In order to prevent the ARCO 3" rotator to touch the internal field flattener of FSQ-106 telescope, we designed this special version of adapter that allows you to connect the ARCO 3" rotator to ESATTO 4" focuser, when installed on the FSQ-106. This adapter optical thickness is 10,5mm so, by adding it to ARCO 3", rotator optical thickness becomes 33,5mm....