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One obvious way to get it within a few mm is to bodge up a mirror
cell on a long yardstick backbone.
Wet the mirror with water if the mirror coating is bad or
non existent...point to the sun carefully and slide a G-clamp with
some white card up and down the backbone board until the
5 cent piece sized focus is smallest.
But be careful...in full sun with a good coating, the paper
will burn with an 8" mirror or bigger.
Steve
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