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Focal modifying lenses

The focal modifier lenses were originally intended to be used to optimize the overfilling of the echelle for different modes of operation, as discussed in detail by Diego & Walker (Montly Notices of the Royal Astronomical Society, vol 217, p347, 1985). In fact, the poor performance of the lenses (see Appendix xiv) means that there are no circumstances under which they would be of use, and they have been removed from the instrument.