the celestial sphere nearest the northeastern horizon. Yet conditions in other parts of the sky were much better. Later in the evening, the light pollution diminished, and on descending into the San Jose area on the way home, I found that sure enough, low-level valley scud was blocking some of the offending photons. Seeing just after sunset was pretty poor, particularly near the horizons, but it improved rapidly; an hour into the night, I was getting diffraction-limited performance out of the Brandon, with excellent splits of gamma Virginis and epsilon Bootis at 164x, and with lots of surface detail on Mars.

I didn't get around to trying any demanding faint fuzzies, but brighter objects showed well. A friend with a newly-purchased 13.1-inch truss-tube Dobsonian (Coulter optics) had excellent views of M81, M82 and NGC 4565 at medium magnification. In the first of these three, I could see the galaxy's elongation easily, but no trace of the inner turns of the spiral arms. A 10-inch showed M104, the Sombrero galaxy, well, with bright portions of the galaxy visible on either side of the dark edge lane, at 56x.

Later in the evening, the owner of that same 10-inch was sharing it with several other observers in an attempt to detect the orientation of the central bar of M83, a face-on barred spiral. I overheard the discussion from my setup nearby, and pointed the 98 mm Brandon at M83, too. At 66x, I could see an obvious elongation, and perhaps a hint that the tips of the bar were starting to curve around, too, so I went over to see whether that was what everyone was talking about. It was, but we then suffered the vexing problem of not having any way to confirm our observation. Someone had a Burnham's _Celestial_Handbook_, with a lovely deep black-and-white photograph of M83, but there was no indication of which way was north on the photo, and I knew from experience that Burnham did not practice any systematic orientation of the photos provided. However, I had Hans Vehrenberg's _Atlas_of_Deep-Sky_Splendors_ at home, and used it later to check: We got it right, including the sense of curvature of the spiral arms. That's not bad for 98 mm aperture, looking at an object not too far off the horizon at a nominally close-in site. Perhaps Castle Rock is worth revisiting.

Our good luck in Hydra prompted me to remember other interesting objects nearby. I swung the Brandon southward and picked up NGC 5128. At 66x, this odd galaxy showed hints of structure, but I did not take time to unravel the details of what was visible. Instead, I dug out my 10x50 UltraView binocular and walked eastward along the south edge of the site, looking for a break in the tree line to the south. At one point I saw what looked like the glow of a flashlight, shining through the branches near the base of a pine. I moved a few steps more, and the glow emerged from behind the tree -- it was the great globular cluster omega Centauri, partially resolved at only 10x, and clearly visible to the naked eye even down among the chaparral. I hollared, and several other people came and got a look. The typical response was, "Just where is it anywa... Oh, WOW!" So much for M13.

My main targets for the evening were in the summer Milky Way or just to the east, so I cat-napped for an hour waiting for them to rise. On waking up, I found to my surprise and dismay that everyone was getting set to leave -- and in the cramped site, that meant that most of us, including me, would have to tear down in order to let people out. What was going on?

Observers had been bitten by the side effects of a summertime meteorological phenomenon in central California that ought to be better-known. When the day is hot and there isn't much wind, solar heating builds up a strong "thermal low", a mass of local hot air, in the Great Valley. After sunset, the hot air cools and contracts, and a strong sea breeze rushes in over the coastal hills to fill the low. In such conditions, winds aloft at the two- or three-thousand foot level sometimes approach 60 knots. We were on the edge of that effect. All the big Dobsons were being buffeted, moist ocean air was dewing car tops and the like, and what's more, wind chill had made people uncomfortably cold.

I was fine. My optics were clear. My Great Polaris mount is sturdy, and was footed on a set of Celestron vibration-damping pads, which really do work. I always keep my warm clothing in the car -- I have never seemed to need it inside my house. And I knew that the sea-breeze jet is typically transient, likely to stop in an hour or two. But all the newbies who were complaining about how fickle the weather was, seemed unable to conclude thereby, that maybe they should stick around and see if it would change. So -- much too early -- we left.

Castle Rock is certainly a viable close-in site, though I think we would do better to seek an area within it that is less cramped.

Jay Reynolds Freeman; last updated: 1997 June 1