The VISTA mirror
The VISTA 4.1-metre diameter primary mirror is the most highly curved mirror ever made of this size and quality — with deviations from a perfect surface of less than 1/3000th of the thickness of a human hair. This mirror has a photographic f/ratio of 1.0, which allows the telescope tube to be extremely short and rigid.
Light from the sky is reflected from the primary mirror up to the convex secondary and from there down into the huge VISTA camera.
The mirrors are coated with a thin layer of protected silver in the facility‘s coating plant. Silver is the best metal for the purpose since it reflects over 98% of near-infrared light, more than the more commonly used aluminium. Up to now, the reflectivity produced by the silver coating, a relatively new and difficult process, is well above that specified and exceeds in quality the coatings on other comparable telescopes.