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Telescope Design, Geodesics And The NGST

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Posted by Jim Bergquist on January 19, 2001 06:15:44 UTC

I am not an expert on telescope design but the use of a corrector plate was a common proceedure before the "problems" were encountered with the Hubble. Corrector plates are used to compensate for the defects inherent in the design of a telescope such as correction for spherical aberration, coma and reshaping of the focal plane. The data that I have indicates that the spherical mirrors of the Hubble were part of the original design. See Astronomical Optics by Daniel J. Schroeder which was published in 1987 by Acedemic Press for more info.

We have a tendency to think in terms of an Euclidean space-time. But if space is curved, as General Relativity assumes, the path that a ray of light takes is bent by the properties of space-time and we have to use geodesics, which descibe minimal paths, to follow the motion of a light ray. The wavelength of the light is also altered by Doppler and gravitational effects. If we tried to look down the gravitational well of a blackhole, what we saw would probably be distorted by gravitational lensing and thus alter the focus. Such effects would have to be compensated for if we were viewing from a distance. If there are largescale effects determined by cosmological considerations then they would affect our viewing of distant objects. Just as we cannot correct for atmospheric diffraction in ground based observation, we could not correct for these effects even if we use a space based observatory. What we can do is try to determine their effect on what we do see.

The design of the Next Generation Space Telescope has not yet been chosen but they are now working of the technology that it will incorporate. Flexible mirrors such as those in the Keck are a possibility. We might still have to compensate for the solar wind or interstellar gas flows for example which might affect "seeing". How the NGST is optimized depends on the factors taken into consideration.



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