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

[Paul Demorest]

Hi Rick,

There is a nice picture of the ALFA beam shape at the top of this page:

http://www2.naic.edu/alfa/gen_info/info_obs.shtml

maybe you've seen this already, but the beams are widely spaced, crossing 
at about -6dB.  They get -3dB spacing in the actual survey by filling in 
the holes with multiple pointings.

To determine an optimal beam spacing (for a given array+telescope) I think 
we want to maximize the integral of (G/T)^2 over the field of view.  That 
should result in the highest possible survey speed.  Then we should check 
the figures we used for the comparison with PMB and Efflesburg surveys and 
see if any claims need to be revised.

I'd guess the survey speed goes down by a factor of ~2 vs ideal 
fully-spaced beams, but maybe the optimization could make this only ~1.5 
or so?

-Paul

On Tue, 13 Apr 2010, Rick Fisher wrote:

> 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