Foam Cone For on axis polarimeter conventional feedlegs produce spurious signal Not good when trying to separate E and B In addition wish to keep mirrors warm to prevent snow accumulation Material Zotefoam PPA30 is legendary in the field for its transparency at these wavelengths Polypropylene copolymer expanded using dry nitrogen Used for cryostat windows... But manufactured only in 6x3' flat sheets How to make a 9' base diameter cone?