Tele Vue Paracorr coma corrector model PCT-2001.
From Tele Vue's literature, "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, we developed the "Tunable Top."
Less than one year old. Used once. I ordered the Paracorr when I ordered my Dobsonian. Upon delivery of the telescope, I found that my Dobsonian didn't need any correction. It has been sitting in my closet ever since. (The Paracorr that is. I use the telescope all of the time.)
$195 plus $10 shipping, insurance. Will ship in the US only.