A scanning system having dual mounting heads (98,102) that define a measurement gap, which has a web entry end and a web exit end that is downstream from the web entry end, and which is configured to accommodate a continuous web that has a first side and a second side and that travels in a downstream direction, which comprises: (a) a first elongated member that extends along a first direction wherein the first elongated member supports a first mobile mounting head that is disposed adjacent to the first side of the web, the first mobile mounting head including a first operative surface facing the first side of the web; (b) means for driving the first mobile mounting head that includes a first belt system; (c) a second elongated member that extends along a second direction that is parallel to the first direction wherein the second elongated member supports a second mobile mounting head that is disposed adjacent to the second side of the web, the second mounting head including a second operative surface facing the second side of the web, wherein the first operative surface and the second operative surface define the measurement gap through which the continuous web travels (d) means for driving the second mobile mounting head that includes a second belt system such that movement of the first mobile head is synchronized with movement of the second mobile mounting head with respect to speed and direction so that they are aligned with each other; (e) an alignment sensor (128,138) that determines misalignment between the first mobile mounting head and the second mobile mounting head and generates error signals representing a difference in alignment between the first and second mobile heads (98,102) as the first and second mobile heads (98,102) scan back and forth along a translation axis; and (f) actuator means (37,47,226) for regulating relative positions of the first mobile mounting head and/or the second mobile mounting head along a translation axis direction, wherein the actuator means is controlled by the error signals and adjusts the tension in the first belt system and/or tension in the second belt system. (a) a first elongated member that extends along a first direction wherein the first elongated member supports a first mobile mounting head that is disposed adjacent to the first side of the web, the first mobile mounting head including a first operative surface facing the first side of the web; (b) means for driving the first mobile mounting head that includes a first belt system; (c) a second elongated member that extends along a second direction that is parallel to the first direction wherein the second elongated member supports a second mobile mounting head that is disposed adjacent to the second side of the web, the second mounting head including a second operative surface facing the second side of the web, wherein the first operative surface and the second operative surface define the measurement gap through which the continuous web travels (d) means for driving the second mobile mounting head that includes a second belt system such that movement of the first mobile head is synchronized with movement of the second mobile mounting head with respect to speed and direction so that they are aligned with each other; (e) an alignment sensor (128,138) that determines misalignment between the first mobile mounting head and the second mobile mounting head and generates error signals representing a difference in alignment between the first and second mobile heads (98,102) as the first and second mobile heads (98,102) scan back and forth along a translation axis; and (f) actuator means (37,47,226) for regulating relative positions of the first mobile mounting head and/or the second mobile mounting head along a translation axis direction, wherein the actuator means is controlled by the error signals and adjusts the tension in the first belt system and/or tension in the second belt system.