Salt Lake City from LandSat

Radiogal writes: "A great view of the area from NASA! See olympics.gsfc.nasa.gov." This is about all of the Olympics you're going to see on the web. :)

Comments

Re:Hubble telescope

[Perdo]

First, how did the Hubbell end up being "myopic". There is a short, quick answer. The optical system used by the Hubbell was designed to operate in the near field where the wavefront from a point source in object space would have a sag across the telescope aperture of several wavelengths, quite large compared to the desired level of correction of the optical system. The Hubbell application, however, for sources at optical infinity, called for a telescope designed for plane parallel wavefronts from object space. The Hubbell optical system was a "borrowed" design. The design was borrowed undoubtedly to "save money", a favorite exercise of the "almost technical" managers of that era.

The technical management of two groups of people had to fail to enable construction of the original "myopic" Hubbell telescope. The government group which contracted with the supplier had to overlook the rather obvious short fall of the borrowed design. Second, the supplier, who had performed the design originally for near field operation, either overlooked the design shortfall or, more likely, had lost the organizational technical legacy of the original design team which would have stopped or attempted to stop the Hubbell "borrowed design" project before it started.

The design criteria for the original optical design for the telescope was based on imaging features on the surface of the earth from a low earth orbit reconnaissance satellite. The potential performance of surveillance systems of this type had been detailed in JOSA in the mid 60s. The performance limitation of an imaging system in earth orbit was shown by Hufnagel of Perkin Elmer to be the modulation transfer function (MTF) of the atmosphere which veils the surface of the earth as viewed from space. In his paper Hufnagel performed trades which related performance to altitude and diameter of the optical system. To achieve the nominal 1 microradian resolution which he projected would require a nominal 2 meter diameter optical system. It is a small step from this fact to the requirement to have a telescope with a customized figure for near field operation. The 2.5 meter aperture used would have to operate at or very near it's diffraction limit of 0.5 microradian to achieve the performance as limited by the atmosphere. To achieve very near diffraction limited performance the optical figure would definitely have to be adjusted to accommodate the large sag, spherical input wavefront.

The production of optics generally requires the building of tooling, test jigs and test plates to insure that the finished product has been correctly produced. The development, building and testing of this tooling is a major part of the expense of producing optical systems. This was, apparently, a major "cost saving" envisioned for the original Hubbell project. The tooling for a telescope designed to operate in the near field was employed to build a telescope to view stars at optical infinity. During initial operation of the Hubbell after being deployed into orbit it's designed-in "myopic" condition was discovered.