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

[Scott Ransom]

For L-band pulsar searches, you would want channels which are a fraction 
of a MHz wide.  Say, 0.25-0.5 MHz at the most.

Scott

On 05/20/2013 03:28 PM, Rick Fisher wrote:
> Brian,
>
> Quite right.  You'd need to anticipate the spectral resolution needed
> for any possble application of the data and provide the data
> storage/bandwidth that goes with it.  We should see what's typical for
> pulsar searches.
>
> Rick
>
> On Mon, 20 May 2013, Brian Jeffs wrote:
>
>> Rick,
>> I agree that you have much more flexibility to try different beamformer
>> designs, detection algorithms, interference mitigation techniques,
>> superresolution, calibration correction, etc. if you store and operate on
>> the accumulated cross products (correlation matrices).  However, you
>> give up
>> the ability to do fine resolution spectral processing.  You are stuck
>> with
>> the coarseness of the correlator's frequency channelization.  I don't
>> know
>> how problematic this is for some applications, such as pulsar searches,
>> where fine spectral resolution may be needed.
>>
>> Brian
>>
>> On May 20, 2013, at 6:38 AM, Anish Roshi wrote:
>>
>>
>> Yes indeed. We can form images with beams with different optimization
>> if the correlations are recorded.
>> Anish
>>
>>
>> On Sun, May 19, 2013 at 9:57 AM, Rick Fisher <rfisher at nrao.edu> wrote:
>>       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
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-- 
Scott M. Ransom            Address:  NRAO
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