Advanced Concepts Studies
The Very Large
Telescope sensitivity increases as the square of the aperture, therefore, for a tenfold increase in sensitivity over the Hubble Space Telescope (HST), an aperture of at least 7 meters is required. A segmented mirror seems inevitable since the technology for fabricating lightweight single piece mirrors above ~ 4 meters does not exist today and is not foreseen.
One of the early HST follow on studies in the 1980’s explored the design of a segmented 8 m aperture UV/Visible telescope. The Very Large Space Telescope (VLST) design projected an eight meter primary constructed from a circular 2.4 m diameter (lightweight HST) central segment surrounded by 8 pie shaped pedals which would be incorporated into an HST type Ritchey Chretien telescope.
The telescope science objectives were basically identical to HST’s objectives for UV/Visible and IR observations. The focal plane instruments were improved versions of the second generation HST instruments.
Because a launch vehicle which could transport the VLST to orbit did not exist, it was proposed to carry the primary and secondary mirror assemblies within the interstage of the Space Shuttle External Tank (ET). On Orbit, with the help of astronauts the mirrors and collapsed mounting structure would be extracted from the ET and assembled into the modified liquid hydrogen tank, which would serve as a light shield and support for the metering structure. Future delivery flights would bring the spacecraft, instruments and other equipment to orbit for mating with the telescope. Obviously this concept involved complex and costly assembly tasks, but would basically be feasible within the technological capabilities which exist.
During the course of this study it became quite clear that it was
not so much the optical system technology, which prevented a larger space
telescope beyond HST, but the transportation to orbit of a ground-assembled
and tested telescope. The transportation problems are the real constraint
for future large, contiguous aperture telescopes.