A Simple Motorized Barn Door for Affordable Astrophotography

A manuscript entitled "Affordable Astrophotography With a Motorized Barn Door" is available for download.  

Motorized Barn Door.doc

Motorized Barn Door.pdf

What follows are instructions for building a simple, motorized barn door photographic system for taking pictures of the night-time sky.  Barn door photography systems have been around for years.  There is a plethora of information available on them.  For general information about barn doors, you may find the following sites useful: 
 

http://www.astronomyboy.com/barndoor/links.html     http://www.philharrington.net/scotch.htm   http://hometown.aol.com/davetrott/page17.htm     http://www.tucsonastronomy.org/barndoor.html   http://www.proweb.co.uk/~ksgreen/Astronomy/Tracker/index.html

Figure 1:  The motorized barn door photographic system (above) can be constructed by following the directions below.

 
 

CONTRUCTION
 
 

STEP 1:  The motor comes with a one-eighth inch steel shaft.  Make a small pulley for it using a 1 – 1/8 inch diameter hardwood dowel rod.  First, carve a 1/8th inch wide groove about 3/32nd inch deep around the circumference of the dowel rod.  Make the groove about 1/4 inch from the end of the long dowel.  You can cut this groove using a band saw with a stop block clamped to the table of the saw to keep the depth of the cuts uniform.  Next, cut the carved end of the dowel rod off so that the groove is centered on a piece a little longer than 1/2 inch long.  A picture of the motor pulley is shown in figure 2.

 
   

STEP 2:  Drill a hole, just barely smaller in diameter than the motor shaft, completely through the center of this small pulley.  You will probably want to drill some test holes in a scrap piece of lumber.  The right size hole is the one that the motor shaft just barely slides into with a firm push.  But be careful not to damage the motor while determining the hole size.

Figure 2:  Motor pulley and drive belt.

   

STEP 3:  Now wipe the motor shaft off with a paper towel and some alcohol to assure there is no skin oil or dirt on the shaft.  Put a drop or two of super glue in the hole, and one on the tip of the motor shaft. Quickly slide them together until the pulley is fully on the motor shaft and the tip of the motor shaft is nearly emerging out of the pulley. Put one or two extra drops of super glue on top of the pulley hole and let it seep down into the hole around the shaft.  Be careful that no super glue drips all the way down the motor shaft or the shaft may become glued to the motor housing which could ruin the motor.  Set the motor and pulley aside to dry well for at least a day.

STEP 4:  The main pulley for the drive system should be constructed next.  Cut two circular pieces, 6 inches in diameter, from a piece of 1/4 - inch birch plywood.  A band saw works well for this.  Now trim the circles down to 5-1/2 inches outside diameter on a saw which has its blade set at a 45-degree angle.  This provides a beveled edge on the circular pieces (see figure 3).  You can make sure the cuts are circular by building a simple jig for the sawing process.  The jig is nothing more than a 3/4 inch thick piece of plywood clamped to the saw table with a finishing nail emerging through it 2-3/4 inches from the saw blade.  Drill a small hole through the center of the pieces so they fit snuggly over the finishing nail.  Make numerous radial relief cuts first and then you should have no problems turning the wood on the nail and cutting out a perfect circle.  To finish this part, glue the two pieces together with their smaller diameter faces touching to create a pulley with a slot.  (The edge of this main pulley can be seen in Figure 2.)  Use a good quality yellow wood glue spread uniformly between the pieces and clamp them together tightly for a full day.

 
 

STEP 5:  When the glue is dry, drill a quarter inch hole through the center of the main pulley and inserted a3-1/2 inch long 1/4 –20 carriage bolt through it.Tighten a lock washer and nut on the other side to secure the bolt to the pulley.

Figure 3:  Main pulley schematic.  Make two of these and glue the small radii faces together.

 

STEP 6:  Paint a single radial line on the main pulley - like the hand on a clock - using glow-in-the-dark (phosphorescent) acrylic paint.The line will glow and act as a marker for keeping track of motion in the dark.The paint is available from Edmond Scientific and others.The line can be seen in figure 4.

Figure 4:  Overhead view of the drive pulleys and speed adjustment.

 

STEP 7:  The other end of the drive bolt will be threaded through a single 1/4 -20 threaded rod coupler which is embedded in the top (drive) board.To embed the coupler, first drill a 7/32nd inch hole in the drive board 11-7/16 inches from the hinge axis.Then use a hobby knife to carve out a hexagonal shaped hole that is the right size such that the coupler can be tapped with a hammer tightly into the hole.Before tapping it in, apply glue to the opening using a good multipurpose glue or epoxy such as Liquid Nails or BOND 527 multi-purpose cement.

 

STEP 8:  After the glue has dried for a day or two, thread the drive bolt through the board and down through the threaded rod coupler.(See figure 5).You will want to lubricate it with household oil or powdered graphite.

Figure 5:  The drive bolt screws through a threaded rod coupler which is glued into the top board.

 
   
   

STEP 9:  Fasten a small metal plate to the top of the bottom board where the drive bolt makes contact with it (see Figure 5).This will provide a solid surface for the drive bolt to push against.Rounding off the bottom of the drive bolt with a file and lubricating it will help to provide smooth tracking.

STEP 10:  A 1/8th inch wide rubber band (about 7 inches long unstretched) serves as the drive belt for this device.The specific rubber band recommended for this project can be bought in an office supply store in a package called “Big Bands”, and is made by the Alliance Rubber Company.But you may have to try several different size rubber bands until you find one that works well for you.If the belt is too thick or too tight, it may bind or work its way off the pulley.   If it is too loose, there will not be enough friction to turn the main pulley.The distance between the rotational axes of the motor pulley and drive pulley is about 5 inches.

 

NOTE:  One problem that I have encountered on occasion with this rubber band drive is that after about 25 minutes of tracking, the drive bolt and main pulley are tipped at an angle and sometimes the rubberband pops off.  You may want to try redesigning the main pulley to make the groove wider and deeper or consider changing the shape of the groove in the main pulley.  Another possibility is to redesign the drive system so that the large pulley could be wound with string.  The string could then be pulled off the main pulley and wound onto the motor pulley much like a cassette tape functions.

 
 
 
 

STEP 11:  Even though the motor drives at a steady 7 rpm, there is still a need to be able to adjust the drive rate.The drive rate will gradually vary during a long exposure as the main pulley and drive bolt start to tip sideways.Adjustment of the drive rate is easily accomplished with a 1k potentiometer, available at electronics supply stores like Radio Shack.Additionally it is wise to add a single-pole, single-throw (SPST) switch so the drive can be turned on and off without removing the batteries.The wiring diagram for the motor, batteries and potentiometer is shown in Figure 6.

Figure 6:  A schematic of the electrical circuit used to control the speed of the barn door tracking system.

 

 

STEP 12:  The potentiometer is mounted on a plate connected to the lower board.You can go a step further and add a handle and glow-in-the-dark scale to the potentiometer if you wish (see Figure 7).An alternate design would be to mount the potentiometer in a small project box and connect it to the switch and batteries with a long lamp cord.This will make the adjustment something you can control in the palm of your hand without the risk of bumping the drive device while it is operating.A small handle and scale could still be added if desired.

Figure 7:  A handle and scale may be added so that the speed is easy to adjust in the dark.

STEP 15:  Fit the entire structure onto an adjustable camera tripod to make it easy to use and quick to set up (See Figure 1).