ST401385@BROWNVM.BITNET (01/27/86)
>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. I've been thinking about this, and I can't think of a good way to make it work. There are two problems. First, as far as I know (but I'm not an expert by any means) 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. It sure sounds like there must be a way to use this phenomenon to cancel out the twinkling of starlight, but it certainly isn't obvious (at least to me) how. --Geoffrey A. Landis
franka@mmintl.UUCP (Frank Adams) (02/01/86)
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. > > I've been thinking about this, and I can't think of a good way to >make it work. There are two problems. First, as far as I know (but >I'm not an expert by any means) 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. >It sure sounds like there must be a way to use this phenomenon >to cancel out the twinkling of starlight, but it certainly isn't >obvious (at least to me) how. My knowledge here is derived mostly from the recent Scientific American articles. Based on that, I think it can be done, but I doubt that it does any good. One could in this way replace the camera and data transmission facilities in orbit, but I don't see how to replace the lenses and mirrors. That is, once one has an image available, one can use this technique to transmit it to ground; but the hard part in astronomy is getting the image. Also, I'm not sure the anti-distortion works properly over those distances. The technique involves sending a light beam from Earth up to the orbiter, and then back down again. If the orbiter is 300 km up, the round trip takes .002 seconds. In that time, the atmosphere is moving; whether it moves enough to noticeably distort the final image I am not sure. Frank Adams ihpn4!philabs!pwa-b!mmintl!franka Multimate International 52 Oakland Ave North E. Hartford, CT 06108
jtk@mordor.UUCP (Jordan Kare) (02/05/86)
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