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

[Matt Morgan]

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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-- 
Dr. Matthew A. Morgan
Scientist/Research Engineer

National Radio Astronomy Observatory
1180 Boxwood Estate Rd.
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matt.morgan at nrao.edu