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. 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: GSO 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. For telescopes that accept 1.25" accessories, GSO 1.25" extension tube increases the distance of an accessory back by 25mm to help achieve focus.

For Varying the Reduction Factor of a Focal Reducer

By using additional spacers or extension tubes with an existing focal reducer, you can vary the amount of reduction you obtain with that reducer. For example, for GSO 1.25" 0.5x Focal Reducer, the reduction provided is as follows:

The GSO 1.25" 25mm Extension Tube therefore helps turn the 0.5X reducer into a 0.25x reducer!