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William Optics are well versed in the manufacture of quality eyepieces, especially in the wide field arena. The 4mm UWAN (standing for ultra wide angle) is a dedicated planetary/lunar eyepiece that will provide high power views when used on any telescope, the 4mm focal length pushes any scope to it's natural threshold.
The UWAN uses more lenses than your average eyepiece, it utilises 7 lens elements spaced into 4 groups, 3 groups of 2 and a single lens. Extra lens elements usually only serve to degrade views but in the case of the UWAN each optical surface has been fully multi coated with a high quality, high light transmitting coating to get the most out of each lens. In the case of the UWAN the extra lenses have been introduced to obtain a wider field of view, the 4mm has a whopping 82 degrees.
The 82 degree field not only means you can view larger objects at higher magnifications but the Moon or any planet passing through the field will stay in view for longer and mean less adjustment to the telescopes orientation will be needed.
Optically the 4mm is impressive. The main complaint of any wide field eyepiece is usually found in the outer field but the UWAN retains sharpness out to about the last 5% of the view. Objects are clear and defined and yield excellent contrast (the UWAN performs exceptionally well on the Moon in this respect). A good colour rendition provides some of the most pleasing views of Jupiter I've ever had.
The UWAN performs visually very well but is very comfortable to look through when you consider the high focal length which is synonymous with small objectives and short eye relief. The extra lenses have enabled WO to get a mighty 12mm of eye relief (which for this focal length is a real feat), meaning there's no need to keep your eyeball pressed against the lens (it also makes the UWAN suitable for spectacle wearers but only just). When coupled with an over-sized objective lens you have an eyepiece that is a joy to look through.
To conclude the UWAN is an amazing eyepiece, with a strong performance optically and the observers eye given the luxury of a decent objective and long eye relief (for the focal length). It's slightly heavier than your average plossl (just over 225g) but not so as to introduce any serious balancing issues when changing eyepieces. At £170 the UWAN is worth every penny and is a definite recommendation.