Tuesday, June 03, 2014

Motorizing a Vixen Polaris Mount

I purchased another Vixen Polaris mount.. for $80 (plus another $100-odd for shipping from the US). Should never have sold the first one.. (this one, with a William-Optics Zenithstar 70ED that I also sold)

The second one arrived in sorry state; rust everywhere, no counterweight (and counterweights for the threaded shaft are impossible to find); and the HA/RA setting circle was busted.  I knew all this, of course - the mount was $80! Vixen Polaris mounts in good condition usually go for $150 to $200.

I ordered a Vixen GP2 counterweight shaft (with white locking nut) which thankfully fits perfectly.  Now I can use standard Vixen (or Celestron) counterweights.

Because I intend to use this Polaris as a travel mount or at least back in the Philippines where Polaris (the star) is actually visible, I purchased a CG-4 polar scope for it.  The CG-4 polar scope fits in the bore, but does not thread in. And it isn't tight, so I had to wrap teflon tape around the polar scope tube for a snug fit.  It works now, but is obviously non-stock.

I had a bunch of Vexta PX243 gearhead steppers (expensive, high-end steppers actually) that I had lying around from my AP600 GoTo conversion (which I have since rescinded).  I hammered together a mounting plate out of 2mm thick aluminum in order to attach the motor to the Polaris.  The motor has a 1:18 gearhead and 200 ppr, giving 3600 steps per revolution of the worm, and 2.5" per step. I don't think I'll bother to half- or microstep the motor.  It is a hybrid stepper though, I hope my Arduino L298 motor shield can drive it without undue heating.

I used the existing bolt hole in the mount for attaching the aluminum bracket. I believe it is an M5 SHCS.

Unfortunately the motor interferes with the mount at low latitudes, which means that there's a large portion of the sky just to the west of the zenith, which is unavailable (kind of like Dobson's hole).  On the other side of the pier things are better, but I intend to mount a Bourns ACE 128 encoder on that side (so I have absolute indexing of the worm position) which will also limit the mount's travel.

The motor is coupled to the worm shaft with a 5mm to 6mm shaft coupler from ebay.

My eventual goal (once the basic motor drive function is working) is to implement some form of absolute PEC (using the Bourns encoder) and then further add atmospheric refraction correction.  This will eliminate the need for RA guiding altogether (and there's no DEC guiding anyway because there's no motor drive in DEC).

Hoping for first light this Saturday.

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