The secondary mirror M2 is an on-axis ellipsoid, giving an f/12.6 F2 focus. M2 is defined as the aperture stop of the whole optical system and has a diameter of ~800mm. This implies that is in the right place to correct wavefront errors in the telescope pupil.
It is part of the current evaluation to figure out if, besides the M2 adaptive optics mode, there should be an uncoupled M2 active mode. This M2 active mode could control the position of M2 along 5 degrees of freedom: one for focusing, two for decentering, and two for tilting. The active optics should always be operative when the adaptive optics mode is used. Besides atmospheric turbulence, the performance of the telescope is affected by disturbances like gravity induced deflections, thermal effects and wind buffeting which cause a modification of the relative position between M1 and M2.
Those low frequency effects should be compensated by the ASM. In particular, M2 decentering is used to cancel coma due to M1-M2 lateral misalignment, mainly caused by structural flexibility or by residual optical adjustment errors. Besides, tip- tilt and M2 focus corrections can cancel tracking error sources derived from the alt-azimuth control system, gravity deflection of the telescope structure, wind buffeting and atmospheric image motion, which do not have a specified direction and vary continuously.