 The most common choice is to change the polar angle ϑ around the axis that is parallel to the slit, and adjust the tilt τ or azimuth φ so that emission from a particular region of the Brillouin zone can be reached.The measured electrons (green rays in images below) have these momentum components in the reference frame of the analyzer formula_15, where formula_16.The reference frame of the sample is rotated around the y axis by ϑ (formula_17 there has components formula_18), then tilted around x by τ, resulting in formula_19.