UFC Telescope-Adapter S70a to RASA 11" – Optical height: 3 mm

This is correct. The UFC S70 adapter for the 11" RASA sits flush on the front lens group, making it no longer possible to install the Celestron light pollution filter. The UFC S70 adapter is intended for later installation of the UFC filter drawer. In the UFC design, the filters are installed in the filter drawer (for quick exchange), eliminating the need to screw the filters into the front lens group. When using the UFC system, all additional glass elements such as the Celestron light pollution filter or optical window must be removed prior to installation.