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How does tip-tilt mirrors correct such issue? What is the physical origin of the error?

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    $\begingroup$ I suspect you have confused several terms related to adaptive optic systems. Perhaps you can provide links to the books or web pages you were reading, and we can try to untangle your confusion $\endgroup$ – Carl Witthoft Feb 19 '18 at 16:26
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Isoplanatism commonly refers to a region of angles over which a ground-based telescope observes effectively the same atmospheric turbulence (e.g. an "isoplanatic patch"), such that a laser guide star provides effective correction of atmospheric seeing.

an-isoplanatism refers to a lack of isoplanatism, or a way in which the science target and the reference laser guide star differ.

A single laser guide star does not provide knowledge of tip or tilt errors since the up-going beam experiences tilts in addition the down-going signal; these two sets of tilts are entangled and in the case where the transmitter and receiver are identical the laser guide star appears stationary. See a detailed overview and history of this problem the introduction of Ragazzoni 1996, http://adsabs.harvard.edu/full/1996ApJ...465L..73R. This is commonly solved by observing a "natural guide star" position, since its light only passes through the atmosphere once, and using that signal to maintain the position of the target star by changing the angle a small ("tip-tilt") mirror.

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  • $\begingroup$ Okay thanks a lot! Just wondering how do they measure the light from the emitted LGS? There must be hardly any light that is reflected back and much less collected by the telescope. If they use the same telescope to measure the reflected light, does this means that this technique can only be used for powerful telescope since it must be powerful enough to measure the little light that is deflected back? $\endgroup$ – newbie125 Feb 27 '18 at 10:44
  • $\begingroup$ It does take a very powerful laser (typically tens or hundreds of watts) to generate a sufficiently bright signal to correct. Since the adaptive system is correcting wavefront across the telescope area, the size of the telescope does not impact the detectability of the laser guide star on the wavefront sensor, but it is more useful for larger telescopes, since they are looking through more diverse columns of atmosphere. For a good intro and list of early laser guide star systems check out this NOAO webpage: ctio.noao.edu/~atokovin/tutorial/part4/lgs.html $\endgroup$ – E. Douglas May 5 '18 at 17:21

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