Stepper motors and slewing a telescope

[jnb]

How do telescopes that are driven by stepper motors manage to slew quickly?

I have an exos2 mount which offers slew speeds of 1x, 8x and 32x. Which is what I would expect from the range of speeds out of a stepper motor. However I have seen scopes, particularly those on goto mounts which seem to offer slew speeds of up to 800x. How do they manage that? because that is beyond the range of most stepper motors.

[1parsec]

To get the speed range of 800x the motors are micro-stepped.  The electronics can position the motor in say 6 small steps between the normal full steps of the motor.

[1parsec]

That should be 16 small steps

[astromerlin]

Hi JNB, Microstepping can be done in 8,16,32,64 steps depending on the resolution you want. Torque is actually reduced in microstepping and advanced drivers use microstepping for the slow movements when you need resolution and then switch to full step at high speed.

[Gina]

Yes, I believe the SW mounts use 64x microstepping, for instance.

[Gina]

I've been looking into making my own lightweight mount using stepper motors - see this thread

Only problem with microstepping is that the torque drops off rapidly as you increase the microstepping ratio but it's frequently used - not just for mounts but other applications too such as 3D printers.  This link explains torque loss with microstepping.  With astronomy mounts, the loss of accuracy due to microstepping can be virtually completely compensated by using guiding, where the difference in real position from wanted position is calculated in software (such as PHD) and extra steps generated by sending pulses to the stepper control electronics.

[Ajohn]

I have just bought an Ioptron mount - no steppers at all.  DC motor plus rotary encoder and feedback to control the speed. Not sure what the max slew rate is but I prefer this arrangement - has any one ever looked at the angular step error that type of motor usually has? I have.  Also a Vixen GP DX with SkySenor - the noise.  That in itself has put me off but mounts do vary in that respect.

The ultimate answer to fast slewing is direct shaft encoding - just spin it around as fast as you like by hand - the cost hurts, even the encoders on their own.

Just looked the slew rate is 4.5 degrees per sec  1 min 20 secs for 360 degrees, 512x. Could be faster with more a more powerful motor but the gearing and probably more problematic the worm and wheel would have to be up to it as well.

John

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[perfrej]

My 10micron GM2000 has ac servos and slews at 20 deg/sec! Perfectly capable if sending a poorly fastened scope into orbit...

[perfrej]

And you may want to read my post on steppers a bit down this section...