Ugly Stuff

Focus shifts due to cone expansion/contraction and primary has small warp
Leads to changing beam ellipticity
Measured, modelled and included in sims
Fixed for 2006 using corrective secondary (still varying width but always round)

A/B beams are not identical - center points offset
Effect measured and included in sims

Ground pickup is significant
Very small in polarization but detectable
Can for sure remove using field difference
May not need to take this hit with better filtering