MAPUG-Astronomy Topical Archive Logo

ETX Modifications

MAPUG-Astronomy Topical Archive     AstroDesigns


Subject: ETX- Simple Drive Mods

From: Michael Hart, Date: Nov., 1998


The ETX is an extremely popular small aperture scope. Any why not? For $595 US you get an all aluminum optical tube (collimated from the factory at the mirror cell), full thickness primary mirror, battery powered drive and flip mirror. This scope is extremely portable. I have looked through several and all had great optics. All showed a nice airy disc when in-focus with at least two diffraction rings- even under not so spectacular seeing.


To check the ETX RA drive rate accuracy, I mounted an ETX equatorially performing a 20 minute drift alignment to minimize RA tracking errors. The drive tracked accurately enough to keep the subject star in the eyepiece at high power. Those using typical rough equatorial alignment will likely find the scope drifts a bit more, however, the drive itself is really not bad.

Having said that, it takes several seconds for the lash to be taken up before tracking starts. At higher powers, this can cause the object to drift out of the field.


I added a simple mod at a cost of $10.50 to an ETX that provides 2X sidereal tracking and slowing of the drive. In this way, one can speed up the drive to remove lash. The speeding and slowing is quite useful for centering objects- very welcome for high power use.

Parts Needed- Available from Radio Shack:

275-1547 momentary pushbutton- normally open 275-1548 momentary pushbutton- normally closed 278-1224 stranded hook-up wire


  1. Remove the metal plate from the bottom of the scope using the same three screws used to replace the batteries.
  2. Attach and solder 6" of hookup wire to both terminals of one normally open and one normally closed switch.
  3. Drill two holes in the base for the switches. Install the switches.
  4. Select either one of the drive motor wires and cut it in two. Attach the wires from the 275-1548 normally closed switch to the motor wires. Solder and tape the wires.
  5. Find the resistor on the circuit board. Those with little electronic experience will recognize it as a small component with 4 colored bands on it. There is only one resistor. Solder the wires of the 275-1547 normally open pushbutton to each end of the resistor.
  6. Install the metal plate back onto the bottom- that's it!


Now, when you want to speed up the drive to remove lash or for centering, push the 275-1547 normally open switch. If you overshoot, merely push the 275-1548 normally closed switch which disconnects the motor allowing the object of interest to drift back.


MAPUG-Astronomy Topical Archive   AstroDesigns   Top