Design of Tenagra Observatory

Figure 21.4 shows the floor plan of the scope and control rooms. The main entrance is marked and is most often used, given the control room's multiple purposes. There are two entry doors to the 3.5 m x 3.4 m (11 ft 6 in x 11 ft 2 in) telescope room. The first door is for access from the control room and the second leads directly to the scope room. The wall between the control and scope rooms has a large window that allows me to comfortably watch the scope from the computer consoles. While I never had any real problems with cables (CCD cameras, dew heaters, video cables, etc.) I never feel comfor

Figure 21.4 Floor plan (not to scale). All dimensions are internal.

table when a high-torque mount slews around without me checking to see that it doesn't get itself into trouble. I have visions of snapping cables and control boxes dragging around in circles! So the window allows a more than full view of the scopes and mounts in all positions.

The building itself is exactly oriented to the compass points and the ridge peak of the control room running east-west with as low a ridge profile as possible (see Figure 21.1). My latitude is just shy of 45° north, so an angle of about 22° for the roof gives me a full view of all circumpolar stars at different times of the year.

I don't consider the obstruction a problem for two reasons. First, I have similar angular obstructions (mountain ridge and trees) to the east and west. Second, my average seeing is far from exceptional and I often prefer to observe only above 30° altitude whenever I can get away with it.

The next choice was the most difficult. I have a relatively unobstructed view to the south (including the ecliptic) and wanted to take advantage of this. If I had opted for the traditional horizontal roll-off roof then I would have limited this view. On the other hand, construction of a horizontal roll-off is relatively simple. I opted for the view, and the associated construction problems as can be seen in Figure 21.1. The

Figure 21.4 Floor plan (not to scale). All dimensions are internal.

Figure 21.5 East section of roll-off (not to scale).

angle of the roll-off is about 10°. Figure 21.5 shows the overall design and dimensions.

There were two main problems that needed to be addressed once I decided to slope the roll-off roof. The first is careful design to get just the right angle to maximize the telescopes' views (i.e. less clearance between scopes and roof when the scopes are parked), properly dovetail the roof with the main structure of the control room, and carefully measure angles for the structure that would hold the roof when it is rolled back. While this may seem trivial, I even went to the extreme of building a wooden mockup of the mount (which had not arrived) and the OTAs. This was placed on the cement portion of the pier to verify the calculations I made to determine the optimum angle.

The second problem with an angled roll-off is roof weight. The third is its secure anchoring when closed. Weight is the main problem. I didn't want a moving roof that would require counterweights or a complicated locking and unlocking procedure for opening or closing. I have wet air in the summers and Pacific Northwest winter rains. So the materials would have to be completely waterproof. I decided to throw away all traditional standard wood construction techniques and use the simple design shown in Figure 21.6. The frame for the roof is made of aluminum 102 mm x 51 mm x 3 mm thick (2 in x 4in x 1/8 in) rectangular tubes. This was welded by a not-so-local shop and moved to the building site (with great nervousness) tied to the side of a rental truck.

Figure 21.5 East section of roll-off (not to scale).

Figure 21.6 a Plan of roof frame. b Section of aluminum. c Section showing roof composition. d Wheel and track assembly.

Once the roof frame was on site it was placed on sawhorses, and rigid insulation was placed in all openings. Then the frame was tapped for screws and flexible Formica was screwed down for the inside covering. A galvanized metal roof was screwed down on the other side. See Section c in Figure 21.6. The objective was accomplished: the roof was easily lifted by four people and placed on top of the telescope room. This design produced the desired light weight, fully insulated and weatherproof moveable roof.

Given the light roof, it was now time to decide how to move it and where to obtain the parts. In keeping with the spirit of over-engineering, industrial garage door straight tracks with matching ball-bearing wheels were used. This method locks the wheels in the tracks (there is virtually no up and down wheel play in the tracks) and serves as the necessary anchor in high winds. See Section d in Figure 21.6. The system is virtually frictionless. Figure 21.7 shows the tracks with the roof down and the scope peeking above. There are seven wheels on each side of the roof.

Once the roof was on the tracks I found that I could, with a little effort, move the roof up and off the telescope room manually and unassisted. Although this is not something I would like to do at

Figure 21.7 The tracks with the roof down.

5:00 a.m. with any regularity, it was nice to know that I could do it in a pinch.

I opted to use a standard winch with a hand controller for raising and lowering the roof (see Figure 21.8a). This small winch, rated to lift 450 kg (1000 lb) vertically, is much more powerful than is needed to move this lightweight roof at its slight angle.

I did discover, though, an interesting effect when the winch was connected by cable to the roof. Many people don't realize that the roof of a building is responsible for much of the structure's integrity. If the roof is not fixed you have wobbly walls.

When I initially hooked up the winch and stopped it before it made contact with the bumpers at the end of the track, the wall on which the winch was mounted would wobble in a frightening manner. This was handily taken care of by placing a U-bolt where the winch cable connected with the roof and placing springs (see Figure 21.8b) to allow for movement in the roof rather than in the south wall. If I have to stop the roof prematurely the halt of the inertia is absorbed by the springs rather than the wall.

The winch is solidly attached to the south wall using connected aluminum plates on the inner and outer sides of the wall. Finally, note in Figure 21.8b that there is considerable overhang to protect the winch (and the rest of the building) from the elements.

Figure 21.7 The tracks with the roof down.

Figure 21.8 a The winch used for raising and lowering the roof. b The U-bolt cable-connection and shock-absorbing springs.

Figure 21.8 a The winch used for raising and lowering the roof. b The U-bolt cable-connection and shock-absorbing springs.

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