Rocky Plains Observatory
- Design -
The first, and for me, one of the most time consuming tasks was the planning and design of the observatory building itself. I'm the type of person that likes to plan every little detail before I begin. I spent a couple of months working out the building architecture and detailed design. The building was completely designed using a 3D CAD system. This enabled me to work out all the bugs before I put a shovel into the dirt.
Images of the observatory under construction, as well as some more specific details can be found in the Construction section.
- Floor/Foundation and Pier
One of my primary goals was to make a minimal thermal-mass structure to minimize impacts on local seeing from lengthy building cooldown (in Colorado, night to day temps are widely varying and there is significant falling temps through the entire night). This led me to select a raised wooden floor over a solid concrete slab. This also was a lower cost solution, and one that was easier to build myself. The floor is isolated from the central pier to prevent unwanted vibrations transferring to the telescope. I made the floor beams oversized (2x10 outer beams, 2x8 floor joists), mainly to impart a very rigid feel so that it doesn't feel like a raised floor.
The central pier for the telescope consists of the base concrete footer, and a bolt on steel pier. The concrete footer is 4 feet deep, and is 3 feet in diameter at ground level, expanding out to 4 feet in diameter at the bottom. The concrete also extends about 18 inches above ground level - I formed this portion using a 24 inch diameter concrete form tube. The concrete is fully reinforced with a rebar cage that extends the full length. Six 3/4 inch J-Bolts were set into the concrete using a plywood disk as a jig. The steel fabricated telescope pier fastens to the concrete using these six bolts. The plywood jig also resulted in a flat concrete surface so that no shimming or grout was needed to seat the steel pier.
The steel pier is made from a 52 inch long piece of 12" ID x 3/8" wall steel pipe (bought at a local salvage yard), to which a 3/4" thick 20" diameter bottom flange has been welded. This flange bolts to the concrete pier through six radial slots (to permit some rotation adjustment for polar alignment). The top surface of the pier was also made of 3/4" steel plate, and was pre-drilled and tapped for several common large telescope mounts (1200GTO and Paramount ME). The basic design goal of the pier was to provide high rigidity, and still provide a clean appearance and freedom from tripping when using the scope. Many commercial and amateur pier designs use smaller diameter pipe, and try to gain back some stiffness through base gussets. These gussets add fabrication expense, and do not provide near the stiffness of simply using a larger diameter pipe (wall thickness is a secondary factor - diameter is the primary consideration in stiffness).
The pier height was chosen to provide a good compromise between being tall enough to allow me to be seated when using a long refractor at zenith as well as when observing 20 degrees above the east or west horizon. I found that this condition occured when the intersection of the declination and RA axis fell approximately at my eye height when standing.
For some pics of the pier, go to the Equipment section.
- Roof Design
I spent quite a bit of time working out the roof and wheel/track design. I began by designing the wheel and track details, as the rest of the roof design really flowed from there. I surfed through all the ideas and designs that others have posted on their sites, and found a wide variety of unique and clever designs. However, none of them directly suited my goals. I wanted a very easy to roll system that added minimal vertical overhead (I wanted to keep the roofs as low as possible to maintain my southern access). I also wanted the roof to be captured from lifting off (wind) no matter whether open, closed, or anywhere in-between. The exposed tracks needed to shed rain and snow. The wheels, tracks, and truss design also needed to be designed to support the potential snow loads in this region, which can reach 30 lbs/sqft (roof projected area).
The best solution that I found was to use commercially available V-Groove wheels running on inverted angle iron. This design provides low friction while still controlling lateral (side to side) movement. Standard wheel/track designs that use wheels rolling in straight sided channels tend to have higher friction forces from the side rubbing of the wheel to track interface. The large 6" diameter wheels enable very low friction to roll the roof, and their bearings were more than able to support the maximum roof weight, though I do not plan on rolling the roof off fully loaded with snow.
Another primary goal of the roof design was to blend in and look as much like a standard (non-rolling) roof as possible . The only obvious clue that the roofs roll off are the two cantilevered track beams. The soffits return up under the tracks, forming a full length capture that prevents the roof from lifting off in any position. The soffits also impart a standard look to the roofs.
For some images that illustrate the roof design details, go to Roof Construction.
- Thermal Considerations
As mentioned above, one of my goals was to make the structure as thermally light as possible. The walls are standard frame construction, and when completed will be insulated with fiberglass batting. The roofs are also insulated with 4 inches of closed cell foam (R20). Framing for an exterior opening is already in place, and will be used to mount a small window air conditioning unit. Rather than try to aggressively ventilate the building (to keep peak summer temps down), I opted to thoroughly insulate the building. Even without air conditioning, this will keep the peak temp below the outside temp - but will slow late afternoon and early evening cooling (with roofs closed). In order to be able to rapidly use the telescope (at equilibrium), I will use the small air conditioner to keep the typical temperature under 80, then pre-cool closer to the evening temps. One concern I have with such a minimally ventilated structure is condensation. So far, I have not experienced any problems (most likely due to my generally arid environment). I am now adding some raingutters to further channel water away from the building. A benefit to having a more sealed structure is the minimization of windblown dust (and bugs!) getting into the building and onto my equipment.
- Misc Details
Though not completed (saved for next Spring), I will be finishing the interior using the darkest wood paneling I can find (or stain my own). In my urban location, I want to create as dark an environment as possible to assist in dark adaptation. I cannot do anything about sky brightness, but by darkening the walls (and using dark gray carpet), I can significantly darken 50% of the area around me.
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