I had been operating with an Optec Gemini focuser/rotator on a commercial Schmidt-Cassegrain telescope. The limited range of focus of the Gemini focuser was not a problem as the SCT had a very wide range involving the movement of the primary mirror. However, when I moved to a Ritchey-Chretien scope, that wide range of focus was no longer available. I found that the adapters available from most dealers were either too long or two short and autofocus with my software failed. The problem was exacerbated when changing cameras. So, it was with a great sense of relief to find smaller extenders at Testar. I purchased both the 7 mm and 5 mm, giving me three possibilities for extending the length. Success!