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:
Wolf359 New Member
- Skill Level
- Time with Product
Feb 5, 2008
- User Notes:
Reduces coma, helps with field curvature in some eyepieces, 1.15X magnification factor.Cons:
Heavy, easy to lose tension washer, 1.15X magnification factor.Comments:
The Tele Vue Paracorr Visual is probably one of the last accessories I would part with as an amateur astronomer. It took me quite a while to get around to buying one, but I can't imagine observing without one now.
Its main benefit, of course, is its coma reducing effects. For me, coma is not much of an issue in my f/6 Orion XT-8, but becomes a real nuisance in my f/5 DBA 12 inch Dobsonian. Different people find it to be objectionable at different levels. For me, it becomes a problem in my f/5 12 inch Dobsonian starting with my 17mm Nagler Type 4 and longer focal length eyepieces. With shorter focal length eyepieces, I don't notice much coma to begin with, so the Paracorr isn't a necessity. However, it doesn't seem to cause any negative effects either (except for possibly one which I'll mention later), so I simply leave it in my focuser at all times.
In addition to helping with coma, the Paracorr has two other properties which I find quite useful. One of those properties is that it seems to help out with eyepieces which show field curvature effects when used in conjunction with my 12 inch Dobsonian. In particular, my 30mm Bird's Eye View and 24mm Baader Hyperion greatly benefit from the Paracorr. In each case, the percent of field which I consider to have bothersome field curvature is halved.
The other additional property which I find useful is the built in 1.15X magnification factor of the Paracorr. I am aware, however, that many astronomers will probably view this as a negative since it takes away some valuable true field of view. As such, I will list it as both a "Pro" and "Con" further down in the review. However, in my case the lost field of view is more than outweighed by the ability to slightly tweak my eyepiece focal length line up. In particular, I like to use the Paracorr to change the focal length of my 7mm Pentax XW to about 6.1mm, which works well with my 12 inch Dob for planetary viewing.
As far as I could tell, the Paracorr has only a couple of drawbacks (in addition to the magnification factor). The first is the obvious hefty weight, which can be compounded by the typically large, long focal length eyepieces most people will likely use with the Paracorr. The second negative is that it can be easy to unscrew the setting adjustment screw for the outer collar. Losing the adjustment screw is not really the concern in this case, as you will likely be holding it in your hand if you unscrew it completely. The real concern is losing the plastic washer underneath, which is used to tension the screw and collar. Neither of these drawbacks were a real issue for me, although losing the washer was admittedly quite frustrating the one time it happened (trust me, after losing it once in a thick field, you won't make the same mistake again!).
So in summary, I would highly recommend the Paracorr to other astronomers. To me it's not just a coma corrector, but a three-in-one tool that I use every time I observe.
Starman1 New Member
- Skill Level
- Time with Product
Jul 11, 2007
- User Notes:
Paracorr is Essential to Short NewtoniansPros:
--Easy to use/adjustCons:
--Kills bothersome coma
--Makes short f/ratio large apertures possible
--The price of a premium eyepieceComments:
--Lengthens focal length by 15%, reducing field size a tad
If you value optical image quality, but your Newtonian scope is f/5 or even shorter, then a Paracorr is an essential part of the optical train. By reducing coma to vanishingly low levels, any other optical errors in the system show through. If there are none, then the star images look as if you are looking through a big refractor (instead of looking at a jump to lightspeed on the bridge of the Millenium Falcon :-)).
Fainter stars become visible in the outer half of the field because they are not smeared to invisibility. Extended objects seem to focus more tightly and display smaller details. Whatever light loss there is in the Paracorr is more than made up for by the change in tightness of focus. Even planetary images improve in an undriven scope where the image drifts from edge to edge in the field.
Why put up with coma as an aberration when it is so easily corrected?Sort by