Rocky Plains Observatory
- Observatory Planning -
- What kind of roof?
The first question that came to mind when I planned out my observatory was what kind of roof? It's only logical to answer this question before proceeding with plans, as the roof type pretty much dictates many of the design decisions. One of my primary goals was to match the architecture of my home, at least to a reasonable degree. Keeping my wife and neighbors happy was a prime concern.
A dome would have probably 'stuck out' too much for my location, though who can resist the appeal of a dome - that screams astronomy? Domes offer some obvious advantages, notably shelter from wind and more comfort in winter. A dome would also give great insulation from stray light and provide a very dark environment. Of course, there are downsides. Cost is the biggest, as I was unwilling to tackle the design and construction myself. Decent commercially available domes were far too expensive. I wondered about thermal effects (slower cooldown, air currents through the aperture). Also, in the inevitable event that I need to sell my house, a dome could be a detractor (unless I make it easily convertible back to a standard roof, for use as a shed).
Architecturally, a roll-off would blend in and look less obvious (also a security concern). Architectural concerns aside, a roll-off design would be much cheaper and I could do all the work myself. A roll-off has a lot going for it. Being able to quickly open the entire building to the night sky provides rapid cooling. It also comes closest to being out in the open under the stars, and the walls provide enough shelter from surrounding light and houses that I can still feel less like I am in an urban backyard. The mechanical simplicity of a roll-off is a real plus. Movement on rollers is such a simple approach, and standard roof construction is easy to pull off with materials that are easy to find.
I spent some time considering less standard designs. Most revolved around different ways to hinge open or clamshell the roof(s). The challenges of supporting, counterweighting, and dealing with snow loads added complexity and structural challenges, without any significant benefits. Some commercial segmented clamshells appear to offer a neat compromise, being able to fully or partially open, but again cost becomes a factor, as does architectural look.
In the end, a roll-off became my obvious choice and early on I settled on this approach. Rather than a one piece roll off roof, however, I opted to split the roof in two. One half of the roof rolls to the south, and the other half to the north. This has some real benefits. Having to roll only half a roof reduces the mass that I need to roll at one time. A split roof also allows you to close the roofs down except for the region you are viewing, which is nice on very windy cold nights (almost like a variable width slit). Architecturally, the 'outriggers' that the roofs roll out on extend from both sides of the building - but are only half the length of a single roof, and can be cantilevered from the building without requiring vertical support columns.
One thing I learned (and was constantly reminded of by others) was to make the building as large as possible. Practically, however, I did have some limits. Given the space I had available, and some local building codes, I settled on a 10'x12' size. I had been in some other small observatories, and found that 10x10 was about the minimum I could tolerate, and 10x12 offered just enough more space to be able to locate a computer at one corner and still have room for some shelves and seating area for visitors. I planned to use some pretty long scopes as well, so 10 x 10 would have probably been too small. One other consideration was the local building codes. At 120 square feet and smaller, no building permit was required for a structure of this type and purpose. Though I could certainly have gone larger and gotten a permit (and the necessary plans and inspections), being able to skip that hassle was a plus. In hindsight, I wish I had gone a little larger (12X14), but 10 x 12 worked out nicely and isn't too imposing in my yard.
- Building Height
One of the tradeoffs that I had to make was how high to make the walls and how pitched to make the roofs. I wanted the walls to be tall enough to obstruct some of the surrounding environment without blocking too much horizon. Also, the taller I make the walls, the higher the roof peaks would be. I wanted to maintain a decent southern horizon, especially for low objects like Mars this past apparition (it was very low this year). I settled on a wall height of 76 inches (to the rail mount surface), which has worked out pretty much as I had wanted. When seated at my scope, I am very isolated from all but the night sky above, and the wind is directed just above my scope. One consideration that I learned of was the impact of wall height on door size. Given my wall construction (4x4 top beams for the roof rails), a standard door had to be cut down about 5 inches. Since I wanted a steel (foam core) exterior door, I had to cut the door down (a real pain). Commercial custom steel doors are very expensive, so I had to do this myself (a weekend's worth of work). If I had it to do over again, I might consider making the walls just enough higher to not need a custom door.
The roof pitch was another question. I settled on a 4/12 pitch, as it was about the minimum that drained well, using composite shingles, and it had enough pitch to blend with my house. Any steeper and I would lose too much southern horizon. As it was, I ended up extending the south rails so I can roll the south roof off 2 feet past the edge of the building.
I had originally dreamed of building an observatory on land in the mountains, or out on the surrounding plains away from city lights. However, as I talked to friends with observatories and gave this some serious thought, I realized that to be truly useful for me, I needed a building as close as possible - in my backyard. What a joy it is to be able to so quickly enter the building, put on some background music, roll off the roofs, switch on the mount, and head into space. City lights are certainly a problem here (on the best nights, the summer milky way is faintly visible when overhead). However, there is plenty I can do from my backyard. Planetary and Lunar viewing is a primary interest, and having an instrument close to thermal equilibrium that I can use at a moments notice is a real key to being able to catch nights of decent seeing. Though deep sky views are not too impressive, the brighter nebula, star clusters and double stars still offer plenty to view. Of course, CCD imaging still works surprisingly well in moderate light pollution. So, in the end, a backyard location was my choice, and after a couple months of usage I know I made a good choice.
I had already decided where in my yard the building would work best, by being farthest from surrounding trees. A small bed of shrubs had to be taken out (some of the bushes had some serious roots!).
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