PrimaLuceLab SESTO SENSO 3 SC Robotic Focusing Motor

Robotic Focusing for SC, EdgeHD, and RASA Telescopes

The SESTO SENSO 3 SC is specifically designed to automate the internal focusing mechanisms of Schmidt-Cassegrain, EdgeHD, and RASA telescopes. These telescopes focus by moving the primary mirror.

With the included adapters, compatible telescopes include (not limited to these):

• Celestron Schmidt-Cassegrain: Fits models from 6" to 14".
• Celestron EdgeHD: Compatible with 8", 11", and 14" models. Note: It is explicitly not compatible with the EdgeHD 9.25".
• Celestron RASA: Fits models from 8" to 14".
• Sky-Watcher: Compatible with the MAK180PRO.

Direct Pulley Clamp (DPC): Backlash-Free Connection with Manual Override

The SESTO SENSO 3 SC uses the Direct Pulley Clamp (DPC), a pulley-driven coupling designed for telescopes with internal mirror focusing. You remove the telescope's rubber focus knob and the motor locks onto the focuser shaft. The coupling is backlash-free.

The DPC also lets you focus by hand at any time with the manual focus knob on the motor, while the integrated encoder keeps track of the focuser's position, even after manual adjustments.

2 Million Steps of Precision: The Integrated Position Encoder

The SESTO SENSO 3 SC has a high-resolution motor with 2 million steps for precise control of internal mirror focusers, which ensures control regardless of the telescope's mechanical reduction.

The integrated position encoder continuously monitors the motor shaft, even when you turn the knob on the DPC by hand, so the focus position stays known whether you switch between visual and imaging setups or make manual adjustments. The encoder tracks motor shaft rotation only: it cannot detect focus shifts caused by mirror flop or internal mechanical play.

Automatic Calibration: A Critical Feature for Internal Focusers

The SESTO SENSO 3 SC has an Automatic Calibration mode, which is not available in other SESTO SENSO 3 models. Working with the encoder, it finds the minimum position, then the maximum, then moves to the middle position. Unlock any primary mirror locking knobs first (EdgeHD, for example). If the motor turns but the focuser knob does not, or excessive friction stops the motor from reversing at the end of travel, use Manual calibration: turn the knob by hand to each end of travel, without forcing at the mechanical stop, and store the zero and maximum positions.

This lets remote observatories calibrate the focuser without manual intervention. The Automatic Position Recovery function, if enabled, restores the previous focus position when focuser movement is detected, after a delay you set from 0 to 20 seconds. It is suited to public or educational observatories.

Dual Control: USB-C, WiFi, PLAY, and ASCOM Integration

The USB-C port connects to an EAGLE computer or a Windows 10/11 PC with the included cable; the reversible plug is easy to connect in the dark. The unit also creates its own WiFi network (SESTOSENSO3 plus the serial number): connect a smartphone, tablet or computer, open 192.168.4.1 in the browser and the Virtual HandPad appears, with a slider, step buttons and nine buttons to save and recall focus positions. The Virtual HandPad stays active when the unit is connected to a computer.

PLAY, PrimaLuceLab's astrophotography software, controls the focus position, including AutoFocus, focus presets and firmware updates. The ASCOM driver (tested with ASCOM platform 7) lets third-party software control it; the manual shows Sequence Generator Pro, N.I.N.A. and MaxIm DL. Only one program can be connected at a time.

Automatic Temperature Compensation: From Internal and Optional External Sensors

As the temperature changes, a telescope's metal parts expand or contract and focus drifts. The SESTO SENSO 3 SC has an internal sensor that reads the motor temperature and a port for the optional external temperature probe, which focus temperature compensation requires. This matters most with fast optics (f/3 or faster), where focus can drift within a single exposure.

The calibration runs in PLAY: record at least 3 points at perfect focus with a temperature difference of 5°C, and PLAY works out the temperature coefficient (steps per degree Celsius). Once enabled, compensation applies with PLAY and with third-party software through the PLL ASCOM Focuser driver. The manual notes it may be less accurate far above or below the calibration temperatures.

12V Power Requirement: Why USB Alone is Not Enough

The SESTO SENSO 3 SC requires an external 12V supply; USB alone is not sufficient. Maximum consumption is 800mA at 12V, through a 5.5/2.5mm center-positive port. Use the included cigarette-lighter plug cable with a 12V battery, a 12V AC adapter with at least 1A, or the EAGLE power cable to power it through the EAGLE.

A blinking power LED on the unit indicates that it is receiving USB communication but lacks the required 12V power or has not yet been calibrated. Always connect 12V power before connecting the USB cable to your computer.

Seamless EAGLE Integration: GHOST and DARK Mode

With a PrimaLuceLab EAGLE computer: GHOST mode allows the EAGLE to control the focuser without a physical USB cable connected, simplifying your setup. This mode also acts as a fail-safe; if the USB connection is lost, the EAGLE can maintain control, preventing disruption to your imaging sequence.

DARK mode dims or turns off the status LEDs, through PLAY or the ASCOM driver. When used with an EAGLE and PLAY software, DARK mode can be triggered automatically at the start of an imaging sequence, creating a fully automated, light-controlled environment.

Installation on Your Telescope: Adapters and Mounting

Position the telescope horizontally first, so the primary mirror does not shift. Pull off the rubber focus knob, loosen the screws of the retaining ring around the focuser shaft and remove the ring. Then fit the matching SESTO SENSO 3 SC adapter with the same screws; it replaces the retaining ring.

Center the focuser shaft in the adapter and tighten the three screws lightly, in sequence and in several passes, to avoid stressing the internal focuser bearing: tight enough to remove lateral movement, not so tight that they bind the focuser. On most 6" to 9.25" models, insert the provided shaft adapter into the rear of the motor. Push the motor onto the shaft, turn its manual knob to line up the locking screw, and tighten it with the Allen key. Rotate the motor body to suit your cables, then tighten the two screws that hold it to the adapter.

• Small Adapter: For Celestron SC (6” to 9.25”), EdgeHD (8"), RASA (8”), and SkyWatcher MAK180.
• Large Adapter: For Celestron SC, EdgeHD, or RASA models in the 11” or 14” size.

Care, Limits, and Safe Operation

Do not disassemble the unit, drop it, expose it to rain or use it below -20°C or above +60°C. Focusing precision depends on both the motor and the mechanical quality of your telescope's internal focuser; the encoder cannot detect mirror flop or internal play. If your focuser shows significant backlash, set the backlash value in the software. If the step position no longer returns to the same focus, lower the Acceleration, Run and Deceleration values in the advanced settings.

Before powering on, turn the focuser knob by hand. It is normal for it to feel stiffer than before, but the motion should stay uniform; if it is irregular, repeat the installation or check the focuser's mechanical condition.

What Sets the SC Model Apart

The SESTO SENSO 3 SC differs from the other SESTO SENSO 3 models in its Direct Pulley Clamp, which mounts in place of the telescope's knob, and in its Automatic Calibration mode.

Together with the encoder, these let you calibrate the focuser remotely and move it by hand without losing motor position.