| The Paracorr (Parabola Corrector) Visual is an auxiliary lens system that fits into the 2” focusers of all carefully-collimated f/4.5 and slower 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.
While optimized strictly for visual observing with f/4.5 and slower scopes, the Paracorr Visual will still work with scopes as fast as f/4. For the new very large and faster focal ratio “ladder-less” Dobs in the f/3 neighborhood, the new Paracorr Type 2 would be recommended.
As an example, this fully multicoated 16 oz. four-lens Paracorr 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/4.5 scope perform like an f/12.
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, this link will take you to TeleVue’s recommendation guide for choosing the best tunable top height setting for various TeleVue eyepieces to get optimum coma correction with the Paracorr.
CAUTION: Do NOT use Barlow lenses with the Tunable Top Paracorr, as the Barlow may contact the Paracorr lens. | 
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