The Bower SLY2X T-Mount Telephoto Extender is a secondary lens which is mounted between the camera and a photographic lens, doubling its focal length.
Reviewed by 1 customers
Yep, it's a 2x extender tube, and if you're using a telescope you're going to be using it with a tripod so can afford some longer exposures. Granted, it does what it says on the tin - it doubles the focal length of the telescope. However, you've got to be prepared for the increased exposure time necessary. In other words, this plus a telescope isn't going to give you the same performance as just paying the price for a full-sized lens of equivalent focal length.