[Bicep2-list] effects capable of causing T->P

Hi Chris, SPT are worrying about their T->P in their sims. In a pair differencing analysis what effects can and can't cause T->P? A/B beam mismatch of course can. I think non-orthogonal pair angles can't - that is only an additional calibration of the pair diff and getting it systematically or randomly wrong across pairs can only mix E<->B. But what about polarization efficiency (epsilon)? If this differs between A and B of pairs systematically or randomly that will leak T-pol in the timestream. And our analysis would not fix this even if we knew the per detector efficiencies perfectly? However this effect is indistinguishable from relgain and would therefore deproject out. Where is this possibility mentioned in your paper? Sec 9.7 talks about "Cross-polar response" but seems to say that this causes only E->B. I note that what I have written above is specifically contradicted in our Instrument Paper where it says: "The crosspolar response enters the analysis only as a small adjustment to the overall gain of the E and B polarization, but cannot create any false B-mode signal." That's certainly true so long as chi is equal for A and B of each pair. But is it really true when is differs between A and B? Clem -- ********************************************************************** Clem Pryke - Associate Professor - Physics University of Minnesota Room 318 Physics and Nanotechnology Building 115 Union Street SE, Minneapolis MN 55455 Tel: 612-624-7578 Fax: 612-624-4578 email: pryke(a)physics.umn.edu **********************************************************************