Celestron Omni 4mm Plossl Eyepiece
|The Celestron Omni 4mm Plossl is a high power eyepiece at a bargain price. Plossl eyepieces of less than 9mm focal length have small eye lenses and very limited eye relief, making them unsuitable for anyone who must wear eyeglasses while observing. The 4mm high power Omni eyepiece is a 4 -element premium symmetrical Plossl optical design. Features include blackened lens edges to minimize internal reflection and maximize contrast, multi-layer coating group on each lens surface to maximize light transmission and contrast, and each Omni goes through a two-step anodizing process to prevent reflection from the top eyepiece barrel. Threaded barrels accept 1-1/4" thread-in filters. |
Please note that Plossl eyepieces are a scaled design, meaning that everything about a Plossl eyepiece scales up or down with the focal length. This includes both the size of the eye lens and the eye relief. The eye lens on any 4mm Plossl will be very small. This is not necessarily a negative aspect of the design, as it is easy to achieve the proper on-axis eye placement.
Eye relief is the optimal distance from the top eyepiece lens to your eye that will allow you to see the entire field of view. By definition the eye relief of a Plossl eyepiece is 0.8X the focal length. Celestron claims that the eye relief of this eyepiece is 6mm, which presents a rather obvious inconsistency. Nevertheless, one must accept the fact that the eye relief of this eyepiece will be rather tight, meaning that you are going to need to get in close in order to use it.