As I’ve explained elsewhere in this blog, segment support assemblies are what hold in place each of TMT’s primary mirror segments. They also let each segment be positioned in real time to provide the best possible imaging capability.
Staff at the TMT Project Lab in Monrovia, California, are building six segment support assemblies to determine whether the design can be manufactured in a repeatable, problem-free manner. Here’s a photo of the first parts we’ve received, along with some of the key staff that have made this happen.
Glass is heavy. Because the weight of each segment of glass is so substantial, we really need to be careful with how we support the segments. Otherwise, we could unintentionally induce different levels of force across the face of the glass. These differential forces would distort the glass and degrade its optical performance; we call this gravity-induced print-through. Therefore each segment support adopts a “whiffletree” as its main structural element. Although this may sound like a species of vegetation invented by Dr. Seuss, here the term whiffletree refers to a mechanical support structure designed specifically to distribute the weight of the glass evenly.
A further challenge is that when we point the telescope further up or down, the mechanical support structure has to be able to counteract the effect of any sag under the influence of gravity. This means we need to design the support to be as stiff as possible in the vertical axis, while remembering that the supports themselves are part of the moving mass of the telescope and therefore cannot be too heavy. As such, we increase the stiffness in the vertical direction using 27 support rods.
In total, each segment support assembly contains 537 precision mechanical components, sensors and actuators. Once these assemblies have all been put together by our engineers, we’ll run them through optical testing with a prototype glass segment. If they are satisfactory to be part of the final observatory, we will send them over to Japan, for integration with the first polished pieces of glass that have come out of Canon’s production run.
Many thanks to our suppliers:
Advanced Technology Machining
Industrial Manufacturing Products
I’d also like to thank Fred Kamphues and Robert Anderson for providing supporting input into this blog entry.
At First Light: TMT’s Wide-Field Optical Spectrograph
Stressed Mirror Polishing