STI Autoguider Focusers are available for the SBIG ST-4 dedicated autoguider and for the STV model. These autoguider focusers effectively use a 180 line/inch ronchi grating for focusing. Please choose your model on the pulldown menu. If you already own a Series IV focuser, you may easily convert any of our autoguider focusers into a regular modular mount with the addition of a converter ring listed below.