To photograph animals you need a telephoto lens. To photograph
athletes you need a longer lens. To photograph airplanes you need an
even longer lens. To photograph space, you need a trapezoid.
The trapezoidian equatorial mount allows digital cameras to see very
very far. It's similar to the Scotch mount in that it uses hinges and
the angle reduction of a long lever to track over a short distance.
The similarities end there. The trapezoidian design is dictated by its
need to be portable and accurate.
It uses aluminum rods instead of sheet wood, a pulling corkscrew
instead of a pushing corkscrew, supports polar alignment without a
second tripod head, mounts the camera on the end of a declination arm,
and has much higher accuracy than a Scotch.
Because of this, we feel the trapezoidian is to the Scotch what the
Dobsonian is to the Alt-Azimuth.
Much has been made of the tendancy of Scotch mounts to drift during
long exposures. Since digital cameras don't need the length of
exposures that film cameras needed, the drift isn't as much of a
problem. The trapezoidian uses a fishing line drive that naturally
forms an isosceles triangle. Extra accuracy can be gained by
programming the microprocessor to change speed based on an odometer.
The moving part in a trapezoidian is 2 pieces of aluminum arranged in a
right angle. One piece is 5 ft, points east, and is raised to achieve
about 15.04° of rotation per hour. This is the rate at which stars
track across the sky, not the rate at which the Earth finishes a day.
The perpendicular leg is 3 ft, and points exactly on the Earth's axis
of rotation.
Of course, unless you live between two certain latitudes, the
declination arm can be too steep or to shallow without raising the
fulcrum.
Additional members support the camera platform. The right ascention
arm is held up by 5lb fishing line and hangs in mid air.
The fishing line is pulled by a servo and corkscrew. The corkscrew
must be stiff enough to resist bending, which makes it the heaviest
part.
The servo speed is regulated by a tachometer and electronics
package.
MORE ABOUT THE ELECTRONICS PACKAGE
Unfortunately the right ascention arm can only move 15°, limiting
your observing time to 1 hour before needing a reset and reacquisition
of the subject. Also it's not accurate enough for any more than 300mm
images up to 30 seconds.
The hardest part is aligning the altitude and azimuth.
FABRICATING A TRAPEZOID The dimensions
and materials used in the trapezoid are available here.
Trapezoidians aren't just for talking about on web pages, they're for
seeing far. Go to the
The trick with
the trapezoid is the long member with the angle reduction is supported
by 2 short members. The 2 short members control azimuth and altitude
alignment.
A
trapezoidian locked & loaded can be carried up mountains by people
other than heroines.
Diagram of moving
part.
Typical
trapezoid assembly
Right ascention arm.
Electronics package.
A
windscreen is critical for using the trapezoid. Read about the WINDSCREEN ODYSSEY
Timelapse movie of a trapezoidian 1440x1080 5.1MB
TRAPEZOIDIAN GALLERY.