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 rings bridge the gap between the focuser and the focal plane slightly (between 5mm to 30mm). They are the ideal solution whenever the focus lies further outside than you can reach with the focuser. For telescopes that accept 2" accessories, GSO 2" extension tubes come in 3 lengths: 35mm, 50mm, 80mm. Choose the one with the back focus that you need. If you need even longer back travel, please look at GSO extension tubes ranging from 35mm to 80mm (also available at Thai Excite).

SPECIFICATIONS

Threaded for standard 2" filters;
5mm version: Total length = 8.5mm, Extension provided is 5mm
10mm version: Total length = 13mm, Extension provided is 10mm
20mm version: Total length = 23mm, Extension provided is 20mm
30mm version: Total length = 33mm, Extension provided is 30mm

The extension rings are machined from solid aluminium. They are black anodized. The 2" barrel has a safety undercut to prevent the extension tube from dropping out of the focuser accidentally. Of course the ring also has a 2" filter thread (M48x0.75) on the side of the telescope.