An optional part of their modular Astro T-2 system, the Alpine Astronomical Baader T-2 15mm Extension Tube can be used throughout the system to add the proper spacing between different components. The extension has internal T-2 threads on one end with external T-2 threads on the other for easy use. You can use the extension tubes anywhere you need to precisely space and couple any of the T-2 components, in configurations of your own design.