Making the mirror surfaces
The largest single mirror that has been produced by polishing
is the 8.4-m mirror installed at the University of Arizona in the United
States. There is no plan to make a larger mirror, as it is commonly understood
around the world that future large telescopes can only be produced through
use of segmented mirrors. All telescopes planed to have a primary mirror
larger than 10m adopt ideas of segmented mirrors, although there is variety
in sizes and shapes of planned segmented mirrors. The major problems
encountered in use of segmented mirror systems are associated with
control technologies for making non-axisymmetrical mirror surfaces
and ensuring that mirror segments function as one large mirror when
assembled. However, these technologies have been demonstrated to be feasible
in the Keck 10-m telescope mirror (consisting of 36 segments) in the United
States, showing that they can be used for future extremely large telescopes.
Japan thus far only has experience in producing up to 1-m-class mirrors.
In terms of international competitiveness, Japan is a long way behind in
mirror production technology. However, for extremely large telescopes,
if technology for making many small, non-axisymmetrical mirror segments
rather than building a single large mirror can be developed, then the gap
will be narrowed and Japan will be able to compete with the rest of the
world on equal terms.
The primary mirror of a 30-m-class extremely large telescope will consist
of approximately 1000 axially asymmetric, aspherical mirror segments of
approximately 1m in size. These elements will be assembled to form a large
mirror. However, if traditional polishing technology is employed,
a single polishing machine will require one year to prepare all the element
mirrors, at a cost of 50 million JPN. Furthermore, using normal polishing
methods it is only possible to make axially symmetric mirrors; a number
of extra production steps need to be introduced in order to make a segmented
mirror. Therefore, the present plan has tended toward using grinding rather
than polishing as the leading candidate mirror-making technology.