10 seconds of arc equals approximately .0005"/10" or .0006"/12". A second of arc is 1/3600 degrees. The sine of 1 second of arc is 4.848 x 10^-6 (.000004848) That times 10" is .00004848". For very small angles, the sine, tangent, and arc in radians (2π radians equals 360º) are close to equal so 10 seconds of arc = .00048"/10".