Per the owners manual and by actual measurement, the "modifier disk" ID is 35mm. This reduces the resolution by about 30% so that the fine detail including the shimmer/glint effects of the mirage are less visable (but then so is the target detail). Also note that use of the modifier disk also reduces the exit pupil area and light transmission by about 55% (but since the mirage tends to be bad only in very bright conditions this is not a problem).
My preliminary testing suggests that in very bad mirage with rapid moving details, using the modifier disk may make an otherwise useless image adequate for reasonable aiming. The loss of resolution limitss 6mm bullet hole resolution to about 420 yards versus 620 yards without the modifier disk.