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To make "hinges" for all the mirrors, I put a small glob of RTV silicone
caulk at the corner of each mirror. Since this glue blob must stand up
and not be smashed flat, I cut a number of round wooden toothpicks into
1/8inch
segments, placed a small dollop of silicone sealant at the corner position
of each mirror, laid each toothpick sideways into the silicone glob as a
spacer, then laid
down the mirror on the correct spot on the wood. The silicone lightly
attaches the corner of a mirror to the wood, while the toothpick segment
will lift up the corner of each mirror; tilting each
mirror and
giving it a greater adjustment range. The toothpick spacers also keep
the mirror
from smashing the silicone into a too-thin layer. Carefully perform this
operation for all your mirrors. The toothpick in the corner will tilt
each mirror one way,
and when you later add the screws to the corners, the screws can lift the
mirror as
desired.
Let the mirrors harden for 24hrs. When the silicone has cured, screw all
your screws into their holes just enough to let them touch the mirrors.
Programming the array as a solar furnace
Cut up some Post-it(tm) notes, and stick a square of paper to each mirror
to block the light. LEAVE ONE MIRROR NEAR THE CENTER OF THE ARRAY
UNCOVERED. This one mirror will act as a reference for adjusting all the
others. (As the position of the sun changes, the position of this spot
will also change, so you shouldn't need to move the whole device to track
the sun while adjusting mirrors.) Take your mirror array out in the sun,
and position it a few
feet from a convenient target. The distance between the mirrors and this
target will become the focal length of the Solar Furnace.
First use your screwdriver to adjust the single bare mirror ship so it is
no longer tilted by the silicone glob. Bounce some sunlight from this
small mirror towards your target, and move the whole device so the spot of
light hits the target. Now remove the Post-it(tm) note from one other
mirror chip, and use the screwdriver to adjust this other mirror so the
spots of sunlight from the two mirrors combine together. Cover this
newly-adjusted mirror, and uncover the next one. Repeat the adjustment
process, then cover it and go to the next. Do not of course change the
setting of your central "reference" mirror. When each mirror has been
adjusted to combine with the "reference" mirror, peel all the paper from
the mirrors and see what you've accomplished. All the bright spots should
now shine on the same place. Tweak any stragglers to put them in line.
If you wish, go through and glue down each mirror permanently and remove
all the screws.
A 6X6 array is pretty safe for experimenting. It won't set anything on
fire, but in the summer sun it will heat a black garbage bag almost to the
melting point. Once you have the whole process learned, try making a 16 X
16 array (256 mirror chips), or even larger. The above process lets you
slowly "coat" any flat wooden surface with solar-furnace arrays. With
thoughtful planning you could even cover a non-flat surface with a solar
furnace array. If you had enough time, you could build one of ANY size
and temperature. ***WARNING*** if you build a big one, keep it covered
when not in use. If you leave it around the house, moving sunbeams might
unexpectedly bounce off it and start a fire!
WOODBURNER
Once I had a brainstorm, and when I tried it out, it actually worked: send
intense light into one end of a fiber-optic cable, and use the other end
as a woodburner. I used an expensive glass-fiber cable 1cm in diameter
which was about 1/2m long, and I placed one end of the cable at the focus
of a 12-in fresnel lens in summer sunlight. The other end of the cable
could char a wood surface, but just barely. A bigger solar furnace might
have made it impressive. Sign your name as charcoal! Note: if you try
this stunt, realize that you probably will damage your fiber optic cable,
so don't try it with one that you can't afford to lose! Plastic
opto fibers might melt, so use glass if anything.
MIRROR SIZE AND LONG FOCAL LENGTH
The smaller your mirror chips, the smaller and hotter the focus. After
all, the hotspot is approximately the size of a single mirror. An array
of 1in. mirrors a foot across will make 144 beams, but if you use 2in.
mirrors for your 1ft furnace, the hotspot only receives 36 beams.
However, if you use small mirror chips and adjust your solar furnace for a
very long focal length, you'll find that the hotspot grows larger,
fuzzier, and cooler. This occurs because the sun is not a tiny point,
instead it
is a disk, and the mirror-facets act as the pinholes of a "pinhole
camera." Small mirror-chips form an image of the sun, rather than an
image
of the mirror-chip shapes. Each little square of light will develop a
blurry edge, and only the center of each square image will get "full
sun."
To compensate for this blurring effect, use larger mirrors. Here's the
rule:
choose a focal length which is lots shorter than 120 times the width of a
mirror-chip. (This 120 comes from 1/tan(.5deg), the sun being about 1/2
degree in angular size.) For example, the 1in. mirrors would give a
blurry
hotspot if F.L. was longer than a few feet, and at 9.5ft the blurred
regions
swallow the hot center of the hotspot. This "blur" is an image of the
sun. If you want to burn objects from 120ft away, you'll have to build a
furnace using mirrors which are wider than 1ft each.
The size of the sun-disk is the cause.
(If our sun was tiny, but still just as bright, then this blur would be
gone, and you could form its light into an intense parallel beam like a
laser!)
For a 2-ft solar furnace:
| MIRROR CHIP | MAX F.L. | BRIGHTNESS |
| 1 in. | 9 ft. | 576 | | 2 in. | 19
ft. | 144 | | 3 in. | 28
ft. | 64 | | 4 in. | 38 ft. | 36 |
MIRROR ART
Rather than "programming" your adjustable array to form a solar furnace,
you could instead use it to form any desired pattern of light spots. For
example, make it project your initials. First adjust all the mirrors so
they are flat. In the sun, the array should reflect a square grid of
light spots. Now choose one-half of the mirrors and adjust them so they
form one of your initials. Do the same with the other half, for your
second initial. Now you have a "magic mirror" which looks like a bunch of
little squares; like an almost-flat mirror, but when exposed to sunlight
it creates a giant projection of the initials of your name.
Chinese "magic mirror"
Some science museums have ancient Chinese "magic mirrors" on display.
These look just like a flat mirror, but if you bounce the sun off them,
you'll see a picture in the projected sunbeam. The surface of these
mirrors contain tiny wobbles. We can program the solar furnace to do
something similar. First program your solar furnace so it's perfectly
flat; so it projects a square array, and so it acts like a big flat mirror
when you use it to look at your own face. Then go out in the sun, aim the
beam at a far distant wall, then program the mirror chips to make a
tiny pattern (such as a number, or your initials). Now use the furnace as
a mirror to look at your own face. It acts like a flat mirror, no?
The deflections used to produce the pattern are tiny, so the mirror is
ALMOST flat, just like the Magic Mirrors.
MISC
DeathMeK says to use old CDROMS (or free ones from junkmail) as
zero-cost mirrors. Unfortunately the silicone glue used as a "hinge" just
makes the thin mirrors bend, so you'll have to attach them some other
way. Or perhaps use silicon glue, but only use an incredibly tiny
dot of the stuff.
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![[Mirror array panel, 8 inch plywood square with 36 small mirror chips]](/amateur/MIRROR2.GIF)
![[Drill Guide: screw holes in opposite corners of each mirror chip, glue dot at 3rd corner]](/amateur/MIRROR3.GIF)
![[Detail of one facet]](/amateur/MIRROR.GIF)
