This medium focal length Celestron Ultima LX is an excellent medium to medium high power eyepiece with a very wide 70° field of view. It is usable with any telescope type, under virtually any seeing conditions – from fair to superb. (Rain, snow, or fog might limit its usefulness, however.)
With short focal length refractors and reflectors, it’s a low to medium power eyepiece. With an 80mm f/6 refractor, for example, it provides 36.9x and a 1.89° field that’s nearly four times as wide as the Moon. This is well suited for observing open clusters such as the Pleiades and large nebulas such as the Lagoon. Both will fit nicely into its spacious field, with a framework of black sky around them to set them off.
It is also excellent for medium high magnification observing with long focal length refractors, reflectors, or catadioptric scopes. On a 7” Maksutov, for example, it yields 205x. This is an excellent magnification for planetary and close-up lunar observing and binary star splitting. Its performance makes it a much better choice as a second eyepiece than merely doubling the power of most scopes’ standard 25-26mm eyepiece with a Barlow.
With a usable eye relief of 16mm, the 13mm Ultima LX will provide nearly unvignetted views for most eyeglass wearers, although some vignetting is unavoidable if you must wear eyeglass while observing due to severe astigmatism. There is a slight loss of contrast when compared to an eyepiece with fewer elements, due to the additional lenses needed to achieve the unique combination of a medium focal length and very generous field. The loss is minimal though, on a par with the contrast loss in an eyepiece/Barlow combination with the same effective focal length.
Dual concentric barrels let you use this eyepiece in either a 1.25” star diagonal/focuser or a 2” star diagonal/focuser.
At 21 ounces and 5" long, the 13mm Ultima LX is a very substantial eyepiece. Be sure that your star diagonal is very firmly locked in place in your scope on your scope before inserting this eyepiece to avoid the chance of the diagonal swiveling unexpectedly and the eyepiece falling to the ground.
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