Re: brand new to this stuff, need starting advice

Mark Christensen

Issues like periodic and non-periodic error are why you either:

0. Clean and adjust the mount
1. Use PEC
2. or Guide

Cleaning/Adjusting/Guiding is the simplest and most robust solution, in my opinion, with guiding being essential as it compensates for all kinds of errors and effects outside of the mount. Even with focal lengths like 300mm.

But not for planetary imaging. For that the TMC chips with 256 point interpolation (between the microstep pulses) are better than the others - it will give you smoother short term (like a video frame rates) motion.

Your CG-5 has, I believe, a 10 minute RA worm period. That means one worm rotation is 600*15 =9000 arc seconds (at DEC=0). So even 32 microsteps (without interpolation) would result in (with direct coupling) 9000/(32*400) = 0.7 " per microstep, which you will (believe me) see in planetary video. And if you have to run at 16 microsteps per step it is 1.4" per microstep without interpolation. But the interpolation will give you 8X improvement, or 0.088" per increment, which you will never see.

Finally, if you must, must use 1/4" bore pulleys (aka 6.35mm), then you can get bushings. But as Khalid pointed out, there are plenty of 6mm pulleys out there.

Pulleys have an advantage (esp for the CG-5 type mounts) because you can then tuck the motor under the axis, whereas direct coupling means it will be sticking out and may unnecessarily limit motion. Using a reduction will give smoother short term motion (which is what you care about for video) no matter what you do. It will, of course, then limit your max slew speed and you have to run the numbers to decide how much you care.

It sounds like your intended use has little to do with GOTO, focusers, Bluetooth, and all that, by the way. If you haven't done it you might want to clean and adjust your mount - I've never seen a CG-5/LDX-75 class mount whose tracking and response to guiding didn't benefit from doing that. There are several websites with How-To info on that (Google "AstronomyBoy", for example).

Mark Christensen

On Sun, 1/7/18, Khalid Baheyeldin <kbahey@...> wrote:

Subject: Re: [onstep] brand new to this stuff, need starting advice
Date: Sunday, January 7, 2018, 6:41 PM

You can get pulleys in
6mm bore.

The reprap stuff
for 3D printers made this stuff readily available on


Search onĀ  "GT2

The issue is
that with any transfer mechanism, be they gears or pulleys,
you can introduce non-periodic error.

See the small pulley in this video at 5:50

That wobble will introduce
elongated stars.

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