In article ,
Earl Colby Pottinger wrote:
People are still struggling to make imaging interferometry work well at
distances of a hundred *meters* with both telescopes resting on solid rock.

Henry, correct me if I am wrong. But I thought with present day computers
you don't need to hold the distance steady between the scopes. Rather you
needed to know the distance to a fraction of a wavelenght between the scopes
at the time the signals are recorded.

The key requirement is that you need to be able to measure the relative
phase of the photons arriving at the two scopes. There are two ways you
can do that, in principle.

One way is to bring those photons together so they interfere, and measure
the results of the interference. That gives you a direct readout of the
relative phase. The length of the optical path that brings the photons
together must be stable -- or at least continuously measurable -- to a
fraction of a wavelength, because any change in its length adds spurious
phase differences between the photons.

The other thing you can do is *record* the phases of the photons at each
scope, along with very precise time references, so you can compare them
later at your leisure, essentially simulating the interference in the
computer. The problem is, we have no way to do that for *light*. The
required speed of the phase measurement and recording system is four or
five orders of magnitude higher than that for radio.

Opps, just realized with long radiowaves even the phase shift can be measured
- if you need the phase of visible light to be recorded at both scopes at the
same time I don't think we have reach that tech yet.

Exactly.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |