German type equatorial mount


The mounting pier holds a German type equatorial mount. Main parts of this mount are the polar axis comprising a couple of radial and thruss bearings, the declination axis that is constructed around a ball bearing pillar and bush (used in plastic molding machines) and the RA-drive comprising a stepper motor and gear train with a 160mm diameter wormwheel attached to the polar axis.

The brass wormwheel is made on a lathe (the principle is depicted here): Using an M12-cutting tool in the jaw-chuck, the worm-pattern is cut into the wheel. This is done while pressing the freely rotating brass wheel against this cutting tool. The worm itself is a piece of an M12 threaded bolt which is pressed against the wormwheel and which is spring-loaded. As with every wormwheel (and especially this self constructed one...) there is a periodic error that is corrected for by using the guider telescope with 4Q-position sensor.
The worm is driven by a stepper motor (taken from an old inkjet-printer) via a gear train. The speed of the motor is controlled by a microprocessor based on the east-west error signal of the 4Q-sensor on the guiding telescope. The motor can be disengaged to position/adjust the polar axis by hand.


Images below show the mount and focus on the polar axis and RA-drive. From left to right:
  • A schematic cross section of the polar and declination axis of the mount.
  • Different parts and features of the German mount are shown and annotated in this photograph.
  • The polar axis as seen from below, i.e. from the side of the 160mm wormwheel and the RA-stepper and gear.
  • The wormwheel with mounted polar axis clamp.
  • The gear train comprising three worm/wormwheel gears: 1:32, 1:24 (not visible) and 1:288.
  • Detail of the RA-stepper, the first worm/wormwheel gear and the knob to manually adapt the RA (after disengaging the stepper motor).
  • The LED-illuminated RA setting circle, seen via a mirror on the top of the pier. 0 is due South.
Schematic drawing of the German mount
Different parts of the mount
Polar axis seen from below
The home constructed worm wheel
Detail of polar axis gear train
Detail of the stepper motor, worm and 32 teeth gear
The RA_circle is observed via a mirror

The declination axis can be adjusted by hand using a micrometer screw or using a small 12V DC-motor which is coupled to the micrometer screw through a slip-coupling. The large length of the declination correction arm enables accurate corrections to be made. The pillar of the declination bearing measures 40mm in diameter. It is relatively short and extended with a 22mm diameter rod on which the declination setting circle, the counterweights and the guider telescope are mounted.
The polar axis is 30mm in diameter and rotates on two standard ball-bearings which are mounted in a 80mm diameter steel tube.
At the position where the declination axis is supported by the polar axis, a thruss ball-bearing is constructed comprising a ring of 4mm balls around the polar axis. A small metal ring retains the balls in their circular track. Near this position the RA setting circle is located which shows the RA-deviation from the exact South orientation. To ease reading of this circle it is printed mirrored while the (LED-illuminated) scale is observed via a mirror.
The RA-stepper motor, the DC-motor for declination adjustment and the LEDs illuminating the scales, are controlled from a box attached to the west-leg of the mounting pier and containing a microcontroller and drivers. The 4Q-sensor of the guiding telescope also connects to this box. The mount controller is connected to the 'system controller' .


Images below focus on the declination axis and the mount controller. From left to right:
  • The two parts of the declination axis taken apart. On the left the bearing with pillar, axis extension and telescope mounting plate. On the right the bushing (i.e housing) of the bearing. Indicated is the location where a ring of 4mm balls, contained by a metal ring (not shown), act as the upper thruss bearing.
  • The declination setting circle. This part is clamped on the declination axis. It holds the lower thruss bearing and locks up the mechanics of the declination axis. A magnifying glass and LED-illumination helps my ageing eyes to read the scale ;-).
  • The declination clamp and declination arm (the latter: attached to the telescope mounting plate) with the micrometer screw (and two tension springs) for manual declination correction.
  • The mount controller that is connected to the 4Q-sensor of the guiding telescope and controls the RA-stepper.
  • A photo showing my new (2021) controller and the old, bulky controller (the latter, containing X-tal clock, TTL-circuits, a DC/AC converter to drive the 230VAC synchronous motor and a voltage-to-frequency convertor that was coupled to a star-guiding duo-diode).
  • Finally: The schematics of the controller.
The bushing, pillar and bearing of the declination axis
The declination circle (with magnifying glass)
Declination vernier control
The mount controller
The new mount controller and its outdated predecessor
Schematics of the mount controller