[Pafgbt] save PAF cross-correlations rather than formed beam outputs

[Rick Fisher]

Brian, Karl,

In trying to understand the ASKAP data processing architecture, I'm 
beginning to understand the fundamental importance of saving the 
cross-products between array element outputs in our own PAF data 
processing.  In forming beams you throw away a lot of information in the 
array's field of view that can be recovered only by forming many beams 
with very close spacing (much closer than HPBW/2).  This has important 
consequences for the sensitivity to point sources, as in the search for 
pulsars.  Hence, I would suggest that the most important archived outputs 
from your signal processor are the cross-products rather than formed 
beams.  For a given data storage volume, there's more information in the 
cross-products than in the formed beam outputs.  In some respects, the 
"beam" concept is a holdover from a waveguide feed where there's only one 
output, and most of the information in the focal plane is reflected back 
into the sky.

Rick