pack@sri-unix (05/27/82)
Extremely long baseline interferometry does indeed promise fine
resolution pictures. The June 82 Scientific American article didn't
mention some of the problems inherent in extending the system to even
longer baselines. Time standards must be more accurate as spacing
between telescopes increases. In addition, a higher data rate is needed
to fully define these finer fringes. Present methods (atomic clocks
and video tape units) would have to be reevaluated to determine their
adequacy for even longer baselines such as earth-moon distances.
Another point to remember is that many different baselines are
needed to allow a unique inversion of fringe data and a picture to be
made. The terrestial baseline's projection on the sky varies on a
daily basis. A space system based on the earth-moon system would
have a natural 28 day period.
Finally, the limited "light" gathering power of individual dishes
must be considered. If steerable dishes in an imagined earth-moon system
were to be limited to about 200 feet in diameter (as present units are),
the increased resolution would not be accompanied by increased signal
power. Hence such a system would be limited to studying objects with
small angular detail coupled with extreme radio brightness. It therefore
seems unlikely that such a system could identify stellar planetary systems.
--Dan Packman
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