This 37mm adapter allow you to install SESTO SENSO 2 on focusers that come with 37mm diameter large shaft, like the 3,5" FeatherTouch and FSQ-130 (or other focusers with 37mm diameter large shaft). SESTO SENSO 2 as standard connects to focusers that comes with 25mm diameter shaft (that are the most of the focusers on the market): in order to connect it to focusers that comes with 37mm diameter large shaft, you need to add this adapter.