The extension tubes 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 barrel also has a 2" filter thread (M48x0.75) on the side of the telescope
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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.
these 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.
The 80mm extension tube is the most popular model. It will work in most telescopes. One particularly important application is to convert a photographical telescope into a visual telescope by reducing the back focus distance accordingly.
The "optical length" includes only the length of the extension tube that acually extends out of the focuser. The length of the 2" barrel of the extension tube is not included in this figure since the barrel normally disappears entirely into the focuser.
The eyepiece is clamped firmly but without scratches through a brass compression ring. The advantage is that eyepieces are held more securely and the risk of the eyepiece being tilted out of the optical axis is minimized.