"Any dish mirror that brings the suns rays
to a single point focus
must be tracked in two axes very precisely,
just like a telescope, which,
come to think of it, uses a parabolic dish mirror."
Please see:
http://www.geocities.com/davidmdelaney/scheffler-precis/scheffler-precis.html
An extract from that text:
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The optical system of Scheffler's community
solar cooker forms a heliostat.
An article on heliostats in the eleventh edition (1910)
of the Encyclopaedia Brittanica defines
a heliostat as "an instrument which will reflect
the rays of the sun in a fixed direction
notwithstanding the motion of the sun.
The optical apparatus generally consists of
a mirror mounted on an axis parallel
to the axis of the earth, and rotated with
the same angular velocity as the sun."
The heliostat of Scheffler's cooker
comprises a primary reflector,
a secondary reflector, and a clockwork
powered by gravity or photovoltaic panels.
The primary reflector produces a converging
beam of sunlight aligned with an axis of rotation
which is parallel to the axis of the earth,
and which passes through the centers of
both reflectors. The clockwork rotates
the primary reflector around its axis of rotation
at a rate of one revolution per day,
keeping the reflected beam aligned
with the axis of rotation as the sun moves.
The fixed secondary reflector reflects
the beam from the primary reflector onto
a cooking pot or frying surface.
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This kind of movement needs seasonal adjustments,
however. The movement around the axis must be
precise of course.
Scheffler reflectors are are providing an example,
which kind of simple powerful solar devices can
be constructed. Even a 'normal' small steam engine
boiler can be powered with these reflectors, of course.
- Hannu
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