Walt, one last comment on this post: In your section 3 list of systematics which may enter differently in Rx1 compared to B2 or other Keck receivers, I would add "pattern of DK angle coverage on the sky" for which Sarah's post is one reference: http://bicep.caltech.edu/~spuder/keck_analysis_logbook/analysis/20130401_pr… I know of no complete theory here (which is true for many of your other bullet points as well), but note that Rx1 is unique among B2 and other Keck receivers in that its Q map is formed from nearly equal contributions of A-B and B-A. (The next closest in this regard is Rx0...which is the second worst spectral jack.) This may be nothing, but I think it is worth thinking about. J On 10/21/13 6:39 PM, Walt Ogburn wrote: -- ___________________________________________________________________ John Kovac jmkovac(a)cfa.harvard.edu Associate Professor, Harvard University Astronomy Department 160 Concord Ave rm 310, Cambridge MA 02138, 617-496-0611