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Thirty Meter Telescope

The Project Manager's Corner: The Next Steps
Gary Sanders, TMT Project Manager

January 2007

The figure shows four copies of TMT on a mountain summit. This arrangement evokes the placement and setting of the two Keck telescopes in the Keck Interferometer on Mauna Kea, or the striking array of four unit telescopes that the European Southern Observatory operates on Cerro Paranal as the VLTI. These are powerful interferometric tools for astronomy. The figure suggests a TMTI, a stupendous combination of four TMT telescopes providing amazing resolution capability. Alas, while visually exciting, a TMTI would be prohibitively expensive.

The figure, courtesy of M3 Engineering, actually represents something more mundane but still very important. It is a trade study comparing four arrangements of summit buildings with varying facility arrangements and roof heights, placed next to our enclosure with its ventilation openings. It is input to aerodynamic modeling of the flow across these structures. What is notable about this is that it marks a kickoff of the next, and more detailed, phase of TMT design.

As I described in my December column, TMT has just completed an exhaustive cycle of conceptual design review and refinement. We also carried out a value engineering study and have modified our design, preserving capabilities, to meet our Board’s construction cost targets. After review by our Science Advisory Committee (SAC), described in December, our Board has now authorized the next steps in the design, adopting the revised configuration and reduced costs as the point of departure in this more detailed design study.

Before we describe these next steps, it is worth describing some of the design changes. We have shortened the physical length of the telescope by changing our secondary mirror from a concave design to a convex design, specifically changing from an Aplanatic Gregorian optical layout to a Ritchey-Chretien layout. This shortens the system by 6 meters and shrinks the secondary mirror by about 0.5 meters. Such an arrangement is a bit harder to test optically but the reduced dimensions allow us to shrink the diameter of our enclosure, from 78 meters to 66 meters, and the size of the footprint of the site improvements. This is a big reduction in expensive construction.

Our Field of View (FOV) has been reduced from 20' to 15', following recommendations of our SAC. This shrinks the size of our tertiary mirror by about 0.5 meters. Reductions in some facilities and development of some upgrade technologies, reducing some software functionality related to operations and a general scrubbing of the budgets has reduced the cost envelope to the range desired by our Board.

We also took the opportunity to make a few other design changes that represent optimizations separate from cost sensitivity. It is disruptive to make design changes mid-stream and we wanted to do this all at once. The very important primary mirror segments have had their dimensions increased from 1.2 meters across the hexagonal shape to 1.432 meters. Thickness is slightly increased from 40 mm to 45 mm. This change reduces the number of primary mirror segments from 738 to 492, simplifying the serial production counts of segments, supports, actuators, edge sensors, cabling, etc. The observatory is integrated in a shorter period of time and operations are simplified. This change increases some of the technical challenges in segment fabrication but we concluded that the added risks are manageable. The result is a simpler TMT observatory with roughly 2/3 the number of parts in the key subsystem.

We trimmed and opened up the Nasmyth platform region where instruments are supported, promoting airflow across the primary mirror, essential to maintaining a uniform optical atmosphere about the mirror. The aerodynamics and thermal environment of TMT is demanding and we have written about it in a previous issue of the Newscast.

With the reduced costs, optimized conceptual design and our SAC and Board's endorsement to proceed, we are launching the next steps of detailed design and development. The figure shows the start of detailed aerodynamic studies taking into account the summit building role in airflow. But we also have design revisions in the enclosure that take into account the reduced size. Our design firm, AMEC Dynamic Structures has optimized placement of the azimuth track separating the fixed and rotating enclosure, adjusted placement of the ventilation openings, and introduced a clever innovation, movable flaps about the shutter opening enabling us to preserve wind protection of the upper part of the telescope with the smallest possible enclosure. We will write about this new feature when we complete the detailed computational fluid dynamic studies that map out the aerodynamic behavior.

The telescope optics and structure, the adaptive optics systems and instruments will also start new design studies during the next year. We will describe these programs as they progress. There will be much to describe during these next steps.

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