Many Schmidt-Cassegrain telescope owners contact us asking whether the SESTO SENSO 3 SC robotic focusing motor is compatible with their optical tube. SESTO SENSO 3 SC is the focusing motor for SC type telescopes with internal focuser we specifically developed for Celestron and SkyWatcher models to provide a precise and reliable motorized solution. Compatibility depends on two essential factors: the diameter and shape of the he internal shaft of the focusing mechanism, and the rear-cell geometry, which must allow correct installation of the adapters included in the SESTO SENSO 3 SC box. In this article, you will learn exactly how to verify both aspects. We explain how to measure diameter of the internal shaft of the focusing mechanism (21 mm or 12 mm), and which optical tubes the system is designed for.
Note: this article applies only to the “SESTO SENSO 3 SC robotic focusing motor for SC type telescopes”. If your telescope comes with an external focuser – such as the one commonly found on refractors or Newtonian telescopes, then you need to use the SESTO SENSO 3 standard version. That model is built for external focusers and follows a different installation process, so the information you find below does not apply to it.
SESTO SENSO 3 SC is designed for internal-focusing telescopes by Celestron and SkyWatcher, whose mechanical design matches both the Direct Pulley Clamp and the rear-cell adapters included in the box. These telescopes use an internal shaft of the focusing mechanism with a precise diameter and have a rear cell that provides the correct mounting points for SESTO SENSO 3 SC. The system is fully compatible with the following telescopes:
- Celestron Schmidt-Cassegrain 6” to 14”
- Celestron EdgeHD Aplanatic SC 8” to 14” (except EdgeHD 9.25″)
- Celestron RASA 8” to 14”
- SkyWatcher MAK180PRO
A crucial aspect of compatibility is the diameter of internal shaft of the focusing mechanism (you can expose the focusing mechanism shaft by pulling off the rubber focus knob from the telescope’s focuser). Celestron and SkyWatcher internal-focus telescopes use two standard sizes, and SESTO SENSO 3 SC has been engineered specifically for them. Larger SC and EdgeHD models (11” and 14”), as well as RASA 11 and 14, usually come with 21 mm focusing mechanism shaft and SESTO SENSO 3 SC connects directly to this knob size without additional adapters. Smaller models such as 6”, 8” and 9.25” SCs, EdgeHD 8”, RASA 8”, and the SkyWatcher MAK180PRO usually come with 12 mm focusing mechanism shaft and for this diameter, SESTO SENSO 3 SC includes a dedicated 12 mm adapter that ensures a precise mechanical interface. Both configurations allow the Direct Pulley Clamp to engage firmly. To verify compatibility, simply measure the external diameter of the focusing mechanism shaft using a caliper. If it’s either 21 mm or 12 mm, and the telescope belongs to the compatible families listed above, SESTO SENSO 3 SC will fit your setup.
In addition to the focusing knob diameter, the second element that determines compatibility is the rear-cell geometry of the telescope. SESTO SENSO 3 SC must be firmly attached to the back of the optical tube, and for this reason each SESTO SENSO 3 SC includes a set of dedicated rear-cell adapters designed specifically for Celestron and SkyWatcher SC type telescopes. These adapters match the bolt pattern of the compatible rear cells, ensuring that the focusing motor can be installed and secured. Since every telescope brand uses its own mechanical layout, rear-cell adapters must be precisely engineered for the specific optical tubes they are intended to support. For this reason, the adapters included in the SESTO SENSO 3 SC box fit only Celestron and SkyWatcher models listed above, and cannot be installed on rear cells with different bolt patterns or dimensions.
Within the EdgeHD family, the EdgeHD 9.25” is the only optical tube that cannot use SESTO SENSO 3 SC. This model features a larger rear threaded ring and reduced space around the focusing knob. As a result, the focusing motor cannot be connected to this telescope. For the EdgeHD 9.25”, the optimal solution is the ESATTO 2” robotic focuser, which offers full compatibility and great focusing performance.