Old camera tele-extender lens work very well but you need to take a couple of precautions. The first is to use a lens that was designed for the focal ratio of your telescope or faster. Many of the older items available were designed for use with convertable telephoto and slower camera lenses and might not be suitable for use with an f4.5 mirror commonly found on todays Dob. The second and most important precaution is to maintain the spacing of the rear of the lense to the focal plane. Any deviation will upset the corrections of the tele-extender lens and it will not perform at its best.