Building a Deep Space Observatory

Building a Deep Space Observatory

Technical notes:

The dome (10 sides) and the walls (6 sides) have fundamentally different shapes and do not play well together. Worse, they conspired to let water in. In this build, the dome spins from the floor of the observatory (rather than the conventional 'top' of the walls). I chose this approach for safety reasons. Placing the dome (even a relatively light dome) onto the top of walls can add a great deal of stress on walls the moment the wind picks up. Instead, here, the dome is placed on the equivalent of braced stilts and are tied directly to the concrete piers at ground level. At high wind (a cyclone passed within 900m after completion with winds gusting 150 - 200kph) the dome structure flexes and distorts (before snapping back into shape) but does not break.

Four supports hold the dome about 6 inches above the top of the wall (allowing for additional side windows and better water run off).

There are two separate wheel systems in play: the guide system and the float system. The guide wheels follow a track on the floor to ensure smooth rotation, under wind and temperature stress. The float wheels provide 360 degree movement, making it easy to move manually or by motor.

The supports are braced and stressed to provide for slight flexing. Connecting the top of all the supports is a single 'L' shaped metal bar, in the shape of the dome base and firmly connected to the dome.

A series of clamps lock the dome securely to the deck after use and in the event of wind.

Finally, solar panels feeding a marine battery were added to power the electrical system and internal lights added. The structure was painted (a cream exterior and black interior).

During construction a large separate facilities building was delivered to the site (sitting under the east facing horizon) to house remote sensing and launch equipment.

The remainder of this post sets out options etc that may be of interest to others thinking of building a dome.

Bamboo is great for a quick, one day build, to be covered with tent-like material. Metal is great for permanent structures.

Weight and force

When complete, the bamboo structure spent a day in a stiff wind settling into a perfect form. To test the weight bearing capability of the structure I added mass to the top of the structure measuring effects on the shape and stability of the structure. Under external vertical stress the structure flexed but showed no signs of collapse when i stopped adding mass at 250 kg (which is the breaking strain of the light wire i was using) - giving a maximum vertical stress rating of about 2000+ Newtons (F=m*a, a=9.8m/s2. m=250kg). Each structure will be a little different, and performance will vary according to the cure strength of bamboo and the type of wire used.

The vertical stress of a similar metal frame would be far higher.

Because this 'strength' is systemic, it seems fair to assume that similar results would apply to lateral force as with vertical force (once the lower elements are braced. Removing any reliance on tie-wire by providing internal bracing at joints (structural) and between nodes (stress wire), will significantly increase stress performance. A final clad mass of 50kg (where the cladding contributing no additional strength) would give it a constant gravitational load of 500 Newtons, well within the simple stress test above.

Calculating the pressure under different local wind conditions, is a more complex process. Given the shape of the structure set in an orchard any calculations will be a bit hit and miss (and will vary as trees grow). A hollow semi-sphere looks streamlined but actually has a bit more drag coefficient (.38) than an ordinary family car (.29) - and the wind pressure will impact significantly differently on parts of the structure - particularly vertical elements. At ordinary (98% of the time) wind speeds <10kph, the pressure is 2-4 Newtons. At 50kph (<1% of the time), this raises to 65-130 Newtons and at 100kph (once every few years), 260-500 Newtons. Because pressure grows exponentially with wind speed, it will finally hit 2000 Newtons at 200 - 280kph. At 280 kph, i expect that the dome will split into 6 small flying saucers and they will travel across the Tasman sea before landing somewhere in New Zealand.

I was not to know that cyclonic winds would test the structure within months of it being built. A cyclone passed within about 900m of the observatory with winds that destroyed a number of houses.

A mass of 30 - 50 kg appears well within the capacity of the structure. A final consideration is that i need it to rotate as well. Too much weight will make that difficult.

Dome Options

I chose hammered tin over fibreglass as the major building component after considering a wide range of possibilities (organised by weight).

1. Do nothing (1kg, $0) A viable option is to do nothing other than throwing a tarpaulin over the bamboo framework after sessions. This does not deal with the wind problem.

2. Tent/parachute skin shell using industrial strength velcro. (3kg $500) Instead of a rigid dome, one possibility is a simple, removable skin. With minimal interior wire bracing, the bamboo structure might withstand significant wind gusts, however, it would be problematic with hail. Still, it leaves open the possibility of the frame being used to put up temporary wind shielding. +Matthew Fowlerreported "I have seen guys at star parties with wind shelters created out of stakes and canvas, which seems to work well for a mobile setup. Usually with a slight lean to divert the airflow upwards rather than block it." Velcro might be used to attach panes or groups of panes to the frame as required. This has the singular advantage of allowing the dome to be stripped if wind conditions permit (thanks +Nina Anthonijsz).

3. Embedded** fibreglass and cotton (14 kg $1000). One low weight option is to deliberately enclose the bamboo in a light media that will take up the fibreglass resin. Based on a spider's web, in one test i wrapped cotton tape around the polyhedra structure which was then embedded within the fibreglass structure. This configuration retained the structural strength of the bamboo frame using half the amount of fibreglass/resin. Cotton does not take up too much resin and retains its shape (unlike cardboard), creating a nice light relatively firm structure. A similar result can be obtained from used unwaxed stock-feed bags and perhaps hessian.

4. Embedded** fibreglass and MDF* (19 kg $1500) MDF provides a rigid crafting surface - but, in an embedded shell probably contributes little to structure while absorbing a small amount of resin. It is probably a better external shell option (using light wooden MDF composites supported by wire between each of the triangles of the structure) - in this form the framework could be used to cast an indefinite number of polyhedra.

5. Hammered tin and perspex* (20 kg $0) While recycled tin can be used, the preparation time for each piece is high, as each triangle has to be cut using tin snips. I had initially discounted roofing tin as a candidate, even though i have a large amount carefully stacked for building sheds as i need another one. Tin has a lot of benefits. I initially looked at this as a simple way of hardening the join points and found that integration with bamboo is relatively straightforward and window/etc elements could be hinged as required. Tin has some disadvantages. It can both tear and bend. Tin snips leave jags that cut and tear the skin. With maximum length of 65 cm, and along all 3 sides of each triangle a facing piece at a slight angle to the first, the structure braces the tin components - which is then interconnected by cold rivets and bolts. Flat roofing tin or clear perspex cut into triangles and riveted into place provides a very strong shell.

6. Embedded** fibreglass and cardboard (21 kg $1200). Cardboard has been used in both external and embedded shell structures. In an embedded shell structure the cardboard absorbs a lot of resin adding a bit to final rigidity. However, in testing it lost its original shape, became heavy (absorbing a lot of resin - hence the extra cost) and sagged. In an external shell, the cardboard is protected by light plastic from being incorporated into the fibreglass mix. Some nice and many horrible structures have resulted.

7. External** fibreglass shell (28kg, $1600 plus cost). I was taken back by the cost of fibreglass and limited retail supply. Sourcing on the internet gave greater options and halved the price. Surprisingly, a number of local commercial suppliers of fibreglass have recently gone out of business. Initially i assumed i would get satisfactory results with 2 coats of relatively light fibreglass sheets. I found i needed heavy sheets and a lot of resin to get a rigid structure. To create a shell, i superglued cut-up waxy stock fee bags to the bamboo frame, applied resin, then the fiberglass cloth with more resin - and repeated. When the fiberglass cured, the frame and stock bags could be pulled away from the shell.

8. Embedded** fibreglass and 3 ply* (29 kg $1800) as MDF, but heavier

*note 1: if using wooden composites or tin to help with the form, the question must be asked why bother with the bamboo at all? i think bamboo is useful because it creates a simple stressed framework without having to glue lots of relatively heavy wooden/tin triangles into the right configuration. The bamboo structure creates a 3D form for crafting and fitting other elements. Others have had great results from wooden/metal structures not using formwork like bamboo but the work involved is intensive.

** note 2: In finalising the dome, there are two technical considerations.

1. External shell: The first is to cast the fibreglass (or other permanent skin - tin) on the exterior of the framework (not including the bamboo and any material used to support the fibreglass during curing in the final structure). I have seen fibreglass used in this way to build kayaks - the original boat is covered with a waxed material (wax paper, light plastic, some types of 'duck tape' or gel), the fibreglass is applied to the hull and when cured cut away from the original. Finally the new shell is glued together and you have cloned your original kayak. However, if you do not use enough fibreglass, the structure will have the structural integrity of a plastic bag.

2. Embedded shell: The second option is to cast the fibreglass around the framework, embedding it in the final structure. This increases the final weight and imparts whatever structural integrity/weaknesses the form work has into the final structure. Sometimes, to overcome this problem, the formwork is sandwiched between two layers of fibreglass.

These results came as a bit of a shock. On a weight/cost basis fibreglass is difficult to justify as the main building material.

As i tested the bamboo framework, a traditional 'slit' opening started to look like it would be a major problem. It would cross a large number of polyhedra (the traditional approach would have involved cutting a viewing slit 300mm wide across 4 polyhedra making up the shell creating obvious structural problems that would need to be compensated for within the dome build - using metal bars and stressed wire). I considered a couple of different options.

1. Vertically Split dome shell: splitting the dome into two halves would allow the halves to be moved to reveal the sky. There are a couple of variants. The sides could be both folded away, one could be folded (leaving a wind shield) or they could be slid apart. Each of these create structural integrity problems and complex patterns of wind turbulence.

2. Two Hinged Polyhedra: Following on from the last option, instead of having a slit, two of the polyhedra might be hinged (the top 5-sided polyhedra and a 6-sided polyhedra). To preserve structural integrity, the shape of the polyhedra would need to be duplicated in light metal and integrated into the main structure. A negative byproduct is the need to put some structural elements into the viewing area to maintain integrity (*this option was chosen, and it ended up requiring significant internal bracing.)

3.Umbrella: A late possibility was an umbrella type structure, capable of morphing from a low profile into a solid roof. This is not a frivolous suggestion - bamboo has been used to create just such a structure which is extremely light and portable. However, I did not pursue this because the design is dependent on the surface material creating a flexible but rigid surface - something difficult to reconcile with the need for an opening.


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