The invention relates to a method for assembling two or more mirror plate stacks into a rigid unit in turn comprising a plurality of mirror plates and a base plate onto which the plurality of mirror plates are stacked. To improve assembly accuracy of the mirror plates, a base plate is provided with a first mirror plate mounted thereto, a handling tool is provided with a second mirror plate and a spacer is provided to a first surface of the second mirror plate and positioned to align the second mirror plate with the first mirror plate. This alignment is based on a measured position and shape of the first mirror plate to compensate for a deviation from a pre-defined position and shape. The plates are then attached to each other with the spacer; the handling tool is removed and the position and shape of the second mirror plate is measured.