This little refractor features an ED achromatic objective
lens of 3.1-inch (80-mm) aperture and 720-mm focal length (f/9).
But it's also long enough to avoid the extreme false color that sometimes exists with inexpensive achromatic objectives
. An unusual feature of the Infinity 90 is that it includes a 90[degrees] image-erecting prism that shows objects in correct orientation rather than mirror-reversed as with most refractors used with a conventional star diagonal --very handy for terrestrial viewing.
The typical achromatic objective
designed for visual use has yellow-green light (with a wavelength around 555 nanometers, where the eye is most sensitive) come to focus slightly closer to the objective than blue (around 486 nm) and red (around 656 nm).
An achromatic objective
, which consists of two pieces of dissimilar glass, brings the focus points of the colors much closer together than a single lens can.
The two elements of the refractor's achromatic objective
were misaligned when a first set of plates was taken from South Africa in the 1930s, says Platais, but not when a second set of photographs was taken four decades later by the telescope, which by then had been reassembled in Australia.
When English optician John Dollond introduced the achromatic objective
lens in the mid-18th century, competitors came out with forged "Dolond" and "Dolland" telescopes in the hope of cashing in on the invention!