Nonmarket valuation of water quality in a rural transition economy in Turkey applying an a posteriori bid design

A contingent valuation study measured citizen willingness to pay (WTP) for an improvement in surface water quality in Latvia. The average respondent was willing to pay 0.7 per cent of household income for the environmental improvement, but that amount was much less than needed to finance the required investments in treatment facilities. While the income elasticity of WTP for the average resident was low (0.56), it increases as income increases, reaching 0.9 at an income level double the current average. As real incomes increase in Latvia, the demand for environmental quality by citizens can be expected to increase substantially.

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Practical Correction of Three Fold Astigmatism in the Philips CM TEM

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100164556 ◽

1996 ◽

Vol 54 ◽

pp. 418-419

Author(s):

Arno J. Bleeker ◽

Mark H.F. Overwijk ◽

Max T. Otten

Keyword(s):

Optical Properties ◽

Phase Contrast ◽

The Other ◽

Objective Lens ◽

Symmetric Part ◽

A Posteriori ◽

Contrast Transfer Function ◽

High Brightness ◽

Rotationally Symmetric ◽

Brightness Field

With the improvement of the optical properties of the modern TEM objective lenses the point resolution is pushed beyond 0.2 nm. The objective lens of the CM300 UltraTwin combines a Cs of 0. 65 mm with a Cc of 1.4 mm. At 300 kV this results in a point resolution of 0.17 nm. Together with a high-brightness field-emission gun with an energy spread of 0.8 eV the information limit is pushed down to 0.1 nm. The rotationally symmetric part of the phase contrast transfer function (pctf), whose first zero at Scherzer focus determines the point resolution, is mainly determined by the Cs and defocus. Apart from the rotationally symmetric part there is also the non-rotationally symmetric part of the pctf. Here the main contributors are not only two-fold astigmatism and beam tilt but also three-fold astigmatism. The two-fold astigmatism together with the beam tilt can be corrected in a straight-forward way using the coma-free alignment and the objective stigmator. However, this only works well when the coefficient of three-fold astigmatism is negligible compared to the other aberration coefficients. Unfortunately this is not generally the case with the modern high-resolution objective lenses. Measurements done at a CM300 SuperTwin FEG showed a three fold-astigmatism of 1100 nm which is consistent with measurements done by others. A three-fold astigmatism of 1000 nm already sinificantly influences the image at a spatial frequency corresponding to 0.2 nm which is even above the point resolution of the objective lens. In principle it is possible to correct for the three-fold astigmatism a posteriori when through-focus series are taken or when off-axis holography is employed. This is, however not possible for single images. The only possibility is then to correct for the three-fold astigmatism in the microscope by the addition of a hexapole corrector near the objective lens.

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