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

[Rick Fisher]

Admittedly, my question is ill-posed, but it sounds like the limitation of 
closely-spaced beams is not a show-stopper.  As a waypoint on the path 
toward many more Nyquist spaced beams it makes even more sense.  Thanks 
for the comments.

Rick

On Tue, 13 Apr 2010, Matt Morgan wrote:

> To add a third perspective, you might consider what expands the parameter 
> space the most (compared to other available instruments, that is). PALFA of 
> course is a feedhorn array, so I suspect their beam spacing was not a 
> strategy but a limitation. Mapping large areas with spaced beams is something 
> feedhorn arrays like PALFA can already do. Mapping smaller areas fast with 
> uniform sensitivity and tightly spaced beams is something only a PAF can do. 
> I'd emphasize what makes you unique.
>
> That's purely an engineer's perspective, but I think its the right way to 
> optimize a general-purpose instrument. Optimizing for a particular usage case 
> is more appropriate I think for application-specific or experiment-driven 
> instruments. So to me the question is, which type of instrument is this -- 
> general-purpose or application-specific?
>
> Matt
>
>
> Paul Demorest wrote:
>>  Rick,
>>
>>  At this level, I think it's kind of a matter of opinion.  I know we had a
>>  few emails expressing varying thoughts on the topic already ;)  However,
>>  survey speed is definitely the most commonly used metric for these
>>  comparisons.  We could always try it both ways, maximize survey speed vs
>>  maximize expected number of sources and see how different the answer is..
>>
>>  -Paul
>>
>>  On Tue, 13 Apr 2010, Rick Fisher wrote:
>>
>> 
>> >  Hi Paul,
>> > 
>> >  Do we want to maximize the integral of (G/T)^2, or should the pulsar 
>> >  population as a function of flux density be factored in?  If 
>> >  completeness is an issue, a steeper log(N)-log(S) curve would favor 
>> >  closer spacings since more pulsars would be discovered near the 
>> >  sensitivity limit.  This would be particularly true for new pulsars.
>> > 
>> >  I realize that PALFA uses interlaced pointings to fill in closer 
>> >  spacings, but their "47 pointings to cover one square degree" still 
>> >  implies -3 dB crossings (assuming 3.3 arcmin HPBW).
>> > 
>> >  Rick
>> > 
>> >  On Tue, 13 Apr 2010, Paul Demorest wrote:
>> > 
>> > 
>> > >  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
>> > > > 
>> > > 
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