Improving the energy resolution of a hemispherical spectrograph using a paracentric entry at a non-zero potential

We demonstrate by ray tracing analysis that the focusing properties of a he mispherical spectrograph depend critically on the placement of the electron entrance position R-0 and the value of the potential V-0 at R-0. Improveme nts over conventional spectrographs using R-0 = (R) over bar, the mean radi us and V-0 = 0 are predicted for particular combinations of R-0 and V-0. A spectrograph with (R) over bar = 101.6 mm and an entrance at R-0 = 82.55 mm using a zoom lens and a two-dimensional position sensitive detector (2D-PS D) with 40 mm multichannel plates and resistive anode encoder has been used to confirm the predicted qualitative behaviour. These results suggest that improvement in energy resolution may be attained without the use of fringi ng field correctors. (C) 2000 Elsevier Science B.V. All rights reserved.