Telescope Reviews: Re: Arc seconds per step for high end mounts.

Jim,

Can you provide a little bit of information on what you plan to build? Is it going to be DC perm-mag motor based or micro-stepping stepper based? Also, what type of controller (intelligent handbox) and protocol you plan to use?

The parameter "ticks per arc-second" is the resolver--gear train precision. For Autostar, a good range is somewhere between 2 and 6. (I would suggest 3.5, this is about 0.3 arc-sec/tick). It affects the GOTO precision and fine-tune in autoguiding.
Larger number means better precision with the trade-off that either the motor has to spin very fast during fast-slewing GOTO (coffee grinder) or a high-tick count encoder + higher-performance CPU has to be used (which is not the case for Autostar).

Use LXD75 as an exapmle:
- a DC perm-mag motor is used
- 50:1 mechanical gear box
- 37:35 further mechanical gear down
(above two mechanical net gear down is 52.857:1)
- 144:1 worm in mount's RA axis
- 108 slot encoder wheel, using quadrature to yield 432 ticks/rev

So its tick/arc-sec parameter is

432 * 50 * (37/35) * 144 / (360*60*60) = 2.537
(which is about 0.4 arc-sec/tick)

However, it may not affect the motor tracking smoothness, if it is DC perm-mag based. The servo PID loop determines that. Speed change is continuous. The ticks/arc-second value is just for math.

For microstepping stepper based, if it is the open-loop type (like EQ-G), the pseudo-ticks/arc-second value indeed indirectly reflects the finest microstepping size, e.g., 64 microsteps/step. The smoothness during tracking is partially governed by that. I.e., a 64-micropsteps/step has the potential better than a 16 microcteps/step stepper system, providing that the driving waveform has been tuned with the stepper motor's own characteristics to form a better final sinusoidal (instead of jittery stepping) shaft movement.

{Note: Speed adjustment in this case is not dependent on that pseudo-tick/arc-sec parameter either. It is done by internal timing loop.}

Use EQ6-Synscan as an example,
- a 200 steps/rev stepper motor is used
- later version has 64 microsteps/step (electronic gearing)
- 47:12 mechanical gear down
(the above mechanical net gear down is just 3.9167:1)
- 180:1 worm in mount's RA axis

So its pseudo-tick/arc-sec parameter is

200 * 64 * (47/12) * 180 / (360*60*60) = 6.963
(which is about 0.1436 arc-sec/pseudo-tick)

Clear Skies!

ccs_hello

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