DIY barn door star tracker

Started 10 months ago | Discussions thread
Alen K
Alen K Senior Member • Posts: 1,333
Re: DIY barn door star tracker

RustierOne wrote:

Alen K wrote:

RustierOne wrote:

You might benefit from the information in this thread:

DIY Barn-door Tracker

Nice description. You might want to update the link in the article to Gary’s webpage describing his tracker since your link doesn’t take you there directly. This one does.

One point: You say voltage variation from the battery causes the drive rate to change. True, but preventing that is the whole reason for the voltage-regulator circuit. It will maintain the same voltage to the motor until the battery voltage drops below, in the case of the LM317, about 1.5 volts higher than the regulator’s output voltage at 25C and 20mA output (but should use 2V or more to be on the safe side). See https://i.stack.imgur.com/ICFoT.png from the original National Semiconductor datasheet. You are using a 3VDC motor and a 9V battery, so you have quite a lot of headroom for the battery to discharge before the regulator stops functioning, even in cold weather. If you are experiencing premature voltage dropout, check your circuit.

My own tracker (I posted the same link in an earlier reply to the OP) uses a 12VDC motor, just because it was what I happened to have around when I built the tracker in 1998. (Note: Nine years before Gary wrote his article. How did I do it? I jumped in a time machine and came to what was then the future, of course.) Hence, I ended up using two 9V batteries in series in front of the regulator circuit. Note that my circuit has the same amount of headroom as yours. Yet I never experienced voltage dropout even after several nights of operation. The battery drain does depend on the motor’s current consumption, of course, but these heavily-geared DC motors draw very little current, even under load (by virtue of the gear ratio).

Temperature variation does remain a problem. But it’s the regulator circuit itself that is the main culprit, and there are techniques (extra circuitry) for temperature compensation. But I never felt the need. I would argue that it is not really needed when using focal lengths most appropriate for such a tracker, say no more than 135mm, and sub-exposures no more than, say, a minute. Back in the bad old days of film, we needed to use 10-minute exposures minimum.

Thanks, Alen K. I don't check into this forum very often. But I appreciate your remarks. Is it possible the voltage could be dropping due to the resistors in the VR circuit changing value as temperature drops? In any case, it isn't much of a problem, since I don't use the tracker much of late. Last time was for Comet Neowise last summer. This was just 30-seconds at 135mm f/2.5, crop-sensor.

Nice image in only 30 seconds!

The resistance of the resistors will certainly change with temperature, especially in the case of a typical ten-turn potentiometer. But I would not expect quite that much change. If you used carbon-composition or carbon-film resistors, change them to metal-film resistors: much lower TC (temperature coefficient). Another improvement might be to use a multi-turn potentiometer with a low TC, like this one from Vishay.

Yet another improvement would be to use the potentiometer in a voltage-divide configuration rather than simply as a variable resistor. Both resistive legs of the voltage divider have the same TC, which should cause less drift.

It is also possible that the regulator is not as good as it should be. Everyone makes LM317's (it's a very old design) but not all manufacturers make them as well as the original National Semiconductor parts (speaking here of manufacturing tolerances). If you have other samples, try swapping. You might find one with a much lower drop-out voltage.

I don't use my barn-door tracker at all nowadays. It has been completely supplanted by the Astrotracer feature in my Pentax K-3II. That came in handy for Neowise (ten-minute total exposure, 30 x 20s).

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