Telescope encoder or no telescope encoder? #question


I'm in the process of constructing a 30" dobsonian telescope with friction drive on azimuth (5" friction wheel against a 30" disc) and altitude (large side bearings (54") on two horizontal shafts (1.4")), using Faulhaber motors including motor encoders, gear heads and harmonic drives (50:1).

Friction drives may result in slip. Are telescope encoders (Gurley) recommended, or are there experiences with systems like these without telescope encoders? Just installing them 'in order to be sure' is an expensive experiment.

KR, Frank

Don W

Hi Frank,
Friction drives can slip, but depending on the design and the materials used, they may not slip.  Mesu Mounts are a good example of a design that seems to work well - my understanding is that they do not rely on mount encoders.

Low cost low resolution encoders like 4K to 10K tic encoders  can only detect slippage to the nearest 5.4 to 2.2 arc-minutes.  That is pretty rough.  These are the hundred dollar range encoders.   The Gurley 320K and 500K encoders are pretty good but cost over $700 (I'm not sure of current price - I bought mine 14 years ago).  They work very well.

if your design is such that a Gurley can be added later on each axis, then when you get it running under the stars, you can check for slippage by moving back and forth between a couple of targets to see if there is any slippage.  Then decide if you need the mount encoders.

Don W