This Celestron Ultima LX is an excellent very short focal length high power eyepiece with a wide 70° field of view. It is outstanding for medium to medium high magnification viewing with short focal length refractors and fast focal ratio/short focal length reflectors, as well as providing about the maximum usable magnification attainable with a typical f/10 Schmidt-Cassegrain.
It will generally be useful with long focal length refractors, reflectors, or catadioptric scopes only under very good or better seeing conditions. Average or worse seeing conditions would limit its usefulness. Using it with a 2000mm 8” f/10 Schmidt-Cassegrain, for example, will result in 400x (50x per inch of aperture) and a 0.5mm exit pupil (about 1/50th of an inch across). Both of these are right at the rule of thumb maximum values of magnification per inch of aperture and exit pupil size for useful observing under good to very good seeing conditions for a reasonably experienced observer.
With a usable eye relief of 16mm (quite long for an eyepiece of this focal length), the 5mm 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 very short 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 very short 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 22 ounces and 5.5” long, the 5mm 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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