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reflector


Spars:
        The purpose of a spar is to reflect sunlight onto an evacuated tube suspended normally one foot above the array. The tube is almost six feet long, 1.8 inches wide and will be between 1 foot and 1.4 feet away from the mirror.  The normal mirror to use is 1.5 inches wide by six feet long and flat.  To save cost, one can use fewer and wider mirrors if the mirrors can be bent.  The optimal bend would be a circular bend in the short axis with diameter of 4.8 feet.  This would have a focal length of 1.2 feet.  Putting a single triangular bend into a mirror is a good compromise and also improves the rigidity of the spar.  The spar must be rigid to be able to focus and must be suspended by the ends only.  Fewer and larger spars save cutting and assembly costs, but increase the cross section to wind.  It is not required that the mirrors be a single six foot long piece if its easier to cut shorter pieces, see the photo above.
        To test a spar, hold it by the ends in sunlight and reflect the light onto a wall.  If the light is in a narrow strip that will be inside 1.8 inches or less when focused on the tube, its a good spar.  If the light is snake shaped or smears, its not a good spar.  Bends on the spar will tend to bend the strip as well.
        For reflectors, there are a few materials to choose from.  Stainless steel is weather resistant, lightweight, not too expensive, and can be ordered cut to size.  Number 8 finish is a true mirror, this makes a sharp reflection in both axis.  However number 8 is normally 20 gauge and is heavy.  Also the best stainless steel mirror looses about 1/3 of the sunlight from absorbtion.  When dirty, the mirror looses more.  On the postitive side dirt does not like to stick to stainless, it will never rust, shatter or scratch in normal use, and its easy to work with.  Cutting number 8 makes it twist a bit, bend it flat by hand before mounting.
        Some stainless finishes other than number 8 can be used as well, these will be lighter and cost less. The trick is the grain (lines visible on the surface) must go the short way across the spar not the long way.  Normally metal suppliers like to cut with the grain, you need them to cut against the grain creating lines that are 1.5 inches long.  If you cut it this way, the light will be blurred in the long axis which is less important but sharper in the short axis, so most of the light will end up on the tube.  Some finishes are too course,  test a sample by reflecting light onto a wall from a rectangular sample.  Two sides of the light spot should have a sharp border, the other two will be soft and blurry.  If the sharp border is sharp enough, its a suitable material to use.
       Other popular reflectors include mylar.  The heavy mylar from Mcmaster Carr can be bonded directly to the support and hang over the edge, but it wont focus in any kind of wind.  Mylar bonded to a stiffener material like aluminum sheet stock will remain flat in wind.  Use a permanent spray on adhesive, not the repositionable which allows the mylar to creep and wrinkle over time.  Mylar is very reflective, over 90% when clean.  As an insulator it also has a static charge and tends to attract dirt.  Mylar is the cheapest of all reflector materials.
        Aluminum is also very reflective.  When polished, aluminum is the most reflective of all mirrors.  Spray on a clear coat after polishing to prevent oxidation,  Krylon makes a good product for this.  After treatment, it is delicate so treat with care.  Puncturing the krylon will allow the entire surface to oxidize, and polishing is very labor intensive especially in 1.5 inch strips.  Clear coat must go all the way around and be applied after cutting.
        Glass is the best performing of all mirrors.  Glass is durable, cheap, very reflective, not very static generating.  However it is heavy and brittle.  Thin mirrors in short segments may be an option.  Do not cut existing mirrors, the coating on the back will degrade from the edge inwards over time.  Mirrors must be coated in their final shape.
        Chrome plated steel is another option.  Chrome plate is reasonably cost effective in large batches.  However it also absorbs about 1/3 of the sunlight.  Chrome plating is a small local business available in many cities, bring your spars in for coating.  Chrome is a bit delicate depending on the coating thickness, never cut after plating.
        Under the reflector is a structure to hold it rigid.  This is a 3/4 inch aluminum angle stock 1/16 inch thick.  There is no better choice.  In high wind areas  consider 1/8 inch thick stock.  To evaluate a material for a stiffener, hold it by one end, put the other end on the ground, look down its length to see how much it bends and twists.  The structure is held by the ends using an element sometimes called a coupling nut or sometimes called a standoff.  It is a rod with a threaded hole drilled down the center, in this case for a #6-32 thread.  Use a high strength epoxy to fasten the standoff to each end of the angle stock.  Use glue or pop rivits to fasten the mirror to the angle stock.
        The spar is held on the top end rigidly by the tilt mechanism, but must still pivot.  The screw on the top side will be tightened to hold it against the tilt mechanism.  An unthreaded standoff forms the bearing which allows the tight rigid screw to pivot against its array mount.  On the bottom side, the screw is loose and held by a liquid lockwasher in the standoff so it wont back out completely.  The screw itself forms the bearing which pivots at the bottom rail.
   
stainless mirrors