Meade 35mm verlengbuis

Different telescopes can have very different back focus distances. In most telescopes the focal plane is deliberately moved further out, allowing users to attach cameras and other accessories more easily.
Refractors always have a rather long back focus since they are usually used with a star diagonal or erecting prism.

The Problem:
As a result of this you often cannot directly focus the telescope if you do not use it with the above mentioned accessories. You would have to move the focuser further out, but you reach the end of the focus travel before reaching a focused image.

The Solution:
TS Optics extension tubes bridge the gap between the focuser and the focal plane. They are the ideal solution whenever the focus lies further outside than you can reach with the focuser.