Parabolic dishes to a point ONLY when they are pointed EXACTLY at the sun. But the sun moves and that point quickly spreads all over the place unless your are watching over your solar cooker all the time.
My conclusion after a couple of months of working with a prototype is that a parabolic dish is the wrong shape!
This instructable is about the shape I found to replace the parabola and how you can use it to make a simple reliable "Compound" parabolic solar cooker. Mine will focus the heat on your cooking pot for up to 3 hours without moving it!
http://www.youtube.com/watch?v=uX9Z-nsUHiA Here is a video link to help too.
You can skip to the 7th page if you just want to see a template for the cooker.
Use the template to make "petals" from any sheet material.
Then tape the template together with masking tape or duct tape and stick your reflective coating on it.
Finally a use for corrugated election signs when the election is over!
Step 1: Problems with parabolas!
The tracking solar accumulator
It was a solar cooker that consisted of a pot of material and a parabolic reflector that turned slowly to track the sun on an equatorial mount. This parabolic reflector was not the bottom of a parabolic bowl as most reflectors are but rather, a section from the side of the bowl.
I did this daily tracking with a low tech tracker that I made.
It was just an adjustable waterclock (also on instructables)
The sun moves higher and lower in the sky with the seasons and I intended to do this adjustment twice a week to keep my dish pointed exactly at the sun and the light exactly on my pot of food. I had to make a "mechanical mathematician" for making my section of a parabolic dish, and I had to make a low tech tracker too.
One problem was that every time I adjusted for the sun's change for the season, the center of gravity changed and I had to readjust the balance of the dish. Time consuming!
Another is that parabolic dishes are precision devices.
If you aim slightly wrong at the sun or if your tracking mechanism is slightly inaccurate, or even if you bump your dish a few times (as I did) you will not get the hoped for results.
This are some of the reason that I needed a more forgiving shape than the parabola!