On Sat, 20 Aug 2011 22:45:27 -0600, Chris L Peterson
wrote:
tan(12.7 arcmin / 2) = tan(0.1058 deg) = 0.0018. Times 2 times 5

That was än unnessesarily complex way of calculating what really
requires only one multiply and one divide;

25 microns / 6.76 mm * 5 m = 18 mm