SESTO SENSO 3 for external focusers is designed to work with the focusing systems typically found on refractors and Newtonians, including Crayford focusers, Rack & Pinion focusers, and similar mechanical designs. So, if you want to know whether SESTO SENSO 3 for external focusers is compatible with your telescope, the first and most important requirement is that your focuser must include a fine-focus mechanism. If your telescope has only a single direct-drive knob without a fine-focus reduction, SESTO SENSO 3 for external focusers cannot be installed. Once this requirement is met, the next step is to check the diameter of the pinion shaft, which you expose by removing the fine and coarse focus knobs. Let’s look at the procedure in detail.
Note: this article applies only to the “SESTO SENSO 3 focusing motor for external focusers”. If your telescope focuses by moving the primary mirror internally—such as Schmidt-Cassegrain telescopes, Celestron EdgeHD models or Maksutov systems, then you need to use the SESTO SENSO 3 SC version. That model is built for internal focusing mechanisms and follows a different installation process, so the information you find below does not apply to it.
Once you confirm that your focuser includes a fine-focus knob, the next step is to look at the pinion shaft diameter. This is the most important factor in determining compatibility. SESTO SENSO 3 uses a mechanical coupling system called the Self Centering Clamp 2 (SCC2). This system automatically centers and clamps onto the focuser shaft without requiring any external adapter plate. Thanks to this design, SESTO SENSO 3 directly supports four different shaft diameters: 25 mm, 26 mm, 33 mm, and 37 mm. These dimensions cover the vast majority of external focusers used today.
To check your focuser, simply remove the coarse and fine focus knobs and measure the diameter of the exposed shaft using a caliper. If your measurement corresponds to one of the supported diameters, then SESTO SENSO 3 for external focusers is compatible with your telescope. After confirming the measurement, installation is straightforward. Inside the box you will find six color-keyed bushings and the matching internal adapter for your shaft size. By inserting the correct color-keyed bushing and shaft adapter, you can slide SESTO SENSO 3 onto the focuser shaft. The Self Centering Clamp 2 automatically aligns and allows you to clamp the system securely, without the need for any external adapter plates.