The question that I'm asked most often is: "Why can't I use an old satellite dish?"
Answer: The reasons are many and varied. Let's start with a few of the highlights"
Windage - The typical C-band dish is mounted on a stalk. This puts the entire lateral windage loads onto a single point of stability. Three-legged stools are stable in all axis. Two-legged stools are stable in one plane or axis. A one-legged stool has no stability at all. Moving the edge of the dish even 1/2 inch will put the target off-center by at least that amount. Grab any dish edge, and it can be moved at least several inches. Wind and snow loads only aggravate those motions.
Shape - Any surface that takes parallel input and focuses it to a point must, due to the physical laws that rule our universe, be made into a surface that can be mathematically described as what we call a PARABOLA. The formula for plotting a parabola is: Z = (X^2 + Y^2) / (4 * F); where Z is the depth, X is the width, Y is the height, and F is the focal point (it's easy to make a spreadsheet that describes this math). This causes the resulting surface to be a continually changing plane that does NOT lend itself well to working by commonly available hand tools. Specifically, you CAN NOT "just swing a finishing tool from the focus" and hope to produce a functional shape.
Given the parabolic shape, such a dish is not easily covered with reflective material. Most such material comes in flat sheets. You CAN NOT make a flat sheet conform to a parabolic surface.
[NB: Technically, the suns rays are *not* exactly parallel! But it is so far away that for all intents and purposes, the rays can be dealt with as if they were parallel]
Surface - TANSTAAFL! (There Ain't No Such Thing As A Free Lunch!) You can't get something for nothing. Anything we do that's less than 100% reflective begins the process of robbing you of energy. Simply reflecting light back towards itself--even if there was such a thing as a 100% reflector--uses up some energy. The best materials available today are probably quartz covered electro-deposited aluminum, which is about 98% reflective. Mylar and other surfaces are less reflective and suffer from dimensional errors (they're two-dimensional, and a parabola is 3D).
Using glass mirror segments adds a large weight penalty and requires very accurate alignment (hard to do well). The use of light or reflective paints is far down on the reflectivity index scale. Even bright shiny paints yield less than 30% of the energy that impinges the reflector--and that light is biased towards the shorter wavelengths.
Aiming - A satellite dish is intended to track satellites located in synchronous orbit over the equator of the earth. They usually extend in an arc, called the "Clark Arc" from east to west. They will always be that location. The sun also traverses an arc, but seasonal variations, known as declination, will be a daily variation. Although they can be adjusted, satellite dishes are not designed to be adjusted on a daily basis for this change--certainly not on an automatic basis without considerable re-engineering of the mount and pivot adapter.
Dangerous - A dish is heavy. Loaded with mirrors and the almost certainly necessary counterbalances, it becomes very heavy. This alone is already dangerous. But an even greater danger is the concentrator beam. Skin and paper flash into flame instantly, wood in seconds, even thin metal can be melted in moments. Able to cause sever burns on exposed flesh in fractions of a second, the beam can cause blindness, fire, and destruction of anything that the beam touches in an unprotected manner.