| The 4mm Vixen Lanthanum eyepiece is a very short focal length eyepiece with exceptionally long eye relief for eyeglass use. The 4mm Vixen Lanthanum uses seven optical elements in four groups to achieve high power without reducing its eyeglass-friendly 20mm eye relief. It provides unvignetted very high magnification/medium field viewing with short focal length refractors and fast focal ratio/short focal length reflectors. It can also be used with longer focal ratio Schmidt-Cassegrains and Maksutov-Cassegrains, but its exit pupil becomes small and the images dim with these scopes. For example, using it with a 2000mm 8" f/10 Schmidt-Cassegrain will result in 500x (63x per inch of aperture) and a 0.4mm exit pupil (about 1/64th of an inch across). Both of these are slightly past the rule of thumb maximum values of 50-60x per inch of aperture and a 0.5mm exit pupil, but are still usable by a reasonably experienced observer under very good, and accordingly rare, seeing conditions. Average seeing conditions would generally keep these maximum values from being attainable. The optical performance is very respectable, on a par with that of a good Plössl. There is a slight loss of contrast when compared to a Plössl alone (and to a lesser degree when compared to the Plössl/Barlow combination needed to achieve the same focal length), due to the additional optical elements needed to achieve the unique combination of very short focal length and very long eye relief of this Vixen. The loss is minimal though, and this Vixen still has the definite benefit of much longer eye relief, particularly at a cost half that of other 20mm eye relief eyepieces.
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