A pie tin, a sheet of aluminum foil, and a little glue to make a spherical reflector. I call it a Reflexor.
I first made one of these around 1980 and I am unable to recall the circumstance of my inspiration. It's quite easy to make and the results can be quite good depending on the care you put into it and the materials used.
This demo uses household aluminum foil which has the drawback of being unpolished resulting in some difusion of the sunlight.
I once made one with the blank side of an aluminized mylar balloon and it proved to be a very good reflctor. I remember that I was able to reflect a near perfect inverted image from the window onto the wall beside the window.
Caution is advised! Focused sunlight is VERY intense, and this is so lighweight, you can flash yourself while handling.
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Step 1: Puncture the Pie Tin.
I had originally made this many years ago with a home baking cake tin, but what I had on hand, and sufficient for this demonstration, was a Marie Callender's pie tin. Adding a small hole with a straight pin was not difficult. I hammered it in with a spoon.
Step 2: Lay the Foil Out.
If you want to be perfectionist, you can stretch the foil out a bit with tape to make it slightly taut.
For this demonstration, I just wanted to get it done quick for photos.
I did take care to tear the foil carefully to avoid wrinkles.
Step 3: The Adhesive.
I used E6000 for this, but later found that it didn't cure well between the tin and the foil.
It worked well enough, but I recommend epoxy, or some other adhesive that doesn't require evaporative curing.
Put it all the way around. If you use a 5 minute epoxy, you want to get it all on in a couple of minutes.
Alternatively, you could put invert the tin on the foil first, then epoxy the seam.
Step 4: Invert the Tin.
Just place the tin on the foil. If you use the alternative suggestion from the last step, add epoxy now.
Step 5: Press and Cure.
Weight the tin against the foil while the adhesive cures for a good, smooth seal.
Step 6: Trim the Foil.
It looks much better this way, and the excess foil won't get in the way.
Step 7: Prepare the Valve/seal.
The first time I did this, i used black plastic electricians tape and use my mouth to suck air out and close the seal.
Place a piece of tape over the pin hole so the hole is not sealed shut, because next...
Step 8: Vacuum Pump
This is a wine bottle sealer which uses a silicon rubber cork replacement and a hand vacuum pump to pull air out of the wine bottle that you didn't finish drinking.
Step 9: Evacuate and Seal.
The cork is placed in the pump then placed over the tape, being careful not to seal the tape, and pull a few strokes on the pump to remove some air from the tin. The MC pie tine I used actually began to collapse a bit. When done pumping, slide the cork to the side to seal the tape around the hole.
Step 10: Take a Look.
You should see a distinct curvature in the foil. Notice the grain on the foil.
If you can get polished foil, you'll get much better results.
I once used the plain side of an aluminized mylar balloon, it worked beautifully and could actually cast images from the outside onto a wall by a window. Mylar leaks air more than aluminum.
Step 11: Check the Focus.
Be Careful. The focal area is intensely bright and hot.
I did this several years back and obtained such a tight focus that a piece of wood I held in the focal point burst into flame in about 15 seconds.
I have a snow disc hanging out in the garage.
When I get the time.