[JA]

Originally Posted by Jasp05

Hi guys,

Been researching on making Barn door trackers and people always mention "Tracking Error".

So a basic single arm barn door with curved rod is apparently only good for 15-20min before the tracking error is quite bad.

Hi J,

I suspect a lot of people wouldn't recommend more than a few minutes even on a commercial tracking mount, without auto-guiding. So I would suggest 15-20 minutes is quite good if you can get it from a barn door tracker, even with a curved threaded rod drive. Of course the visibility of the tracking error in terms of star trailing will be less evident with shorter focal lengths, so maybe that is where some of that generosity with the 15-20 minute exposure time comes from.

The tracking error becomes worse as the exposure time increases, since there are a few time dependent errors potentially at work, namely:

1. The Barn door tracker is not accurately set or manufactured to the true length of sidereal day as it was not set correctly or accurately enough in the first place or the variation in the length of the sidereal day (although tiny) is such that it no longer is close enough to that assumed in the design of the mount. Also possible is that the gear ratios are close enough, but not quite good enough, or similarly that the radius of curvature of the threaded rod is NQR, thread pitch NQR, hinge to drive shaft distance NQR .......

2. There is time dependent variation in the voltage supplied to the drive motor, as perhaps in the case of an unregulated power supply to a DC motor drive or slowly discharging battery.

In addition to these errors there will also be periodic errors in the drive, many of which relate to speed variation, backlash take-up ......

Originally Posted by Jasp05

Does this mean you have to redo the polar alignment after 20 mins? or just reframe the shot in the camera due to it "drifting" through the frame.

No, you shouldn't need to, unless you run out of drive rod. Just stop the exposure (or time it to a selected length) and the take another shot provided that the framing hasn't changed so drastically in your viewfinder that it can't be adjusted in post processing, either manually or say stacked automatically .

Originally Posted by Jasp05

And one last silly question. I have a barndoor tracker, and I use it with the hinge on the left hand side of the tripod. I have to use my tracker in reverse (close the doors together rather than drive them apart) to have it track. If I drive apart my star trailing gets worse..

Yes I can understand that. It relates to a slight difference in drive speed in both cases, whether pulling doors together (Left Hinge) or pushing them apart (Right Hinge). In one case: driving (pulling) the barn doors together, it appears that the barn door tracker's angular velocity more closely approximates the true siderial rate of rotation of stars around the South Celestial Pole. In the other case driving (pushing) the barn doors apart, not so much.... This is caused by a load variation between both cases which affects the drive speed slightly. Assuming of course the motor doesn't use speed regulation in the electronics.

Best
JA