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From: jtk@mordor.UUCP (Jordan Kare)
Newsgroups: net.space
Subject: Re: Phase Conjugate telescope
Message-ID: <5379@mordor.UUCP>
Date: Wed, 5-Feb-86 00:15:27 EST
Article-I.D.: mordor.5379
Posted: Wed Feb  5 00:15:27 1986
Date-Received: Fri, 7-Feb-86 06:06:35 EST
References: <8601272039.AA01149@s1-b.arpa> <1126@mmintl.UUCP>
Reply-To: jtk@mordor.UUCP (Jordin Kare)
Organization: S-1 Project, LLNL
Lines: 61
Keywords: Rubber mirror
Summary: Possible, not now useful

In article <8601272039.AA01149@s1-b.arpa> ST401385%BROWNVM.BITNET@WISCVM.ARPA writes:
>   >Why won't the phase conjugation technique work in reverse
>   >to build a large earth based telescope that removes the effects
>   >of atmospheric turbulence  ...  could make the Space Telescope
>   >obsolete.
>
>... There are two problems.  First... phase conjugation only works on
>monochromatic, coherent light (or at least light that is very nearly
>so).  More worrisome, though, is the fact that phase conjugation
>doesn't remove the distortion.  It antidistorts, so that repeating
>the passsage through the atmosphere cancels the distortion.

	Phase conjugation using non-linear optics (as discussed
in Sci. American recently) is (currently) limited to monochromatic
light and to some specific types of correction.  There is another
class of correction based on "adaptive optics": mirrors divided
into segments that can be moved (tilted) by electrical signals.
The "rubber mirror" project in the astrophysics group at Lawrence 
Berkeley Labs (where I got my degree) was an attempt to build
such a turbulence-correcting telescope.

	The size c of a "cell" of atmosphere over which starlight
is "coherent" (deflected the same way) is a few inches; the
"coherence time" over which such cells change is a few milliseconds
(and varies from place to place and night to night, just like 
telescope "seeing").  Thus, one needs (d/c)^2 mirror segments
to correct a telescope of size d -- a few tens to hundreds for
a good sized (say 4 meter) telescope -- and each segment must
be repositioned every few milliseconds.  The berkeley project
cheated by only worrying about 1 dimension, using 8 mirror segments
in a line to correct a modest (10 inch, I think) aperture in one
direction only.  The difference in path for different colors of light
is small as long as one is far from the horizon and not using too
broad a band, so the system works for white light.

	The problem is in figuring out where to move the mirrors.
It turns out that this is pretty easy if you are pointed at a bright
star; you just drive the mirrors one at a time to get the brightest
peak in the middle of the image.  The process converges to a "best" image
quite fast, and the electronics required are pretty modest.

	Unfortunately, one rapidly runs out of photons if the "reference"
star is dim (limit is about 8th magnitude, independent of
just about everything one can control, like aperture size), and the 
"field of view" for which the correction is good is very small -- and
there just aren't many things worth looking at that are that close
in the sky to 8th magnitude stars.  So the rubber mirror project got
dropped after proving (by resolving a close binary star) that the
principle worked.  So far, the problems appear to be fundamental.

	If you could supply the reference light, it would
indeed be possible to make diffraction-limited ground based telescopes
(possible, mind you, doesn't mean practical).  But remember that
anything in orbit (even geosync) would move 
rapidly relative to the fixed stars, so
you can't put your beacon on a satellite even if you could afford to.
Meanwhile, we'll just have to live with ten-meter light buckets
and 2000x2000 CCD detectors doing speckle imaging while we wait
(:-() for the Space Telescope.

				Jordin Kare