Building & Using a Tracking Mount for Astrophotography
Some Further Observations and Conclusions
After using the mount for a few months, I have made some changes in its construction and use.
As noted on page 2, I had moved the location of the ball-head from the center of the top plate to its top edge. The purpose was to avoid problems with my camera’s articulating monitor not being accessible when the camera was pointed up. What I discovered after using the mount for a while is that this move just changed the pointing directing where I encountered the interference. The best solution was to mount the ball head on a short wooden post centrally located on the top plate as shown in the image below. This prevents interference with the monitor for most directions the camera is pointed.
The only down side to this solution is that the camera and ball head are located further from the hinge pin, requiring a bit more effort on the part of the motor to lift the weight. But in this position, I seldom have to remove the laser and its mount for clearance.
On page 8 I noted: “Power supply output voltage (and tracking rate) can vary as battery voltage drops as well as with temperature changes”. With that in mind, I had recommended checking the drive rate before each imaging session. I discovered that this was time-consuming and subject to error as the drive rate continued to change during the course of the evening as battery voltage and temperature dropped.
I believe that the best solution to this variation in drive rate is to determine by experiment what voltage supplied to the motor results in the correct tracking rate. If the mount is made carefully according to plan dimensions, the correct rate for the drive nut is 1 rpm. Once the correct voltage is determined, one merely monitors the voltage during the course of an imaging session, making corrections as necessary. To that end I made a set of multimeter leads that can be connected to the power supply via an extra output cord and quick disconnects. This is shown in the following image.
|Ball head mounted on wooden stalk; power supply voltage monitored by digital multimeter|
One important note on polar alignment: The north celestial pole is not exactly on the North Star (Polaris) – it is located at a point about a degree from Polaris. For best results align on that point rather than on Polaris. For short focal length lenses, Polaris may be close enough. For longer focal lengths, find the pole’s exact location from a good star chart and align there.
|Tracking mount with laser for polar alignment and Multimeter for checking supply voltage|
Overall the mount has been a joy to use. It is quick to set up and polar align. And the results are satisfying. Here’s a nice image taken with a Samyang 8mm fisheye lens at f/2.8, 2-1/2 minute exposure.
|Milky Way From Sagittarius (Lt.) to Perseus (Rt.)|
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