| The Paracorr (Parabola Corrector) Visual is an auxiliary lens system that fits into the 2" focusers of all carefully-collimated f/3.5 to f/5 Newtonian reflectors. It drastically reduces coma and curvature of field in visual observing. There is also a non-vignetting purely photographic version of the Paracorr, the #PARAUNI. As an example, this fully multicoated 16 oz. four-lens system increases the diffraction-limited field of an f/4.5 system by 36x in area and reduces the field edge coma by 75%-95%. The Paracorr essentially makes an f/4 scope perform like an f/8. Stars at the edge of the field change from comet-shaped flares to points of light, so that stars look virtually the same anywhere in the field with a good eyepiece. Faint stars at the edge of the field do not fuzz out to invisibility due to coma and field curvature, but show as tightly focused points as much as six times smaller than their size without the Paracorr. This not only is pleasing visually, but reduces significantly the need to constantly nudge a Dobsonian’s optical tube to keep an object centered in the scope’s “sweet spot." In effect, the entire field becomes the sweet spot with the Paracorr in use. The Paracorr is ideal for premium eyepieces like the TeleVue Naglers and Panoptics, but all quality eyepieces will benefit. It holds 2" eyepieces directly and has an adapter for 1.25" eyepieces. The Paracorr increases the magnification by 1.15x. A unique feature of the Paracorr is its “tunable top." An important aspect to optimizing the Paracorr’s coma correction is to precisely set the distance from the eyepiece focus point (field stop location) to the last surface of the Paracorr lens (within a small +/- tolerance, of course). Typical eyepieces used with fast Newtonians can have field stop locations differing by as much as half an inch. The “tunable top" permits optimizing this critical spacing for all TeleVue eyepieces, and many others, by simply loosening a screw and rotating the tunable top. This raises and lowers the eyepiece barrel height, allowing you to set the proper eyepiece field stop to Paracorr lens distance. For your reference, here is a recommendation guide for choosing the best setup for various TeleVue eyepieces to get optimum coma correction: Tunable top at maximum height and used with 2" eyepieces: 20mm Nagler Type 2, 13mm Nagler (old style), 35mm Panoptic, 27mm Panoptic, 22mm Panoptic. Tunable top at maximum height and used with the 1.25" eyepiece adapter: all Radians, all Plossls, 4.8mm and 7mm Naglers, 15mm and 19mm Panoptics, TeleVue 8-24mm zooms. Tunable top at minimum height and used with 2" eyepieces: 31mm Nagler Type 5, 22mm Nagler Type 4, 17mm Nagler Type 4. Tunable top at minimum height and used with the 1.25" eyepiece adapter: 16mm Nagler Type 2, 12mm Nagler Type 2, 12mm Nagler Type 4, 9mm Nagler (old style). CAUTION: Do NOT use Barlow lenses with the Tunable Top Paracorr, as the Barlow may contact the Paracorr lens.
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