Here is an article I found on making a parabolic mirror.
I have yet to try this but someone with a little more time should be able to tackle this project in a day. Let me know how it goes.
Given: the focal length only. This can be any distance you want to work with and is nothing but the distance from the back of your planned curvature (see Fig. 2)-at the center-to the focus (the spot where the heat is to be directed). Let's say you've decided to use a focal length of four feet. A completed parabolic curve, across the focus, will have a diameter four times that focal length or, in this case, a diameter of 16 feet (4 X 4). A half-curve, then, will have a height of eight feet . . . and here's an easy way to seek that halfcurve:
Draw the focal line out to its required length on a large sheet of smooth paper (Kraft building paper is fine). Fig. 3 shows this focal length-Of-on the sheet of paper. It also shows a second line-fPdrawn at right angles to and twice as long (eight feet) as Of.
We know, of course, that the parabolic curve we're seeking will run, in some fashion, between points 0 and P. And, although we have a rough idea of the area in which that curve will fall, we're not yet sure of its exact course. So we're ready to get down to the finer definition of our curve, and we're going to begin that definition by drawing in a number of lines that are parallel to fP and spaced one inch apart (see Fig. 4). These lines need be put in only in the near vicinity of where our final curve must lie, but they should be measured and drawn accurately. You will, when finished with this step, have a total (counting iP) of 48 parallel lines drawn on your sheet of paper.
Now (Fig. 5) find an accurate straightedge that is at least twice as long as the focal length Of (or, to put it another way, at least as long as fP). Place the corner of one end of the straightedge precisely on point f and-taking care to keep that corner exactly on frotate the face of the straightedge from Of down to fP. As you touch each of the 48 parallel lines from the top down, add one inch to the length (48 inches) of Of and make a dot. (The first dot will be made on the first parallel line down and 49 inches from f, the second dot will be on the second parallel line down and 50 inches from f, etc.) Continue on until you scribe your last dot on the bottom line and 96 inches from f. The series of dots you've just made will define a parabolic curve with a four-foot focal length.
Now connect the dots by very accurately placing a flexible metal, plastic, or wooden strip across them and carefully draw. ing a line from O-cutting through all the scribed points in between-to P (Fig. 6).
This completed line is the curve you seek. It is NOT part of an ellipse. It is NOT part of a circle. It is NOT a hyperbolic. It is a true parabolic curve and you can now use it in the construction of your solar heating unit or cooker. By doubling the drawn curve back over the focal line (pick up point P, swing it over Of, and lay it down again an fP distance on the other side of Of), you can quickly and easily convert the eight-foot-long half-curve you've just drawn into a 16-foot-long full curve. When the focal line of this true parabolic curve is pointed precisely at the sun, ALL the Incoming solar rays which strike the curve and are reflected will focus at f . . . and, believe me, it gets HOT.
OK. Now for some variations. Suppose, for instance, that you want to construct a parabolic reflector which has the same focal length (four feet) of the curve we've just drawn . . . but a smaller diameter. Easy. Starting at the base line fp, just measure up as far as you like, slice off the bottom of the drawing, and keep the rest. (If you cut your drawing as shown in Fig. 7, the remaining portion of the curve can be used to build a collector much like the one shown in Fig. 1a.)
It's a little more complicated-but not really much more difficult-to extend a parabolic "dish" out to some endless dimension . . . while still retaining a specific focal length.
I have yet to try this but someone with a little more time should be able to tackle this project in a day. Let me know how it goes.
Given: the focal length only. This can be any distance you want to work with and is nothing but the distance from the back of your planned curvature (see Fig. 2)-at the center-to the focus (the spot where the heat is to be directed). Let's say you've decided to use a focal length of four feet. A completed parabolic curve, across the focus, will have a diameter four times that focal length or, in this case, a diameter of 16 feet (4 X 4). A half-curve, then, will have a height of eight feet . . . and here's an easy way to seek that halfcurve:
Draw the focal line out to its required length on a large sheet of smooth paper (Kraft building paper is fine). Fig. 3 shows this focal length-Of-on the sheet of paper. It also shows a second line-fPdrawn at right angles to and twice as long (eight feet) as Of.
We know, of course, that the parabolic curve we're seeking will run, in some fashion, between points 0 and P. And, although we have a rough idea of the area in which that curve will fall, we're not yet sure of its exact course. So we're ready to get down to the finer definition of our curve, and we're going to begin that definition by drawing in a number of lines that are parallel to fP and spaced one inch apart (see Fig. 4). These lines need be put in only in the near vicinity of where our final curve must lie, but they should be measured and drawn accurately. You will, when finished with this step, have a total (counting iP) of 48 parallel lines drawn on your sheet of paper.
Now (Fig. 5) find an accurate straightedge that is at least twice as long as the focal length Of (or, to put it another way, at least as long as fP). Place the corner of one end of the straightedge precisely on point f and-taking care to keep that corner exactly on frotate the face of the straightedge from Of down to fP. As you touch each of the 48 parallel lines from the top down, add one inch to the length (48 inches) of Of and make a dot. (The first dot will be made on the first parallel line down and 49 inches from f, the second dot will be on the second parallel line down and 50 inches from f, etc.) Continue on until you scribe your last dot on the bottom line and 96 inches from f. The series of dots you've just made will define a parabolic curve with a four-foot focal length.
Now connect the dots by very accurately placing a flexible metal, plastic, or wooden strip across them and carefully draw. ing a line from O-cutting through all the scribed points in between-to P (Fig. 6).
This completed line is the curve you seek. It is NOT part of an ellipse. It is NOT part of a circle. It is NOT a hyperbolic. It is a true parabolic curve and you can now use it in the construction of your solar heating unit or cooker. By doubling the drawn curve back over the focal line (pick up point P, swing it over Of, and lay it down again an fP distance on the other side of Of), you can quickly and easily convert the eight-foot-long half-curve you've just drawn into a 16-foot-long full curve. When the focal line of this true parabolic curve is pointed precisely at the sun, ALL the Incoming solar rays which strike the curve and are reflected will focus at f . . . and, believe me, it gets HOT.
OK. Now for some variations. Suppose, for instance, that you want to construct a parabolic reflector which has the same focal length (four feet) of the curve we've just drawn . . . but a smaller diameter. Easy. Starting at the base line fp, just measure up as far as you like, slice off the bottom of the drawing, and keep the rest. (If you cut your drawing as shown in Fig. 7, the remaining portion of the curve can be used to build a collector much like the one shown in Fig. 1a.)
It's a little more complicated-but not really much more difficult-to extend a parabolic "dish" out to some endless dimension . . . while still retaining a specific focal length.
