Final Polish of the 6.5m Primary Mirror Has Started !

The primary mirror of the TAO telescope, to which the lights from astronomical objects first enters, has an aperture of 6.5m in diameter.
After the approval of the construction budget in 2013, we have studied the design of the primary mirror and decided to incorporate the light-weighted honeycomb mirror fabricated by the Steward Observatory Mirror Lab at the University of Arizona, which is also used at various large telescopes such as Magellan telescopes at Las Campanas Observatory in Chile. Instead of the large volume of 6.5m in diameter and 0.71m in thickness (0.39m at the center), its weight is as light as 9 tons thanks to its honeycomb structure. Its density of 0.4 g/cc is less than water, and is one-fifth of the thin-meniscus primary mirror of Subaru telescope (8.3m in diameter, 0.2m in thickness, 22.8 tons in weight and density of 2 g/cc )

Together with the support system and accurate measurement system of the surface, we hope the primary mirror will provide us enough accuracy to take the advantage from the excellent observing condition at the summit of Co. Chajnantor, and enable us to carry out the world's best observations.

Steward Observatory Mirror Lab at the University of Arizona

Steward Observatory Mirror Lab (SOML) at the University of Arizona, founded by Dr.Roger Angel, is located under the stands of the american football stadium of University of Arizona. They have been studying the manufacturing technology of the light-weighted honeycomb mirror by borosilicate glass, and fabricated various mirror for such as MMT (1992), Magellan telescope, and Large Binocular Telescope.
Currently, the 8.4m mirror for LSST and seven 8.4m mirrors for Giant Magellan Telescope (GMT) as well as the 6.5m mirror for TAO are under casting and polishing, and they are the most famous mirror company in the world.

▲Figure 1 : The blue crane and the building on its left is the office and the factory of SOML. On the back of the crane is the American football stadium of University of Arizona.

Contract of Mirror Casting

After the approval of the construction budget, we have been discussing the detailed specification of the primary mirror, and signed the contract to cast the primary mirror with SOML on September 2013 (Figure 2) .

After the signing, we have been conducting Skype meetings bimonthly, as well as the face-to-face meeting at University of Arizona in April 2014 and at University of Tokyo in July 2014, for detailed discussions.

▲Figure2 : Just after the signing of the contract, having a pleasant talk.

Current Status of The Mirror Casting

The casting of the mirror progressed smoothly, and the back-side polishing is finished in February 2014. Now the support structure is installed on the back side, and the the surface polishing has just started (Figure 4), which is expected to be completed in the spring of 2015. The fabrication contract of the secondary and tertiary mirrors and their support structures are also signed in January 2014, and they are also expected to be completed soon.

▲Figure 3 : The back side of the 6.5m primary mirror of TAO ©University of Arizona	▲Figure 4 : The front side of the 6.5m primary mirror of TAO ©University of Arizona