Ptolemy's Cafe


Like so many amateur astronomers, I've always wanted to have my own permanent observatory.  Of course, we would all like to have one on top of Mauna Kea but the real world intervenes and we make compromises.  So mine is in my backyard in the middle of the San Francisco Bay Area's huge light dome.  Well, actually, at the edge of it; I'm located in the hills between the ocean and the bay so there's no city to the west of me and little to the south. (My GPS says I'm at 37 degrees 27.6 minutes north, 122 degrees 16.2 minutes west.) My chimney and the two trees to the south go up to about 22 degrees elevation; the big tree to the northwest is 35 degrees.

Thus I have a pretty decent sky to the west and south and at the zenith but the east and north are pretty much lost.  Fortunately (OK, it wasn't exactly an accident), my backyard is west of my house.  My southern horizon isn't great but it is good enough to see Scorpius in the summer. I can usually see the Milky Way and on a good night I can see M31 without optical aid. Not bad for suburbia :-)

I knew from the beginning that I wanted a roll-off design, not a dome. I like the feeling of the open sky above me and the climate here is mild enough (rarely below freezing) that a dome is not a requirement. But more importantly, I wanted it to be aesthetically pleasing. After all, it dominates my backyard. So I decided on a "Japanese tea house" look and designed the rest of my landscaping to match.

There were two major tradeoffs I had to make. First, having the roll-off support structure on the north side as is traditional was out of the question. It would have been in the middle of my lawn. So it is on the west side where it is least visible from my house (and fortunately, not too bad for my neighbors, either). That compromises my view of the sky low in the west but while that is the darkest part of my sky it is rarely worth observing due to the seeing.

Second, between the "tea house" design and the building codes and a pretty conservative structural engineer the structure got more massive than I would have liked. The engineer insisted on making it out of steel which greatly increased the cost (but at least I can use it as a shelter in an earthquake :-) . It wasn't possible to have the whole south wall fold down (as is also commonly done) so I have to make do with looking through fold-down windows. This means that the posts and tops of the walls are pretty thick and obstruct more sky than I would have liked (I did a lot of trigonometric calculations about what part of the sky would be blocked, all of which turned out wrong since the obstructions got much thicker during construction). I partially compensated for this by designing a ridiculously complicated pier which allows me to move the telescope up and down to get different angles thru the windows and over the walls.

Building the observatory was part of a larger project to landscape the entire yard. Getting architects and plans and permits and contractors all arranged took over a year before we started work. So in the meanwhile, I made a temporary pier in the spot destined to be the observatory. Of course, since there was a lot of grading and digging to be done the very first thing we did when construction began was to remove the temporary pier. It was a good thing I had my new 10" f/6 Dob to use while the LX200 was off line.

Bill Arnett
last updated: 2000 Aug 31