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*Subject*: Re: VP Gore's Proposal*From*: clark@xxxxxxxxxxxxxxxxxxxx (Tom Clark -- W3IWI)*Date*: Sun, 15 Mar 98 20:45:57 UTC

Phil said > Actually, I was thinking of optical physics, which limits the angular > resolution attainable with a given aperture and optical wavelength. > The math problem comes in the form of geometry: the farther away you > are, the larger the area on the earth's surface that corresponds to > the angular resolution limit. Just to clarify Phil's comments -- consider a rather small 10 cm (~4") telescope operating in the infrared at 10 microns (micrometers) wavelength. Simple arithmentic shows that there are 10**4 wavelengths in the aperture, so the Airy disk (the size of the diffraction disk in the image) is 1.22*lambda/D = 1.22*10e-4 radians = (approx) 5 arcseconds. If GoreSat is at 10e6 km from the earth, then the smallest area that could be resolved by such a telescope in the Infrared is 122 km in size. Using a bigger telescope, and moving towards the visible wavelengths could achieve resolution of x km, where x is a single-digit number. To snap a picture, the telescope has to have arc-second (or sub-arcsecond in the case of a bigger telescope and/or shorter wavelength) stability for the duration of the exposure. If you take successive multiple images to make a movie, then you need similar pointing accuracy to prevent jumping between frames. All of this in turn probably requires that the spacecraft will have to have a star-tracker to guide the telescope. All this is technically feasible, but it requires a non-trivial engineering effort (and Gore's proposal seems to call for it to be operational before the next election which is less than 20 months from now). 73, Tom

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