Life with Paracorr Type-2
Paracorr is a universal corrector that tightens and intensifies star images on all f-ratios down to f/3! 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 fully 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, Ethos and Delos eyepieces achieve higher levels of aberration correction than competitive models. However, Paracorr is also compatible with other eyepiece makes.
Life without 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, some required difficult installation in the focuser's draw tube. Dedicated coma correcting eyepieces were another approach. However, these were limited to 50 degree apparent fields and would orphan your existing eyepiece collection.
Paracorr also works with DSLR and CCD cameras. Just unscrew the lens assembly from the Tunable Top and attach appropriate threaded accessory tubes and camera mounts.
Mount your DSLR or CCD to the Paracorr Type 2 lens assembly and the quality of full frame photos will be spectacular since star image sizes on the image plane can be less than .001" over the full frame for an f/4.5 mirror. Edge-of-field sharpness is further enhanced since Paracorr also acts as a field-flattener. Works on all f-ratios f/3 and slower without adding any false color or spherical aberration! NOTE: Curves shown are typical for mirror focal lengths in the 1200mm or longer focal length range. For focal lengths shorter than 1200mm, you might find some field curvature. In general, TeleVue recommends APS size formats, which would typically yield 90% illumination over a 30mm diagonal chip.
The Paracorr Type 2 allows separating the optical assembly from the Tunable Top and using it with Tele Vue imaging system components to permit use with SLR and CCD cameras. APS size formats 27mm diameter are recommended to minimize field vignetting. Ideal for mirrors as fast as f/3.
Optimizing the Coma Correction by Using Paracorr's Tunable Top
As you've read, 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, TeleVue 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. For your reference, here is a recommendation guide for choosing the best setup for various Tele Vue eyepieces to get optimum correction:
Article: The New Dobsonian Revolution in Astronomy Magazine
Manual: Paracorr Type 2 Instructions
Chart: Chart (PDF): Spot Size Graphs
Chart: Paracorr Type-2 Recommended Tunable Top Positions and Dimensions