This 10 mm extension tube fits between the focuser and the main tube on the F50G Guidescope. Using this extension tube, the guidescope may be used with our E3023R reticle eyepiece, converting the guidescope into a finderscope. This item may also be useful if your guidescope needs more outward focus travel. Different guide scopes have different back focus requirements. We machined this part to help optimize our guidescope for different cameras.