This short focal length 9mm TMB Planetary eyepiece is from TMB Optical, the company founded by noted apochromatic refractor designer Thomas M. Back to produce state-of-the-art premium optics. The 9mm TMB Planetary has been optimized to show maximum contrast, detail, and definition with a wide variety of telescope types and focal ratios when viewing bright objects such as the Moon and planets. It has very high light transmission and contrast, plus minimal lateral color and light scatter. These combine to provide exceptional high power observing, both on and off axis, with long focal length refractors, reflectors, and catadioptric scopes under average or better seeing conditions. That you can get its truly premium performance at such a reasonable price is little short of amazing.
Its 9mm focal length is nearly ideal for an f/10 Schmidt-Cassegrain, as it provides 222x and a 0.9mm exit pupil that's a close match to the 1mm exit pupil that most observers feel best matches the resolution of an SCT to the resolution of your eye. Combine that resolution with its high contrast and you have an eyepiece that's unexcelled for lunar, planetary, and globular cluster observing with an SCT. (Some of the larger globular clusters, such as M13, will more than fill its 0.26 degree field with such a scope.)
It will work with f/10 to f/15 catadioptric scopes, providing 433x with a 14" f/11 catadioptric. That's about 31x per inch of aperture - the most you can reasonably expect under typical seeing conditions with such a large aperture scope. It will work with 80mm f/6 refractors, providing 53x with a wide 1.07 degree field (more than twice the diameter of the full Moon). And it will work with any telescope type in between. Eye relief, at 12mm, is quite 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 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 degree 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.
