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Re: Focal Length: How To Detemine For A Parabolic Mirror

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Posted by Robert May on October 29, 1999 20:56:34 UTC

Finding the Focal length is not all that hard. Take the mirror outside and place it in the sun placing it so that the reflection of the sun goes back to approximatly where the sun is. Take a piece of wood or paper and find the image of the sun and you will find a place where the sun is pretty well focused. Be sure to be wearing your sunglasses (two sets will be even better) as the image will be very bright on the paper or wood. Use a dark colored paper if desired but remember that the sun may start a fire if you leave the paper there too long. That's one way. The second way is to look at the published F (probably about F6 or so)ratio and multiply that by the diameter of the mirror (8") and if it's a F6 mirror then you will have 8"*F6=48" which is the focal length. This is pretty close as the actual focal length of a mirror is rarely exactly the dimension that is given. Your mirror may be as much as an inch (or more) off in the F ratio and still be the number. Be carefull with the sun method as it is possible to destroy an eye if you look directly at the sun through the primary mirror in a very short time. The light will be concetrated about 5000 times more than looking directly at the sun and will burn very fast.

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