[Pafgbt] PAF beam spacing on the GBT for a pulsar survey

[Rick Fisher]

In writing an MRI proposal for the construction of a PAF for the GBT we've 
run into a conundrum that we should have anticipated much earlier.  Any 
thoughts on the following will be appreciated.

The problem is that the "plate scale" (linear feed offset distance per 
angular beam offset in HPBWs) is roughly twice as large on the GBT as it 
is on the 20-meter telescope where we have done our PAF tests.  Hence, our 
19-element array can accommodate only half as many HPBW offsets as the 
20-meter before the focal spot loses significant power off the edge of the 
array.  The attached plot calculated by Karl Warnick shows 
Tsys/aperture_efficiency as a function of beam offset for the GBT.  Going 
to 37 elements (or even more) is something that we clearly want to do in 
the long run, but it's a big step up in all aspects of the array system 
(Dewar size, number of receiver channels, real-time beamformer size, 
etc.).

I am wondering if the best strategy for this proposal will be to stay with 
the 19-element array as the next logical step and to form 7 beams on the 
GBT with the 6 outer beams spaced about 0.6 HPBW from the center beam. 
This is essentially Nyquist spacing, but it is different from the strategy 
used in the Arecibo PALFA survey.

The PALFA web site says that they are using 47, 7-beam pointings to cover 
one square degree of sky.  This would imply that their beams cross at 
about the 3-dB level, which means that most of the sky is covered with 
sensitivity considerably below peak beam sensitivity.  If we use 0.5 or 
0.6 HPBW spacing we won't cover as much sky in beam areas, but the 
relative average sensitivity within this area will be considerably higher. 
Does this sound like a reasonable enough trade-off to justify putting a 
19-element array on the GBT as the first science instrument?

Rick
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