Wednesday, August 22, 2012

A simple but elegant barn door tracker

Completed barn door tracker
If you've ever tried to take a photograph of the night sky you know that it's not easy.  Even with a good tripod, any exposure longer than a few second will start to result in the stars turning into streaks - commonly called "star trails".  Don't get me wrong - star trail photos can be absolutely beautiful, but if you want to see dim objects, you've got to move the camera in sync with the earth's rotation.

Enter the "barn door tracker".  You can read all about the theory on the Wikipedia page.  I'm here to tell you about my version, which can be made from easy-to-purchase materials and will cost less than $30.00. It's not a fancy motor-driven version - in fact, it's the simplest kind, good for exposures of no more than 5-10 minutes, but if you take multiple 5-minute exposures and "stack" them in software with a program such as StarStaX, you can get some truly inspiring results.

About tracking error

This (simplest) type of barn door mount will start to show errors (in the form of star trails) due to tangent errors.  The maximum exposure time will vary with the focal length of your lens.  Here's a rough guide:
  • Wide-angle lens (35mm or less) - about 15 minutes
  • Normal (50-60mm lens) - about 10 minutes
  • Telephoto lens (70mm or longer) - about 5 minutes

Materials list

(links may be out of date - most hardware is available at Lowes or Home Depot):

ball head ($13-20)
1 8" strap hinge ($6.00)
1 1-1/2" 10-32 round-head bolt  ($1.20 for 5 - includes 5 nuts) - this will give a maximum of about 20 minutes of exposure time.  If you want longer, use a longer bolt (2" or even 3"), but be aware that this type of tracker isn't designed for exposures of more than about 10 minutes, due to tangent error.
1 1/4"x3/4" flat weasher ($0.07 in the store) - that's a 3/4" diameter washer with a 1/4" diameter hole
1 1/2" 1/4-20 flat head machine bolt ($1.20 for 4, includes nuts) - needed only if ball head doesn't come with an attachment bolt
1 1/4-20 wing nut ($1.20 for 2)
2-part epoxy ($4.00)
1 plastic straw (free - go to McDonalds and order a drink!)
1 wooden popsicle stick (free with a nice, cold popsicle - and don't you want one right now?)
1 small piece of flat, smooth plastic (I cut one from a strawberry container, but most anything that's thin and slick should do - an old CD case lid, perhaps?)

Construction details

When making barn-door trackers, there's one critical dimension - the radius of curvature.  This is the distance between the lifting screw and the center of the hinge.  This depends on two factors - the speed  (in Revolutions Per Minute, or RPM) with which you'll be turning the screw and the number of threads per inch (TPI) of the screw - in other words, how far the camera platform rises with each turn of the screw.  Here's the formula (shamelessly stolen from this barn door website, which has an elegant motor-driven tracker that's worth a look):
Radius (in inches) = RPM / (0.004375 x TPI)

We're going to assume 1 RPM (because it's easy to do with a watch handy) and we're using a 10-32 bolt (a #10 size with 32 TPI), so the calculation becomes:

Radius = 1 / (0.004375 x 32) = 7.142 inches (or 181.5 mm)

We're in luck, because the 8" strap hinge just happens to have a hole centered about 182 mm away from the center of the hinge!

Assembly instructions

  1. Using the 2-part epoxy, attach a 10-32 nut to the 3/16"x3/4" washer.  Be very careful not to get epoxy on the threads!
    Bolt and washer attached to strap
  2. Once the washer/nut combination has dried solid, use the 2-part epoxy to attach the washer to the bottom side of the top of the strap hinge. Before it starts to set up, make sure you break out your ruler and double-check the distance between the center of the nut and the center of the hinge!   If you can, set something heavy onto the assembly to ensure a good bond and set it aside to dry overnight.
  3. After the above have dried completely, cut a small piece of flat, smooth plastic to fit over the hole on the other side of the strap hinge, just below where the nut is glued when the hinge is closed.  This will ensure that the bolt head has a smooth surface to ride across.  Attach this to the hinge with glue or double-sided tape.
  4. Drilled popsicle stick
  5. Now thread the 10-32 bolt into the epoxied-on nut.  Open the hinge and thread it from the side where the nut is.  When the hinge is closed, the head of the bolt should hit the plastic piece on the other side of the strap hinge, and the threaded side should come out the top.  Thread it almost all the way in.
  6. Carefully drill a 3/16" hole in the middle of the popsicle stick.  Thread 10-32 nut onto the bolt, run it down about 1/2", then put on the popsicle stick and thread another 10-32 nut on to firmly hold the popsicle stick in place.
  7. Open the hinge and attach the ball mount through the middle hole in the top arm of the strap hinge using the 5/8" 1/4-20 bolt.
  8. Sighting straw attached to strap hinge
  9. Attach a 3" piece of plastic straw to use as a finder scope to the lower part of the strap hinge where it meets the hinge.  You can use tape or glue to keep it in place, but make sure it's as parallel to the hinge as possible.
  10. Use a 1/4-20 wing nut to attach the strap hinge to your tripod using the center hole in the bottom arm of the strap hinge (the one directly below the ball mount).

Using your barn door tracker

To use your barn door tracker, take the completed unit to a dark sky area.  Using the finder straw, find Polaris (the north star, the star in the handle of the little dipper furthest from the bowl).  Now move the straw about 3/4 of a degree toward the top star of the bowl.  Your tracker is now pointed properly.

Now mount your camera to the tracker and screw the screw so that the two halves of the hinge are parallel to each other.  If you haven't already done so, set up your camera.  Here's some of the settings you'll want to use:
  • Aperture:  open all the way
  • Shutter:  the longest possible setting.  If you have one for multiple minutes, use it.  Otherwise you'll need to use the "bulb" setting and open and close it by hand.
  • ISO: 800 or 1600 (you can go higher if your camera doesn't introduce too much "noise")
  • Zoom:  to taste - with lower zoom settings you can shoot longer exposures before the errors inherent in the barn door tracker cause trails
  • If you have a DSLR, turn off mirror lockup
  • Turn off the "review" setting (showing you the image after its taken)
  • If possible, set your lens to manual focus, and focus to infinity
  • Turn off any image stabilization
  • Remove any lens filters for maximum light transmission
  • Shoot in RAW format if possible
 Once you've set up your camera, you're ready to shoot.  Point the camera to the area of sky you want to photograph and open the shutter.  Now start turning the popsicle stick at at pace to match the second hand on your watch.  If you don't want to be moving it continuously, you can  follow this guide, depending on the focal length at which you're shooting:
  • With a wide-angle lens (35mm or less), you'll need to turn the stick 1/2 turn every 30 seconds
  • With a normal (50-60mm lens), you'll want to turn 1/4 turn every 15 seconds
  • If you use a telephoto lens (70mm or longer), you need to turn 1/12 turn every 5 seconds.


Many people have documented their builds, some have provided additional information which has been summarized above.  A shout-out is due - you can use these links to learn more information or build a mount that suits your needs better:

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