Do your eyepieces or cameras get close to (but not quite at) the correct focus with your focuser racked all the way out? If so, an extension tube will help.
Slips into 2 inch focusers or diagonals, providing a 50mm extension and allowing eyepieces to come to focus. These sorts of focusing problems are most common in telescopes configured for photography.
Bintel extension tubes are threaded on the bottom to take 2″ filters – useful if you want to use 1.25″ eyepieces with 2″ filters. They additionally feature brass compression rings to prevent eyepiece barrel damage.
An extension tube can also be used for increasing image scale when doing projection photography.