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Old 14-01-2018, 07:31 PM
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OzEclipse (Joe Cali)
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Join Date: Jun 2012
Location: Young Hilltops LGA, Australia
Posts: 1,177
Barry,

Are you measuring the depth of the curve on the mirror or by measuring the edge tile thickness on the tool? If the latter, this is not a reliable method.

If you are using the mirror curve depth across the whole disk, I have written you an excel spreadsheet to do the mirror curve depth to focal length calculation. You have to input the diameter of the mirror in mm and the curve depth in mm, it will calculate the rest for you.

Personally, I prefer the sun method. As long as it is a sunny day, tack a piece of white card on the end of a long dowel rod. Longer than the focal length.

Slosh water on the ground mirror surface, point it and the dowel at the sun and mark where the sun comes to focus. Measure the distance to the white card and that is the focal length. You can also put distance marks on the dowel to simplify measurements. It's simple and bulletproof.


Joe
Attached Files
File Type: zip mirror curve depth to focal length calculation.xlsx.zip (38.2 KB, 96 views)
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