Alstar 2-Inch Telescope Eyepiece Extension Tube Adapter provide extra focuser back travel sometimes needed to achieve focus with certain eyepiece or telescope combinations. More back travel is often needed for short-range terrestrial viewing or with use of a field flattener, coma corrector, or focal reducer lens. Extension tube provides extra focuser "back travel" often needed for short-range terrestrial viewing through a telescope, or for astrophotographic applications. Fits in 2" telescope focusers, and accepts 2" eyepieces or accessories. Features two locking thumbscrews to secure any inserted astronomy accessory, with a brass compression insh to prevent the marring of your telescope accessories. Machined of high quality aluminum and anodized black, durable for long time use.