Set your focuser to a certain position, any position, say midway in the travel and measure that distance with a digital calipers from a fixed point to the end of the draw tube.
Ask your focuser to move out 1000 steps. Using the digital calipers again, measure the distance it has moved, from the same fixed point and end of drawtube. Divide that distance by 1000 to give you the answer, which will be in the micron range.
Mine, for example, with a Feathertouch FT3545 and a Sharp Sky Pro focus unit moves about 7.5mm for 1000 steps, which is 0.0075 mm per step, or 7.5 microns per step.
You can do this in reverse to calculate backlash in your focuser drive system. This is great for rack and pinion units to set backlash compensation and helps to garner some info on slippage in Crayford systems.
