PVL-2008 Paracorr VISUAL, with Tunable Top
All Newtonian/Dobsonian telescopes can benefit from using a Tele Vue Paracorr (Parabola Corrector) to eliminate coma in the image. One aspect to optimizing this correction is the distance of the eyepieces 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. To accommodate for the differences, we developed the "Tunable Top."
The "Tunable Top" permits optimized aberration control for all Tele Vue eyepieces and many others, by simply loosening a screw and rotating the Tunable Top. The eyepiece barrel height raises and lowers, allowing you to maintain the proper field stop to Paracorr lens distance.
Life before Paracorr
Coma makes stars look like blurry comets whose tails point out radially from the center of the field. The faster your scope's speed, the worse the effect. All parabolic mirrors used in Newtonian reflecting telescopes are limited in field sharpness due to coma. Even a perfectly made 13" f/4.5 parabola has only about 0.1 degree diffraction limited field.
Previous attempts to correct for coma have been limited in usefulness. Some correctors were designed mainly for astrophotography. Dedicated coma correcting eyepieces were another approach. However, these were limited to 50 degree apparent fields and would orphan your existing eyepiece collection.
Life with Paracorr
Paracorr is a universal corrector that tightens and intensifies star images [see performance graphs here] on all f-ratios from f/3.5 to f/8 without adding any false color or spherical aberration! You no longer have to constantly shift a Dobsonian to keep objects centered for sharp viewing. Using a Tele Vue eyepiece, put M-13 at the edge of the field and enjoy resolving its beauty as it drifts across your view.
Paracorr is easy to use. It slips into your focuser, just like a Barlow. To minimize focuser in-travel, due to additional length, a mild 15% Barlow effect was designed into Paracorr to effectively push the focus out. Paracorr is recommended for use with all Tele Vue eyepieces because our Plössls, Radians, Panoptics, Naglers and Ethos achieve higher levels of aberration correction than competitive models. However, Paracorr is also compatible with other eyepiece makes.
Try Paracorr on your "DobNewt." We think you'll agree that the revolution is complete.