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This 2.5mm TMB Planetary eyepiece from TMB Optical gives you exceptional high power views with short focal length/fast focal ratio refractors and reflectors – even under so-so seeing conditions. It has been optimized to view bright objects, both on and off axis, while showing maximum detail and definition. The TMB has very high light transmission, very high contrast, minimal lateral color, and minimal light scatter.
The very short focal length 2.5mm TMB will work with f/8 reflectors and f/10 catadioptric scopes, but its tiny exit pupil (0.3mm with an f/8 scope and 0.25mm with an f/10) and very dim images with such scopes make it exceedingly difficult to use.
Combine the 2.5mm TMB’s 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 f/5 or f/6 focal ratio telescope. Its 2.5mm focal length provides 192x (a very high 62x per inch of aperture) with an 80mm f/6 refractor, about the practical maximum magnification possible under very good seeing conditions. While this TMB eyepiece is optimized for observing subtle lunar and planetary detail and low contrast features, it works equally well for splitting binary stars, resolving compact globular clusters, etc. An 80mm refractor would be about the smallest aperture scope recommended with this eyepiece; refractor apertures of 100mm and larger would be preferable.
Eye relief, at a surprising 14mm, is exceptionally good for an eyepiece with such a short focal length. Vignetting will be relatively minor 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° field while still being sharp and having high contrast, thereby maximizing your observing time.
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.
|TMB Optical Planetary Type II in general|
Many of today ’s premium eyepieces have very wide fields of view to give an exciting “picture window” view of deep space. However, these eyepieces need many exotically-shaped glass elements to achieve a wide field of view – up to 9 lenses in some cases. More lenses mean more cost and more glass (which cuts the amount of light transmitted). More lenses mean more air-to-glass surfaces to scatter light (which gives lower contrast and sometimes multiple ghost images of bright objects). In addition, with all these extra lenses, optical design compromises have to be made that sometimes result in lateral color and light scatter.
Lateral color is an aberration where different colors intercept the focal plane at different points and can sometimes be seen as a haze of false color around bright objects, both on- and off-axis. Light scatter is seen as a distracting halo or glow around bright objects, which can also reduce contrast with the object itself. Light scatter and lateral color are rarely seen in these wide field eyepieces when observing faint deep sky objects, but they are the bane of lunar, planetary, and double star observers. These observers don’t need an extra-wide field of view to see small details and subtle contrast differences on the Moon and planets. While a comfortably wide view that can show all of a planet’s moons in a single field is a bonus, more important is an eyepiece with high contrast, no scattered light, and the highest possible definition.
Until now, orthoscopic and Plössl eyepiece have been the eyepieces of choice for lunar, planetary, and binary star/star cluster observers. Their low number of lenses (four) and air-to-glass surfaces (also four) give visibly better contrast and definition than the more exotic extra wide field eyepieces. A high quality orthoscopic or Plössl eyepiece on a good telescope gives views of Mars, Jupiter, and Saturn that are unforgettable. But even the ortho and Plössl designs can be improved for lunar and planetary use.
The TMB Optical® eyepieces are the first long eye relief eyepieces with a comfortably wide field that have the sharpness, contrast, and lack of lateral color and light scatter for planetary observing of the best orthoscopic and Plössl designs. What they don’t have is the ortho and Plössl’s limited eye relief and restricted fields. The six-element TMB Optical® eyepieces provide a substantial 58° field of view with high contrast, a tack sharp view, and surprisingly long eye relief for relatively unvignetted eyeglass use.
The TMB Optical® eyepieces are fully internally baffled and all internal spacers have antireflection microbaffles on their edges and full flat black anodizing to reduce reflections. The field stop likewise has microbaffles in its edge to reduce off-axis flaring. Every air to glass surface is broadband multicoated for peak light transmission and superb contrast on subtle lunar and planetary detail. Each has a flexible rubber fold-down eyeguard on top of a twist-up eyecup that can be raised or lowered to match precisely the eye relief you find most comfortable. The eye lens is very large, at a 22mm diameter, which many people find more user-friendly than the narrow diameter lens of most short focal length eyepieces. Spherical aberration of the exit pupil is well controlled, so there are no serious blackouts or kidney-bean effects if you move your eye slightly off the eyepiece sweet spot, as you find with many other wide field eyepiece types. Dust covers are provided for both ends of each eyepiece. A safety groove is machined into the black barrel to engage your focuser thumbscrew to prevent the eyepiece from falling should the thumbscrew accidentally loosen while observing.
Tested against such stellar planetary eyepieces as the legendary TMB Super Monocentrics and the Zeiss Abbe orthos, the TMB Optical® designs were equal in sharpness to these 5-6 times more expensive eyepieces, with wider fields and longer eye relief, and virtually their equal in the amount of light scatter – truly amazing, considering the low price of the TMB Optical® eyepieces.
These TMB Optical® Planetary eyepieces are the only Planetary Series eyepieces authorized by the estate of the late Thomas M. Back.
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