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More testing, more refinements, more ideas…

 
05 Dec

At the moment, I’m still testing the tracking system I’ve built; and whilst it’s still not perfect, not by a long way, I’m slowly closing down things that need to be worked up on.

After all, it’s a prototype, and when I’ve got it tweaked up and working to how I want it to get close to then it’s going to be rebuilt from the ground up on a clean sheet with all the things I’ve learned with it.

In the previous bit about it here, I mentioned that I’d noticed some eccentric rotation on not only the hinge centre but the drive shaft. Needless to say, the drive shaft was quite an easy fix; I fixed a screw close to the tripod mount and looped an elastic band from there round the back of the guide rails and then back on to the screw. It just pulls enough pressure on things to hold the drive in the nut a lot tighter. Now that’s marked off the list, probably I’ll build a tensioner with a spring, as it’s a bit of an elastoplast at the moment and I don’t like using elastic bands because they always seem to fail at the most critical moments.

The other thing I’m thinking about is to counter overshoot, that bit where the camera and lens have moved the centre of gravity over a point the actual gravity takes over and pulls things open. A lot of solutions say for an elastic band there as well, but that seems a bit contradictory to me as it’s pulling the top board back in to itself and as the angle of board increases the more pull it’s going to exert on the band and that will either fail or it’s just going to waste energy counteracting the pull with the drive.

I’ve still not got a new hinge on it as before I do that, I’ve got to pull the nasty one I’ve got on there and then fill the holes with epoxy putty and let that cure. No point in having holes where I don’t need them, as it’ll only weaken the structure, but as I’m not in a rush to get out there at the moment with the curfew and also the full moon and forecast for clouds over the next few evenings, I’d only have (at best, assuming the clouds stayed off) about an hour or maybe two to get up the fields before having to be back in doors. Also, the other down side is that the electronic unit still is a bit of a mess; I don’t think I’d trust it further than the other side of the balcony door to be honest. I’m still not too convinced, even though I’ve potted them down that the dupont connectors would stay put.

On the electronics side of things, though, I have been busy with the code for getting the right amount of rotations per minute; from testing the doublesteps the other day, I’ve got that down to 2037.8, and with that (even with the slightly sloppy hinge), I’m down to 15.033 degrees after an hour of running and even without correct alignment (just compass, which probably isn’t 100% as there’s a lot of metalwork in this) and an inclinometer.

Even with that, though, just a random bit of sky with a 70mm for five minutes looks like this.

And 100% view…

That was all good until… Last night I took it out on to the balcony after I’d changed a couple of things; one was actually making proper plates on the end of the drive guides and the other was building up a more robust (albeit not pretty) control package for it. First thing I noticed, with a 200mm, and remembering I’d got a minute out of a 200mm the other day, was sine-wave like stars or periodic error at one minute; so I let it run for a bit to make sure things were tightening and it got worse.

After swapping on to a 70mm, which I’ve had 5 minutes out of, re-aligning and going with that, I couldn’t get more than 1 minute out of it (at best) with noticeable trails but the periodic wave had gone. So I started stressing out a bit as to why something that did work now didn’t; so just as I was about to get the tools out and reset it back to the day previous, work called… Full moon, curfew, Halloween, first day back proper after the leg getting funky…

After a few hours of intense oddness, I was back but the moon was too far round to make anything out, so after ruminating on the adjustments some more, the only thing I can thing of was with changing the way the guide bars inserted had make the carrier loose and the motor was knocking. Sure enough, after pulling the plated bars out, dropping in the ones I had previously and pulling them together over the bottom board with an elastic band the bars at the motor end splayed out a couple of mm and the motor was now locked properly again.

The alignment, that got me; so in the office, I was checking the alignment of the bottom board (41º, as expected), down the hinge (41º as expected) but closed, the hinge was slopping. Now this hinge was the better of the two I’d found in the basement, looks like a bearing hinge, when new felt like a bearing hinge, but just a cheap bit of pot metal with a bit of pig iron packed full of axle grease; and now the grease has shifted with running it up and down a few times (did a lot of that in testing for the correct doublestep ratio) it started pulling to about 2 degrees off (so flopping between 41º and 43º and between 0 and 2º North). I’ve not got time in this to compensate for a bad hinge, so by the time it would have taken me to make something to fix that, I was straight on to the suppliers website and ordered some heavy duty, flush fitting, bearing hinges so we shall see how that goes.

Which leaves me on the list with two more new ideas… Something to space the rods tighter under the motor and rattling something up that sits between the collar of the tripod head and the base of the head plate which, in lieu of buying a proper polar wedge, would just make sliding the tripod head in to degree a bit easier.

Ah well, one step forward makes two more steps to the horizon, eh?

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