Thirty Metre Telescope

Enclosure

Enclosure - General Information:

The primary function of the enclosure is to act as a housing unit, protecting the telescope from the wide array of intense weather conditions present at high altitudes. To get an idea of just how intense these are, consider the worst case conditions of the Mauna Kea site:

Wind Speed - 78 m/s (3 s gust)
Snow Buildup - 150 kg/m**2
Ice - 68 kg/m**2
Earthquake - 1.0g lateral ground acceleration

Designing the enclosure is a complicated task as it must protect the telescope without creating a separate internal environment which would compromise its performance. A separate indoor environment would introduce a different medium through which the incoming light would travel. As a result, the light would diffract as it passes from the air outside to the indoor medium, which effectively acts as a second 'atmosphere' causing poor imaging. Therefore, during hours of observation it is a requirement that the whole structure be in thermal equilibrium with its environment; this is facilitated by ventilation doors. During the day, on the other hand, the heat imparted by the sun on the enclosure must be carried away by some mechanism in order to prevent it from being transmitted to the telescope. Without such a thermal control the heat would cause the steel of the telescope to expand during the day and contract as observational hours began at night, rendering images out of focus. This is an important effect to take into account particularly at high altitudes where the temperature difference between day and night is quite substantial.

Design and Analysis:

Finite element analysis (FEA) is a computational method by which a large structure/object is modeled as composed of small individual elements which are interconnected in a grid or mesh. Specific material properties and geometries can be allocated to certain sets of elements, defining how each physically interacts with the other elements to which they are connected. In this way a perturbation applied to any element or group of elements will propagate throughout the structure; i.e. by applying some sort of external force such as a wind load, the response of the structure as a whole can be analyzed and the design can then be altered accordingly. In conjuncture with computational methods of fluid dynamics the wind and thermal patterns produced by the enclosure can be studied along with how well the structure stands these forces and others such as the distribution of its own weight. With this in mind another set of parameters to vary and hence study are the various configurations that a particular enclosure design can be in – open, closed, cap orientation etc. In addition to this, FEA is also used to predict the stress utilization and structural mass of a given enclosure design.

In general, enclosures are divided into two categories specifying whether the telescope and enclosure move together or separately from one another; these are called co-rotating and free-rotating designs respectively. Using FEA, Empire Dynamic Structures Ltd. analyzed three types of 'free-rotating' enclosure designs and various co-rotating ones. Presented below are the free-rotating "Dome-Shutter", "Calotte", and "Carousel" designs as well as a co-rotating design.

Free-Rotating Enclosure Designs: Dome-Shutter (left), Calotte (center), and Carousel (right)

A Co-Rotating Design
Image Credit: Empire Dynamic Structures Ltd.

The Calotte design was found to be the most efficient at distributing the gravitational load (best azimuth bogie load distribution) and possessed the lowest structural mass, thereby decreasing its cost. It also exhibited the best stress utilization which is particularly noteworthy about the calotte design.

Developed in the recent June 2007 construction proposal, the following is a cross sectional diagram of this design highlighting its salient features.

Cross section of the TMT's enclosure.
Image Credit: Empire Dynamic Structures Ltd.

In order to protect the telescope, enclosures typically have diameters three times the size of their opening (aperture) which for the TMT, with an opening of ~ 30 m, would require a diameter of ~90 m. However, what is particularly attractive about this design is its effectiveness at defending its interior from strong winds, minimizing the diameter of the enclosure to about 66 m and hence minimizing the cost of building materials. The structural components largely responsible for this minimization are the aperture flaps which provide extra wind protection for the secondary mirror (M2 lens). They also serve as the primary seal between the cap and shutter during non-observational hours. Presented below is an illustration of the two configurations which the aperture flaps take on.

This illustrates the open (observing) and closed (sealed) aperture flap configurations.
Image Credit: Empire Dynamic Structures Ltd.

Bogie System and the Cap/Base Interface (CBI)

Bogies are crucial components to the enclosure which serve to distribute the load evenly to the site soil and also act as the wheels or ball-bearings upon which any movable portion of the enclosure rotates – similar to the wheels on a train with the girders acting as the tracks. There are three sets of bogies; the azimuth bogies, the cap/base interface (CBI) bogies, and the shutter bogies. The following diagram illustrates the location of the CBI bogies and the azimuth bogies.

Illustration of the Cap/Base Interface, the location of its bogies and the azimth bogies.
Image Credit: Empire Dynamic Structures Ltd.

Very similar to the bogies in use on the Subaru telescope, the azimuth bogies have two rollers and are mounted onto the fixed base structure, allowing for movement of the rotating base while not contributing to the rotating mass.

Illustration of the azimuth bogies.
Image Credit: Empire Dynamic Structures Ltd.

The cap/base interface refers to the ring of contact between the cap and base structures and is inclined 32.5^o to horizontal. This is by far the highest risk area of the enclosure as it involves a complex angled bogie structure upon which the cap rotates. In this case the loading is changing, making it more complex than the azimuth bogies which handle a relatively constant load at its planar interface. To further complicate things the cap, which rests upon the soft structure of the rotating base, is also the second rotating structure making the problem much more dynamic. When there is such complexity in a structural design one looks to analyze and test it before construction; however, since one cannot test it without building it to scale (which would cost too much), engineers have analyzed the CBI very extensively using FEA. Some of the things they have analyzed are the effect of the cap orientation, wind loads, and thermal loads which led to a modification in the CBI design. Previously, the CBI was designed to incorporate an ‘integrated bogie', meaning that a single bogie consisted of both radial and normal rollers; normal rollers are oriented perpendicular to the plane of rotation while the radial rollers are oriented perpendicular to the axis of rotation. Using FEA engineers noticed that extreme, but possible wind loads could in fact lift the enclosure structure. To prevent this from occurring a different bogie system was adopted at the CBI, incorporating a pair of bogies oriented ±45^o to the CBI plane. Each has a main roller and an ‘uplift’ roller, which now share the distribution of normal and radial loads. At the CBI there are 60 evenly distributed bogies which are mounted onto the rotating base as is shown in the following diagrams.

Illustration of a CBI bogie (above) and their orientation with respect to the CBI (below).
Image Credit: Empire Dynamic Structures Ltd.

Shutter

Engineers have developed various shutter designs; the key distinguishing features are a balanced versus unbalanced structural design, and metallic versus composite material (the balance of the shutter is determined with respect to the axis of rotation).
The unbalanced design has large power/torque requirements which can be resolved by choosing a light weight material such as a composite carbon-fiber. Since the composite material is lighter than the metallic, a light shutter frame can be chosen which decreases the overall cost of the shutter. However, the manufacture/on-site assembly of this composite shutter introduces a degree of technical risk not associated with a more conventional metallic shutter: a composite shutter is built in a single piece which, due to its size, requires it to be built on-site, under harsh weather conditions at high altitude. A metal shutter, on the other hand, can be built in pieces and transported on-site.
In the case of a balanced shutter there are less power/torque requirements (60 hp as opposed to 200 hp) allowing for less risky, albeit heavier material to be used such as steel or aluminum. In the final analysis it was determined that the risks associated with the unbalanced design were not worth its slight financial savings, leading to the adoption of the balanced, steel and aluminum shutter design. This balanced design is presented below - note the counterbalancing portion of the steel shutter frame.

Image of the chosen balanced shutter design.
Image Credit: Empire Dynamic Structures Ltd.

'Flushing': Ventilation and Thermal Control

As mentioned above, during the day a thermal control system is needed to carry heat away from the telescope while at night it must be well ventilated so as to maintain thermal equilibrium with its surroundings. The ventilation system is comprised of three rows of vents all of which are located on the rotating base structure. Each vent is composed of an interior and exterior door, both serving different functions. The interior portion is a hinged double door, providing insulation and sealing while the exterior portion is a roll-up door, providing weather protection and some additional sealing. These vents are presented here:

Vent Composition: Interior doors (left) and exterior roll-up doors (right).
Image Credit: Empire Dynamic Structures Ltd.

The thermal control system cools two regions of the enclosure; the volume between the two ventilation doors and the enclosure’s interior. The region between the vent doors is cooled by a fan, located at the azimuth bearing which continually recycles air from the enclosure’s exterior. The interior of the enclosure is kept cool via an AC unit located below the azimuth bearing. The flow of air is illustrated in the following diagram of the thermal control system.

Illustration of the thermal control system, with the flow of air indicated.
Image Credit: Empire Dynamic Structures Ltd.

Construction

Once an appropriate site has been chosen, construction can begin. The six key phases are outlined here:

Assembly of the fixed base followed by the falsework, which serves as a temporary structural support while the entire enclosure is being assembled.

Ventilation structure modules are then preassembled in jigs at an off site location and transported to the summit where they are pieced together against the falsework

The base shell modules are also preassembled in jigs at an off-site location and then transported to the summit where they are erected with support from the falsework

The base ring girder is erected followed by the installation of the cap bogies (roughly aligned and remain unloaded).

Shutter ring girder and bogies are erected and secured to the cap ring girder at which point the shutter structure is erected against the falsework.

The aperture ring girder is assembled on top of the falsework tower.
Cap shell modules are pre-assembled at an off-site location and then erected in two levels with falsework support.
The weight of the enclosure is transferred from the falsework and onto the bogies.
At this point erection of the telescope structure can begin once the falsework has been taken down.

There are three challenges of the construction process that require particular consideration:
Site Size: When assembling a structure it is usually useful to lay all the pieces out in order of assembly. However, to do this for such a massive structure would require a field of open space which is not available at the peak of a mountain such as Mauna Kea. The engineers will have to devise some construction strategy to mitigate this lack of assembly space.
Safety: Personnel directly involved in the construction will have to deal with a few potential health risks such as intense winds, Acute Mountain Sickness (AMS), and the general precariousness of construction on a small site. An additional risk is the alertness of the builders as construction typically occurs late at night around 3 am when weather conditions are least fierce.
Interference: The difficulty here will be to minimize the interference of particular groups of specialists all working on separate tasks in the same constricted area. This can be mitigated by careful planning of allotted tasks in a physical sense and from a time-management point of view.

Below is a slide show of the construction process over its six main phases - pass the cursor over the image to begin.

Illustration of the enclosure construction process.
Image Credit: Empire Dynamic Structures Ltd.

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