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RE: Help Again,

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Posted by Bill on August 3, 2000 13:55:33 UTC

One way to measure focal length is take the rough ground mirror outside on a sunny day, spray it with water, and get the mirror to focus the sun`s disk on a sheet of paper or on the side of your house. The distance between the mirror and focused sun image is the focal length of the mirror. I have used this method and can measure the focal length to within a few inches or less.

Be careful when using this method. The sun image is very bright and will cause eye damage if the image is directed into someones eyes.

I also agree that you should start with a longer focal length (F7 or F8). It`s easier to figure the mirror into a nice parabola.

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