While such photos can be aesthetically appealing and beautiful, they are not very useful if you want to look at the night sky in detail. The problem is this: In order to get sharp clear images of faint stars or other objects in the night sky, we must counteract the rotation of the planet by rotating the camera in the opposite direction.
The classic Haig mount consists of two boards, hinged on one side, with a threaded bolt going up through the lower board and just touching the upper board on the other side. See Phil Harrington's web site for diagrams and plans for a traditional Haig mount. As noted on Phil's web site, the classic Haig mount is the cheapest possible star tracking device that you can build. However, this simple design suffers from some rather large problems.
The biggest problem with the barn door mount is now accuracy. Assuming your simple clock driven Haig mount is constructed well and aligned properly, it will accurately track the starts for about a minute or so. After that, the stars will start to drift again due to "tangent error". In order for our camera to counteract the planet's rotation, it must rotate at a constant rate of 360 degrees per sidereal day. The stepper motor on a simple Haig mount is running at a constant rate, but it is driving a tangent bolt. Thus the angle between the top and bottom boards is changing at a varying rate. The longer the clock runs for, the more error there will be between the angle of the mount and the desired angle.
One method that people use to reduce the error is to build the mount so that the threaded bolt pivots. See Ray Grover's Quartz Controlled Scotch Mount for a good explanation of tangent error and an informative graph of error vs. time for both the traditional Haig mount and the pivoting bolt design. You will see from this graph that error is reduced but still present.
One last problem with a constant rate barn door mount is that usually the bolt is made to turn one revolution per minute. While this makes the drive mechanism simple, it also completely dictates the dimensions of the mount. Also, any errors in the construction of the mount will result in almost immediate tracking error.
I also decided that my barn door mount would use a pivoting bolt. This was for two reasons. First, I didn't like the idea of the bolt sliding along the top board, and all of the vibrations that would result. Also it seemed impractical attach a stepper motor to the bolt in such a way that the bolt could move relative to the bottom board. Second, the top board would have to be much longer than the bottom board if you wanted to track stars for any long period of time. I thought the pivoting bolt design would offer a more stable base to put a camera on, and would theoretically allow for accurate tracking periods of over six hours, if constructed properly.
The "pins" refer to the pins of the parallel port. Pins 2-5 are the first four data pins, and pin 25 is the ground. Note that pin 25 should be connected to the same ground as our main 12V power supply.
The 10,000 ohm resistors are to protect the fragile parallel port from any backlash caused by the motor, as well as to limit the current drawn from the motherboard. From what I have read, this is the bare minimum precaution you must take to avoid permanently frying your motherboard.
For the transistors I used NPN "darlington" transistors with a rating of 5 amps and 100 volts. This is slight overkill but the electronics store was almost out of darlington transistors so I didn't have much choice. You will note that all of the power needed to switch the transistors is coming from the parallel port. Luckily it doesn't take much to switch a lot of current through a darlington transistor.
The 1A diodes are to protect the fragile transistors from any backlash caused by the motor. If any current runs the wrong way through those transistors they will burn out permanently.
After getting the motor to spin properly, I went a little overboard adding stuff to my simple circuit. I added a power LED, and four LEDs to show the state of the motor windings. I also added a feedback "error" circuit and LED. The plan was to use the error line on the parallel port to tell the software when the barn door mount was closed all the way. I haven't actually hooked it up yet, but the circuit works and is there in case I ever need it.
For my threaded bolt I bought a three foot 5/16" bolt. For the top board nut I originally tried using a 5/16" hex coupler, but unfortunately I found that there was too much friction if the coupler was not precisely aligned with the bolt. It seems the stepper motor I have is a little underpowered for such a large barn door. So for now I punted and used a regular nut. I even allowed a little bit of play to reduce friction.
I mounted the electronics package on the underside of the bottom board, near the motor. As you can see in the above photo I put a DB9 connector on the motor wires so that I could easily disconnect the motor from the electronics box. I also used a simple strap to secure the box so that I could easily remove it if necessary.
The second change I made concerned the tripod. Originally I pictured my barn door mount sitting on top of my trusty Velbon tripod. Unfortunately the tripod proved much too unstable and weak to support such a large barn door mount. As soon as I put it on I could tell the tripod would never work. Using some spare plywood I had lying around I quickly put together a stand for my mount.
The stand cradles the barn door mount at a natural angle of about 40 degrees on a level surface. Two adjustment screws allow the mount to be elevated from 40 to about 50 degrees. This range of angles should cover almost all of the locations on the planet where I plan to take photos. (i.e. most of Ontario.) If I travel anywhere else I will have to build a custom stand for that location. However, given the size of this mount I doubt I will take it anywhere far away.
I don't think it's that critical to get the alignment "perfect", because it will be quite difficult to point the Telrad at true north once I am out in the field, anyway. Plus with wide field astrophotography, perfect alignment is less important.
HANDLE h = CreateFile("\\\\.\\giveio", GENERIC_READ, 0, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
if (h == INVALID_HANDLE_VALUE) {
printf("giveio not running\n");
return 1;
}
// keep the file handle open as long as you want access to the port
// start by making sure all data lines are off
_outp(0x378, 0x00);
// close it when you are done
CloseHandle(h);
As you can see from the above code, you can toggle any of the 8 data lines by setting the corresponding bits on the port.
The rest of the software is just a simple application of the cosine law.
Count the number of threads per inch on your bolt, and measure the distance from the center of the hinge to the bolt on both the top and bottom boards (these two distances should be the same if you have constructed it properly).
Remember that there are 23.9344696 hours in a sidereal day, giving you about 7.29211585 * 10^-8 radians per millisecond.
When your software is running, keep track of how many milliseconds have elapsed since you have started and you can use the cosine law to work out how many times you should have rotated the bolt.
I threw in a Sleep(10) to conserve laptop battery power, and a reset button to restore the mount to the home position when desired.
| Item | Store | Price |
|---|---|---|
| 1x12x4 knotted pine | Home Depot | $9.40 |
| 1x6x4 knotted pine | Home Depot | $3.57 |
| Brass hinges (6) | Canadian Tire | $14.67 |
| 5/16" 3' threaded rod | Canadian Tire | $2.19 |
| 5/16" hex nut | Canadian Tire | $0.29 |
| Vinyl tubing 1' | Canadian Tire | $0.39 |
| Hose clamps (2) | Canadian Tire | $1.78 |
| 12V battery charger | Canadian Tire | $44.99 |
| 1/4"-20 1" bolt to secure ball head | Canadian Tire | $0.42 |
| Amplis S-4 medium ball head | Henry's | $44.99 |
| Stepper motor 12VDC 7.5 degrees/step | Sayal Electronics | $15.00 |
| Lead acid battery 12V 5AH | Sayal Electronics | $19.70 |
| TO-220 NPN darlington transistor 5A 100V (4) | Sayal Electronics | $5.00 |
| Diode 400V 1A (5) | Sayal Electronics | $1.00 |
| DB25M solder type | Sayal Electronics | $1.60 |
| 10K 1/4W resistors (10) | Sayal Electronics | $1.00 |
| Etched board 2.75x3.75 | Sayal Electronics | $2.50 |
| Project box 4.5x3x1.3 | Sayal Electronics | $7.05 |
| Telrad finder | Khan Scopes | $54.00 |
I left out a few extra things like the DB9 connector for the motor and the LEDs because they are all optional. I also left out some items that I already had kicking around such as alligator clips, insulated wire, and a parallel extension cable. I also didn't factor in the cost of a laptop, since I assume nobody would even consider this approach unless they already had one.
The total cost of the materials comes out to $229.54 before taxes, give or take $20.00 for optional supplies. Due to the experimental nature of this project I ended up spending an additional $60.00 on supplies that proved to be useless.
There are a few places where money can be saved. If you already have a battery charger (for your motorcycle or similar vehicle) then you can save there. The battery I bought ended up being too large. Each coil in my stepper motor draws 1/3 of an amp. I run two coils at a time in order to improve torque, so I draw 2/3 of an amp continuously. This means that my 5AH battery can power my barn door mount for about 7.5 hours. It is unlikely that I will ever let it run for so long. A battery half the size would probably be sufficient. A Telrad might be overkill for a simple barn door mount. You can buy a red dot finder for less than $25 and it will probably be just as accurate.
I think it turned out rather well considering I didn't have proper polar alignment. I even managed to capture a hint of one of Andromeda's companion galaxies.
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