Aspherical correction

In case of concave surfaces the aspherical correction of the wavefront is done with a null system. If a convex surface has to be tested, such as the surface of a secondary mirror, testing is performed against a concave matrix glass. 

The original bend and polish idea is due to Bernard Schmidt who built what has become known as a Schmidt telescope with a spherical primary and an aspheric corrector plate that introduces exactly the right amount of spherical aberration to cancel the spherical aberration that would be introduced by the spherical primary (Schmidt). The corrector has to be thicker at the edge in proportion to the radial distance in the aperture to the fourth power to provide the needed correction. 

The results show only a modest variation when the van der Waals radii are changed within reasonable bounds (Table 6.2). As the data were not refined with the aspherical atom formalism, the scale of the observed structure factors may be biased, an effect estimated on the basis of other studies (Stevens and Coppens 1975) to correspond to a maximal lowering of the scale by 2%. Values corrected for this effect are listed in the last two columns of Table 6.2. Since neutral TTF and TCNQ have, respectively, 72 and 52 valence electrons, the results imply a charge transfer close to 0.60 e. 

X-ray EFG values to spectroscopic splittings. But the polarization of the valence shell is at least in part accounted for in the aspherical multipole description. To the extent that the model is sufficiently flexible, the shielding factor Rva,ence is not needed, and the correction equation becomes... 

UV-visibie-near-iR Plane holographic gratings up to 1400 grooves per mm, or spherical or aspherical types with diode array detection Aspherical types correct for optical aberrations... 

In comparison to earlier 4 m class telescopes the NTT primary has a much shorter focal ratio of F/2.2 and the resulting aspherical deviation from the best-fit sphere is in the range of 200 pm. The aspect ratio was chosen to he D/h = 3600/240 = 15 and allows active correction of a few A by the 78 actuators of the axial support system. Because long-wave errors such as astigmatism or spherical aberration can be corrected, the demand for an extremely smooth optical surface becomes much more stringent [4.43]. This goal has been reached with ... 

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