Re: Resolution of a satellite as seen from the ground

From: Allen Thomson (thomsona@flash.net)
Date: Sat Dec 30 2000 - 06:46:51 PST

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    > However, I recently thought about the fact that taking a picture from
    > the ground looking up shouldn't be any different than taking a picture
    from
    > orbit looking down.
    
    In fact, the situations are not the same.  The local variations in
    atmospheric density that cause degraded resolution are much closer to the
    ground than they are to objects in space, and therefore the "lever arms"
    over which they operate in blurring images are much different.   You can see
    this effect by using a piece of pebble glass  -- shower doors are often made
    of pebble glass --  to simulate the turbulent atmosphere:
    
    Place a newspaper  two meters from the door and stand on the opposite side
    of the door with your eye close to the glass; the print on the paper will
    appear greatly blurred. Now move the paper so that it is close to or in
    contact with the glass and back away so you are viewing the paper from two
    meters distance -- at least the headlines will be readable.  The first case
    corresponds to looking into space from the ground, the second to looking at
    the ground from space.
    
    > If satellites can resolve things smaller than a meter
    > across then with good optical equipment, we should be able to do the same
    > when looking at the satellite.
    
    We can, using a variety of tricks to avoid or compensate for the atmospheric
    blurring. Short-exposure imaging, speckle imaging, adaptive optics, laser
    guide stars are terms to seach for.
    
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