Background
The retrofocal dialyte concept underlying the Wall-Wise Zerochromat refractor represents the culmination of a lifetime's empirical research for optician John Wall. In 1998 he discovered a way of going beyond the catadioptric medial design of Ludwig Schupmann, putting the flint corrector downstream from the field lens, which is placed at the prime focus. A 10-inch aperture prototype proved that such a telescope was viable, whereupon John started work on a 30-inch aperture instrument that was completed in 1999. It currently resides at the Hanwell Community Observatory near Banbury, North Oxfordshire. This telescope still represents the largest aperture refractor in professional or amateur hands within the UK and ranks equal fifth largest in the world.
John took Peter Wise, whom he had befriended some years previously, to see the 30-inch refractor at Hanwell. Peter was excited by the concept and vowed to one day build a 40-inch model. Thus was the partnership forged, at which point Peter ray-traced the design of what would become Zerochromat on a computer and declared that it was apochromatic to a very high level of correction. With Peter's optical experience and commercial expertise the quest to create the 'golden telescope' bore fruit in the form of the first production model, hence Peter deserves equal credit for the new telescope.
How does it work?
Unlike most contemporary premium refractors that use high-density fluorite glasses with doublet or triplet objectives, the Zerochromat uses a long focal ratio single element 200mm aperture plano-convex lens as its objective. Two internal fold mirrors reduce the physical length of the instrument to a highly manageable 135cm (53 inches). The patented corrector lens assembly reduces chromatic aberration to vanishingly small levels before reaching final focus, delivering the truly diffraction-limited performance that only an unobstructed optical system can provide. As you'd expect from such a high-performance instrument, the level of optical and mechanical craftsmanship is exemplary. Furthermore, the lightweight, strong and thermally stable carbon-fibre tube ensures that you have an ideal photo-visual platform.
How well does it work?
The 200mm f/12 Zerochromat is the result of exhaustive computer optimisation and field testing designed to give you what we firmly believe to be the finest observing instrument of its aperture. As the following wavefront error diagram shows, the Strehl ratio is in excess of 0.999, and the spot diagram demonstrates superb performance.
Thus for a wavefront error of 0.0042, the Strehl ratio is 0.9993.
Strehl ratio is an excellent measure of the optical quality of a system. An f/12 achromat will typically have a Strehl ratio of 0.8, and an f/12 apochromat, a Strehl ratio of 0.98. The most expensive apochromats may have a Strehl ratio even higher than 0.98.
Under very good sky
conditions, it is possible to use a magnification of 500x, and the Zerochromat
shows bright stars as perfect Airy discs, with faint diffraction rings. Inside
and outside focus is textbook perfect. There is no chromatic aberration visible
at all.