  The lightweighting and surface accuracy targeted for NGST's 8 m primary
mirror has been demonstrated in a 0.5 m prototype. Now a second, 2 m prototype
weighing 40 kg in total is being fabricated at the University of Arizona under
the NGST NMSD program. It will be tested in cryogenic operation in spring 1999.
A third, advanced mirror system demonstrator (AMSD) is needed, the full size of
an NGST segment, for flight qualification, including launch survival and
extremely high reliability. The 3.5 m mirror, 1/6 the weight of HST's primary,
would be made with a 2 mm thick glass face-sheet attached by adjustable screws
to a carbon composite support. It would be figured as a fast telescope primary,
and fully tested cryogenically to verify closed loop figure stability with
simulated infrared starlight. If started in early 1999, this could be completed
in 2001. Construction of the NGST mirror panels could then be undertaken on the
basis of proven technology, and in time for a 2007 launch.
  With a diameter of 3.5 m and a weight of only 140 kg, the mirror could be
incorporated into a telescope and launched to a high orbit by the Shuttle or
number of other vehicles. Such a space mission would complement those already
planned for scaled down tests of the unfolding of the NGST mirror segments
(NEXUS) and the sunshade (ISIS). Much would be learned by running the mirror at
cryogenic temperature, with a new infrared array to make preliminary
observations in the 2 - 4 microns dark sky window, and a CCD to check optical
image quality. This mission would be independent of the NGST, but would lay the
scientific groundwork for the much more powerful telescope, as well as giving
practical experience of mirror operation.
