Here are some photos and a description of a barn door tracker I built a few years ago.  It does a great job taking photos of the night sky at low magnifications.  The milky way photo below was taken in Sheffield, MA.  The bright star in at the top center is rho cygni, and the bright star in the trees (lower left in the frame) is beta casseopeia. The lens was fully open at f/2.  Film speed was 400 ASA, and the exposure lasted 5 minutes.

 

I don't know where the term "Barn Door Tracker" originated, but what it means to astronomers, amateur and pro alike, is a homemade device for taking pictures of the night sky at low magnifications.  Such a device must move a camera so that the earth's rotation is cancelled,  so to speak.   This in turn requires three things: 

 

1.  A way to adjust for latitude.

2.  A way to sight on the north celestial pole. (polaris is close)

3.  A calibrated mechanism for slowly rotating the camera.

 

The device in these photos does all of these things, in a way that makes it possible to relax and enjoy a stellar photo session, without having to contort uncomfortably in the freezing cold in January when the skies are clearest.

 

The ruler in the picture above is a foot long.  The pocket watch is perfect for timing the turning of the the drive wheel, more about that later.  The L-bracket makes it possible to aim the camera at most of the sky.  The hinge is a piece of aluminum flashing mounted behind the two wood strips with the closely spaced rows of nails. 

There are a pair of dowels at the sides which are used to adjust for latitude.  On the left is a device for sighting the north star.  A mirror mounted at a 45 degree angle helps reduce contortions.  There is

a battery powered LED which illuminates a small ring at the far end of the sight.  The table in the photo is not a suitable platform for taking pictures, you need to have a stable surface.  Frozen hard ground in January or February is best.  Or a paved surface, if you can find one far from light pollution.

Above is a closeup of the drive wheel.  There are notches every thirty degrees.  During an exposure, I advance the drive wheel one notch every five seconds.  The pocket watch keeps time.  I stick it in a pair of earmuffs to keep hands free, and it tics 4 times per second.  (It helps to be musical)  The drive wheel shaft is a 1/4-20 machine screw.  It is 11 (and a fraction) inches from the hinge.  This almost exactly matches the earth's rotation for the 2 to 10 minutes it takes to get an exposure.