Motorised focuser for the Meade 5000 80mm APO

[themos]

I've just finished a focuser project so I thought I would share some of my experience. I already had another stepper-based focuser for a rack and pinion Newtonian focuser and I wanted to reuse the control electronics so that meant I had to use a unipolar stepper motor. I found one in an old scanner. I measured the resistance in the coils to be 32 Ohms. I took a guess at the working voltage, needing the current to be well below the limit of the Darlington ULN2003 chip that my existing controller is using, 500mA. At the moment I'm driving the stepper at 7.5V and that seems to work well without missteps. I could raise it to 9V or more if I have to.

Then I worked out the gearing ratio of the integrated gearbox that came with the stepper. The output of the stepper is on a 7-tooth gear that drives 56-tooth gear (1:8) and that in turn drives a bigger gear giving a total of (1:20) reduction. The stepper itself does 7.5 degrees per full-step or 48 full-steps per rev which becomes 960 full-steps per rev at the output gear. The output was equipped with a 20-tooth timing belt pulley. I figured out that the pitch of the timing belt would be 0.080-inch and the width 0.125-inch - this is known in the trade as an MXL012 timing belt. I needed to buy myself a timing belt of the right length and a timing belt pulley to fit on the Crayford shaft (which was 4mm diameter). I settled on a 60-tooth pulley and a 8-inch length timing belt after consulting some online timing belt calculators. I couldn't find a timing belt of the right width so I bought one twice as wide and just cut it in half (giving me a spare one!). The 60-tooth pulley needed an additional insert as its bore was 5mm, luckily this was available, too.

http://www.motionco....c-25_35_48.html

http://www.motionco....c-25_38_27.html

The Crayford shaft I measured at around 4.23mm diameter, giving a perimeter of 13.3mm. The timing belt brings another 1:3 reduction so the Crayford would be turning at 2880 steps per rev. Each full-step would then be 4.6 microns and a half-step would be 2.3 microns. I judged this to be on the small side as the Critical Focus Zone for this f/6 refractor is 20 microns. But better too small than too big!

After I put it together I measured the average (half)step to be 2.169 microns, so not too far from my calculations. One problem is that with the total focuser travel being about 109mm and my stepper controller delivering about 64 steps per second, it takes about 13 minutes (more than 50,000 steps) to go from one end to the other! Luckily, I am not going to be doing that often as adjusting the focus is only a matter of sampling a few hundred steps either side of the focus. My controller allows me to cut the voltage to the coils so that the focuser can be turned by hand easily, if needed.

The trickiest part, I find, is the construction of a bracket to hold the gearmotor at the correct position. The pics show the current arrangement which has been tested and found ok. I used existing threads in the focuser assembly to attach a metal plate from a desktop pc expansion port cover, bent to the right shape. I added a brass wire hook that can add some tension to the timing belt and this may prove robust enough (I was inspired by my recent sailing adventures!).

[Gina]

Nice job

[themos]

Thanks, I consider this Mk1, I probably should strengthen the bracket by replacing the brass wire with a metal piece for Mk2.

[ncjunk]

Nice setup, your mark1 is better than mine which is a bent bit of metal which holds the stepper to the telescope body.

Some good pics as well.

Sent from my GT-P5110 using Tapatalk 2

[themos]

It's hard to get those right, isn't it?