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The 2302 attachment fits over the objective end of our alignment telescopes and converts the instrument into an angle reading telescope.  Normally, the micrometers on an alignment telescope allow you to measure the lateral displacement of the line of sight.  But with this attachment in place, the micrometers read in arcseconds when the instrument is focused at infinity.  In fact, they read directly to one arcsecond over a range of 50 arcseconds.  This effectively converts the instrument into an autocollimator. Maximum usable distance for autocollimation is approximately 30’ (9.14 m).  Infinity setting on focusing knob is approximately 17’ (5.18 m).