SL1-3 120 Spoke design, giving 3 deg rotation. Clearance at centre is 0.20mm, giving a minimum spoke width of 0.004mm at the centre SL1-2 72 Spoke design, giving 5 deg rotation. Clearance at centre is 0.15mm, giving a minimum spoke width of 0.006mm at the centre SL1-1 36 Spoke design, giving 10 deg rotation. The outer diemater of the pattern is 2" / 50mm. There are 4 different patterns to choose from, each having a different spoke count. Material: Chrome on White Glass, 1.25mm thick, available as 4" (100mm). 
Material: Chrome on Transparent Soda Lime Glass, 1.5mm thick, available as 3" (75mm). 
It consists of a pattern of bright "spokes" on a dark background that radiate from a common center and become wider as they get further from it. It is used to identify the aberration of the optical system - determining the resolution of an optical system by noting how close to the center of the pattern an optical system is able to resolve adjacent bars A Siemens star is a device used to test the resolution of an Optical instrument.
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