| This short focal length 5mm TMB Planetary II eyepiece is from TMB Optical, the company founded by noted apochromatic refractor designer Thomas M. Back to produce state-of-the-art premium optics. It is designed to give you exceptional high power views with short to medium focal length refractors and reflectors – even under so-so seeing conditions.
The 5mm TMB Planetary II has been optimized to show maximum contrast, detail, and definition when viewing bright objects such as the Moon and planets, both on and off axis. It has very high light transmission and contrast, with minimal lateral color and light scatter, to provide exceptional high power observing with long focal length refractors, reflectors, and catadioptric scopes under average or better seeing conditions. It will work with f/10 catadioptric scopes, but its small half-millimeter exit pupil and dim images with such scopes will put it at the very limits of usability. That it does so at such a reasonable price is little short of amazing.
Combine its high resolution with its high contrast and you have an eyepiece that’s unexcelled for lunar and planetary observing and binary star splitting with an SCT. Its 5mm focal length provides 400x (a very high 50x per inch of aperture) with an 8" SCT, about the practical maximum magnification possible under excellent seeing conditions.
Eye relief, at 12mm, is surprisingly good for an eyepiece with such a short focal length. Vignetting will only be moderate for those who must wear eyeglasses while observing. This usually won’t be a problem when using the TMB Planetary II eyepiece on the kinds of objects for which it was optimized – planets, binary stars, etc. – as these small objects are generally kept in the center of the eyepiece field, where your eyesight is sharpest.
For Dobsonian users and others without tracking drives, the very low lateral color and other aberrations allow a planet to drift across the entire 58° field while still being sharp and having high contrast, thereby maximizing observing time. While the eyepiece is optimized for observing subtle lunar and planetary detail, the eyepiece’s observing virtues work equally well for splitting binary stars, resolving compact globular clusters, etc.
A light coating of lubricant has been applied to the threads of the twist-up eyecup. If any lubricant is visible on the inner eyepiece body when the eyecup is in the extended position, remove the excess lubricant with a clean wipe, such as a tissue. Operate the twist-up eyecup several times, cleaning the body as necessary until no more lubricant is visible on the eyepiece body when the twist-up eyecup is fully extended.
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