This 33mm adapter allows you to install SESTO SENSO on focusers that come with 33mm diameter large shaft, like the 2,5" and 3" FeatherTouch (or other focusers with 33mm diameter large shaft). SESTO SENSO as standard connects to focusers that come with 25mm diameter shaft (those are most of the focusers on the market): in order to connect it to focusers that comes with 33mm diameter large shaft, you need to add this adapter.