Barn door camera mount.
It is cheap and easy to build, portable to take advantage of distant country dark skies, needs no electricity.
By Phil Holt, Mornington, Vic, Australia, in association with the Mornington Peninsula Astronomical Society
The platform is based on one described in Astronomy magazine September 1985 by Robert Reeves.
The Basic idea
Imagine 2 sheets of timber joined at one edge by a hinge and separated at the other with a screw. If a camera is mounted on the top sheet and the hinge is parallel with the earth's axis then turning the screw at the right speed will move the top sheet relative to the bottom and allow the camera to track the sky by counteracting the earth's rotation.
Construction
The first hinge (latitude control) joins the base plate to the latitude plate. The angle between the 2 being your latitude. A threaded rod between the two helps you adjust this angle.
The second hinge (right ascension) is on the latitude plate at right angle to the first hinge and connects the latitude plate to the r.a. guiding plate. The hinge is parallel to the earths axis if the latitude has been set and it is aligned north-south.
The third axis which is parallel to the first serves as a crude form of declination adjustment. It is attached to the top edge of the r.a. plate and connects to the declination plate which houses the camera and the guiding scope. The camera should have a B setting so that the shutter can be locked open for long exposures. The guidescope on my platform is an old 6x30 right angled finder with a 12mm crosshair eyepiece, giving a magnification of about 20x.
At the wide end of all the axes threaded rods pass through holes in one plate and end in a point that rotates in a notch on another plate. To allow the angles to change the rod passes through a short length or cylindrical curtain rod with a section cut out and a nut inside. The weight of the top plate holds the nut against this rod which rides in wedge on the lower plate to stop slipage.
The tripod is sturdy 3.5 x 1.75 inch timber to reduce flexure and vibration. The heavier and sturdier it is, the less vibration you will encounter. The increased weight of course will reduce its portability, so it can be a case of "what you gain on the swings, you lose on the round-abouts" as motorcycle legend Barry Sheene often said.
How to Use
Set up the tripod on some sturdy ground, adjust the latitude plate so that the r.a.hinge points to the celestial pole (easier in the northern hemisphere). Point the ball and socket mounted camera to the target. Point the ball and socket mounted guide scope at a star in or near the target area. Defocus the star so that it surrounds the crosshairs in the scope. While watching the star turn the threaded rod controlling r.a. slowly to keep the "crosshairs in the middle of the star." Correct for north/south drift by turning the Declination control rod.
Limitations
The declination adjustment fails when photographing areas of the sky close to and of similar altitude to the celestial pole. The R.A. tracking motion and the dec. motion are in the same direction, ie. both result in vertical motion of the camera.
Using lenses longer than 135mm requires real accuracy. The limit of guiding with your eye constantly watching the crosshairs is about 14 to 15 minutes.