# Thread: Calculating width of parabolic reflector sheets

I made a full size mockup using a 25cm focal length parabola and a 60cm full sheet size (30cm for 1/2 of the parabola) and checked the formula prediction (not good I'm afraid) using a piece of wire 30cm long matched to the parabola outline. I've tried to be accurate to about a millimeter. The formula predicts 22.36cm but the actual result is about 28.75cm

Attachment 1943

George Plhak
It's me again.

I used the values to simplify the more genral result of my previous posts. And suddenly my computer works fine ...

2. Computer glich here Pappas which caused me not to see your last message. So please ignore what I posted below which I've left in place because of the background info. I will study the revised program thanks! George

Hello Pappas and thanks.

I always knew that I could make a model and find a measurement but I still want to be able to calculate the intersection of the sheet with the parabolic curve. It seems to me that a circular reflector is not the same as a parabola and although the calculation comes close with f=25, will it match as closely at other values? My current design uses f=10cm for example.

My intent is to try a series of curves to optimize the weight balance of the whole solar reflector assembly. If it is balanced, less energy is required to move it around to follow the sun hence the system is more efficient. The motor can be very small.

So the calculation of the intercept is part of the calculation of the weight distribution.

Does anyone else have an idea how this intercept might be calculated?

Many thanks

George Plhak
some of my solar work

3. Pappas,

Can I please ask you to turn the formula inside out, so that I am solving for a? It would be a lot more useful to me here. My computer does not know how to get from step #3 to step #4. Thanks. George

Pappas,

Can I please ask you to turn the formula inside out, so that I am solving for a? It would be a lot more useful to me here. My computer does not know how to get from step #3 to step #4. Thanks. George
I think it would be best for you to get a spread-sheet where you can vary the values of focal length and pick the appropriate value for the width of the reflector arms and the length of the reflective sheet.

If it isn't possible to attach an Excel-file here I'll be going to pm you.

So I'll try if it is possible to send you the file by pm.

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