Starlight Instruments Posi Drive Motor System

Lifting Capacity for Heavy Image Trains

The Posi Drive Motor System can lift in excess of 56 pounds. On the Feather Touch, the standard attachment drives the pinion shaft rather than the reduction assembly, which eliminates slip and gives precise repeatability with heavier payloads.

That makes it the Starlight motor to consider when the camera, filter wheel and other accessories behind the focuser add up to a heavy load.

The Posi Drive Timing Belt

The PDMS drives the focuser through a timing belt running between a pulley on the motor and a pulley on a shaft extension fitted to the focuser in place of the knobs. The housing is clamped to the focuser's reduction assembly, and rotating the housing tensions or slackens the belt.

• Toothed belt: the belt runs on both pulleys and is tensioned by rotating the motor housing clockwise, then locked with the thumb screw.
• Load: The system can lift in excess of 56 pounds.
• Temperature: Starlight's site says the PDMS has a thermistor built onto the motor circuit board for temperature compensation through the controller.

Seamless Manual Override for Hands-On Focusing

You can switch between motor and manual focusing without removing the motor.

To focus by hand, loosen the thumb screw and turn the motor housing counterclockwise to the stop, which slackens the timing belt. To re-engage the motor, rotate the housing clockwise to tension the belt, turning the coarse focus knob back and forth while you do so to take up any slack, then tighten the thumb screw. Starlight's site also describes the PDMS-FTFF override as letting you adjust focus with the fine focus knob without removing the motor.

Versions and Fit

The PDMS is sold in versions, each for a specific focuser. Choose the version named for your focuser; the fit is limited to the focusers listed below.

• Feather Touch: SI-PDMS-FTFF-2.5 for Feather Touch 2.5" and 3.0" focusers and the Astro-Physics 2.7" focuser; SI-PDMS-FTFF-3.5 for Feather Touch 3.5" focusers and the Astro-Physics 4.0" focuser. The FTFF is for 2.5", 3.0", true 3" and 3.5" focusers.
• Takahashi: SI-PDMS-TAK for the Takahashi stock focuser.
• Sky-Watcher: SI-PDMS-SW for the Sky-Watcher Esprit.
• Celestron: SI-PDMS-RASA36 for the Celestron RASA 36cm.
• Other versions: SI-PDMS-TEC (TEC designed focuser), SI-PDMS-WO (William Optics), SI-PDMS-SV (Stellarvue stock focuser) and SI-PDMS-AT (Astro Tech).

Installation on a Feather Touch Focuser

Starlight's PDMS-FTF manual installs the motor with the supplied tools, without removing the motor from its housing:

• Remove the dust cover, the packing material and the 8-32 button head cap screw, then take the pulley assembly and belt out of the housing.
• Remove the shaft extension from the pulley, remove the gold and black focus knobs, and install the shaft extension with its setscrew on the flat, tightened securely.
• With the clamp screw loose, fit the motor housing all the way onto the reduction assembly (you can see it through the cut out), then tighten the clamp screw just enough to stop the housing rotating. Do not over tighten.
• Loosen the thumb screw, position the pulley set screws over the flats of the shaft extension and tighten both through the access hole.
• Fit the timing belt on both pulleys, rotate the housing clockwise to tension it while turning the coarse focus knob back and forth, and tighten the thumb screw.
• Replace the 8-32 button head screw and the dust cover.

Control and Automation via Focuser Boss II and EFS

The Posi Drive Motor System is a component of a complete focusing solution and requires a controller for operation (sold separately). It runs on the Starlight Instruments Focuser Boss II or Electronic Focusing System (EFS), which power the stepper motor and connect it to your computer.

Both controllers have ASCOM drivers, so ASCOM client software can control the motor; Mac users of the Focuser Boss II can also use TheSky X and Equinox. The controllers provide temperature compensation using the thermistor on the motor's circuit board.

PDMS vs. HSM: Choosing the Right Starlight Instruments Motor

Starlight Instruments offers two motor systems for its controllers.

The Handy Stepper Motor (HSM) clamps onto the pinion assembly of specific Feather Touch focusers (HSM20, HSM30 and HSM35 models), and turning its housing disengages it for manual focus.

The Posi Drive Motor System (PDMS) is what Starlight offers for heavier image trains and non Feather Touch focusers, with a lifting capacity in excess of 56 pounds.