Summary of chop-nod analysis telecon of Dec 3, 2008

Mike, Giles, Lero, Tristan, Hiroko, John

_____________________________________________________________

Mike's analysis of NGC 1333 IRAS 4A:

Mike went through his Dec. 2 posting (please refer to this posting while 
reading my summary)

Part 1 of Mike's posting:

statistical weight determined from minimum error in Q/U, across the map.

for the two bins, these errors are balanced to within 10%.  the first bin 
has about twice as many files

threshhold is "nominal 3 sigma"

we agree with Mike that the Part 1 analysis is encouraging but not 
definitive.


Part 2 of Mike's posting:

crops are 6 sharpcombine pixels in radius

this analysis seems rigorous, and the result seems important!  i.e., we do 
seem to have an extended uniform field in this region, parallel to the 
SMA-map's magnetic axis!

the displayed map may be a final, publishable result.  we agreed to give 
ourselves a couple of days to think about this to be sure we're not 
missing anything.

there was some discussion of whether we should try to see if the lack of 
vectors at the peak of 4 B is due to lower polarization values there.  
Mike notes that this question is not central to his interpretation, so it 
should be kept in mind that this is a side-issue.  Giles suggests using 
the feature of polsharp that can plot 2-sigma upper limits given a 
threshold, and he suggests setting the threshold to around 1.5 percent.  
If circles appear, this will be evidence of a drop in P toward the core.  
John suggests a more rigorous method which is to plot the average debiased 
P as a function of radius from 4 B.  we agreed to try the first suggestion 
first.  if it does not work, no point trying the second one.

Giles notes that Mike should correct for polarization efficiency, which is 
given in Li et al. (2008).  (I think its 93%).  John notes that polsharp 
can do this via the -eff flag.

Two additional things for Mike to document in a third memo:

- list discarded files, provide explanation for decision to discard

- list all flags he is using for sharpinteg and sharpcombine, for future 
reference.

_____________________________________________________________