Later that night, I grabbed the 350D and the tracker and headed outside for some practical tests, to see if they backed up my earlier conclusions. A full moon made setup easier, rare that I'm grateful for its presence when astro-shooting!
After a few test shots to check focus (oh Live View, let me count the ways I do miss thee) I fired off seven 2-second shots of Sirius, each approx 10 seconds apart, to give me a full cycle of the motor. A quick zoomed-in review showed some disappointing drift. It was too late to get them into Photoshop, more detailed analysis was going to have to wait.
Lunchtime Easter Friday, and what better to do while eating my Hot Cross Buns than stack last nights shots and see what we had. Into Photoshop, push contrast and black levels to remove all noise, label times on the layers, and drop into a timeline in the Photoshop 6 beta. Resulting animated GIF below :
On the up side, it looks like we're tracking fairly well overall - the stars are staying put. Clearly, there are still wobble issues however, within each turn. Quite where the issues lie will be the next challenge. Is the weight of the camera affecting the motor? Seems unlikely, as the camera doesn't move. It could simply be that the motor doesn't turn smoothly over its cycle, it's heavily geared internally and might have its own issues. So next steps :
1. Test unloaded motor, see if it turns smoothly through a 360 cycle
2. Test loaded (camera attached) tracker
3. Run some longer tests, maybe motor will warm up, and also to check overall tracking accuracy for long exposures
I need to come up with a test setup that doesn't require the stars to be out! Looks like they're gone for the next few days. Perhaps a laser attached to the rig, and projected onto a distant wall? Video it climbing the wall and see if it's climbing evenly. That might be a bit slow-motion to be practical or accurate. Worth a rack though, as it sounds like fun.
Anybody got any other ideas? Anybody reading this, or am I just thinking out loud? That would be ok, it helps me work through things, but 10,000 page views would suggest we're not alone here :)