RASC-KC Barndoor Tracking Project
With project funding from the Youth group, we purchased material for 35 Type I
barndoor astrophotography mounts on January 31st, 1999. Construction started
on February 1st and completed on February 16th. 120 man-hours were spent on
the project (basically all available free time evenings and weekends).
We decided on a Type I design for its simplicity and even then highly underestimated
the time required to complete the project. Sometime in the future we would build
more accurate (and more complex) Type 2, 3 or 4s).
[
Setup |
Operation |
Upgrades |
Cost |
Construction Info |
Parts List |
Log |
Links
]
BarnDoor Tracker for Astrophotography
What Is It?
A barndoor in the simplest form is made of two boards hinged together, with the hinge pointing towards
the polar axis. One board is kept fixed, and the camera is attached to the other. A screw is used to slowly
push the boards open at just the right rate so that the camera will track objects in the sky as the Earth
rotates. With this tracker you can now take longer exposure photographs with your 35mm camera
without getting star trails. Typical exposures will be on the order of 5-10 minutes.
We have designed and built 30 manual BarnDoor Type I Trackers for the youth group (with another 5 going
into the equipment loan program). To use one you will also need a standard tripod and a 35 mm camera
with a tripod mount (should be standard). Here is a photo of a general type of barndoor tracker:
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Setup:
The barndoor can be used in two ways:
-
have the hinge on the left and polar align the hinge to Polaris. The arms will
have to start together and the dial will have to be rotated CLOCKWISE, opening the arms (the top arm
will swing east to west). This method provides the best accuracy but a worse balance point on the
tripod.
-
have the hinge on the right and polar align the hinge to Polaris. The arms will have
to start apart and the dial will have to be rotated COUNTER-CLOCKWISE, closing the arms (the top arm
will swing east to west). This method provides the worst accuracy but a better balance point on the
tripod.
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Operation:
To operate the barndoor the main drive bolt must rotate at one RPM (rotation per minute). We
approximate this by rotating it 1/4 turn (90 degrees) every 15 seconds. This can be tedious after a few
minutes and requires attention to detail but the results are quite spectacular! The mechanical limit
of the barndoor is approx 70 minutes but the design of Type I barndoors gives a large tangental error
(the longer the exposure the worse the tracking).
The formula governing the tangental drive is theta=arctan (a/r) where r= the distance from the hinge to
the bolt (11 7/16"), a is the distance between the two arms and theta the angle the two arms subtend.
Over time the drive bolt does not push the arms apart fast enough and the tracking begins to lag behind.
At 30 minutes the lag is 19 arcseconds. (Which is unreasonable). At 15 minutes the error is approx 2
arcseconds (reasonable). So we recommend a maximum tracking of 15 minutes on this
design, but feel free to experiment on your own. The other factor is how accurately you have
polar aligned the barndoor... the closer the better.
We will display some examples at the February Youth Group meeting of long exposure astrophotography
on a tripod with no tracking, and using the Type I BarnDoor Tracker.
Depending on how you have polar aligned it, you must rotate the dial in a particular direction.
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Upgrade projects:
After using the barndoor for some time you may want to improve on it with some projects of your own.
Here are some suggestions:
-
Replace the string with some kind of elastic connector to allow for constant tension on the drive bolt
-
Create a better polar alignment guide (straws? Eyebolts? A finder scope?) And align and mount it.
-
Add a red LED and power source so you can see the dial and perhaps a watch or other timepiece. Or
make it longer so you can also see the dials on your camera.
-
Add an easier timing system. Do you have a portable cassette tape player? Record a 5, 10 and 20 minute
piece of music and every 15 seconds call out the time (so that they are mixed together on the same tape).
You can then play it back, listen to music and have built in time cues. Another member built a small
electronic box that beeps every 15 seconds and the number of minutes every minute.
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Cost:
The approximate cost of the Type I is $20. There are other design types that are more accurate in
tracking and allow for longer exposure times, but they are also more expensive. Once you have used this
Type I model, you can look around and find a design you like and built a type II, III or IV.
Construction Notes: All parts were sanded and coated with outdoor
varathane (2 coats) to improve dew
resistance. String is used on the two arms so the camera weight does not
cause the arms to flip apart and possibly break things.
Assembly Instructions (PDF)
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Parts List
|
Qty |
Unit |
Total |
|
Note |
| Wood |
6x1x40" |
$1/LinFt |
$3.65 |
|
Clear Pine 10% wastage allowance |
| Piano Hinge |
6" |
0.11 |
$0.66 |
|
$8 per 6' section |
| A-Nuts |
1 |
0.16 |
$0.66 |
|
Tripod Link |
| T Nuts |
2 |
0.12 |
$0.24 |
|
Main Drive + Azimuth Lock |
| Carriage Bolt 1-1/2" |
1 |
0.13 |
$0.13 |
|
|
| Carriage Bolt 3" |
1 |
0.21 |
$0.21 |
|
|
| Screws 1-1/4 #7 |
5 |
0.05 |
$0.25 |
|
$5 per box of 100 |
| Screws 3/4" - #6 |
8 |
0.05 |
$0.40 |
|
$5 per box of 100 |
| Wing Nut |
1 |
0.25 |
$0.25 |
|
|
| Hex Nuts |
5 |
0.07 |
$0.35 |
|
|
| Posi Nuts |
2 |
0.23 |
$0.46 |
|
|
| Thumb Screw 1-1/2" |
2 |
0.47 |
$0.94 |
|
|
| Washers |
5 |
0.03 |
$0.15 |
|
|
| Washer Wide 1-1/4-2" |
1 |
0.21 |
$0.21 |
|
Wide Fender Washer |
| Carriage Bolt 4-1/2 |
1 |
0.28 |
$0.28 |
|
|
| Drive Pate |
1 |
0.29 |
$0.29 |
|
|
| EyeBolts |
4 |
0.074375 |
$0.30 |
|
Polar Alignment (Est) |
| Sanding |
|
|
$0.57 |
|
$20 Sandpaper / 35 units (Est) |
| Finish (Urathane) |
|
|
$1.51 |
|
$40 4Litre/35 units + $6 Brush |
| Glue |
|
|
$0.50 |
|
Estimate |
| Rubber Grip |
1 |
1 |
$0.40 |
|
Estimate |
| Blades |
|
|
$0.06 |
|
$2.00 / 35 units |
| String |
|
|
$0.14 |
|
|
| Total |
|
|
$12.61 |
|
|
| Total (Taxes In) |
|
|
$14.51 |
|
|
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Activity Log for barndoor Project
- 2000 Sep 5 : added assembly instructions in adobe acrobat format
- 1999 Feb 16 : 32 Barndoors completed and in storage. 3 remaining with replacement parts that had broken during assembly.
- 1999 Feb 14 : 4-6 hours/day every day since the last entry. Mostly completed 32 assemblies. Broken segments account for the remaking of 3 units. Still need to tie the two arms together and provide a sighting mechanism.
- 1999 Feb 8 : 6 hours of sanding top assembly, cutting parts for L bracket, cutting parts for L bracket support, countersinking and drilling L bracket
- 1999 Feb 7 : 6 hours cut, glue and assemble top assembly
- 1999 Feb 6 : 3 hours sanding
- 1999 Feb 5 : 3 hours
- 1999 Feb 4 : 3 hours of sanding and 1st coat of varnish
- 1999 Feb 3 : 2 hours of drilling later, the tripod mount and main drive holes are set. Varnishing the lower body is next, followed by assembly of the hinges and mounting of the A and T Nuts.
- 1999 Feb 2 : Major body of barndoors have been cut to size.
- 1999 Feb 1 : Parts for 30 barndoors have been purchased with an estimated completion date of Sunday Feb 21st. These are Type I barndoors slated for the Youth Group. An additional 5 units will become part of the Loan Program.
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