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Re: VP Gore's Proposal




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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