[evlatests] How Fast Can We Step?

[Rick Perley]

     I think the basic argument is right -- the unrecoverable loss is 
the delay smearing within a single channel.  Since this is the same for 
all the higher frequencies, one can imagine a phase/delay stepping 
interval which is actually larger than the primary beamwidth (!).

     But, by stepping faster, (or scanning slower), one could average 
over frequency, and lower the computing costs.

     Rick

On 03/09/2015 03:38 PM, Steven T. Myers wrote:
>
> IMO this is probably fine. I'm not sure what the driver is for 
> resetting the
> delay centers on a fast timescale. I think the delay center can be set 
> (and
> fixed) to within a distance theta of the primary beam center given by 
> the usual
> bandwidth smearing value (Bridle lecture) - essentially
>
>   theta < beta*(Freq/DeltaFreq)*SynBeam_FWHM
>         = beta*(Freq/DeltaFreq)*(D/Bmax)*PB_FWHM
>
> where DeltaFreq is the channel width (e.g. 2MHz) and where 
> SynBeam_FWHM is the synthesized beam, or equivalently in terms of 
> D=25m and max baseline Bmax (36.4km for A config) and the primary beam 
> PB_FWHM.
> The usual error limits apply (e.g. no more than beta=0.33 for a 5% 
> error).
>
> Example: 2MHz chans at 3 GHz in Aconfig :
>
>   d/PB_FWHM < beta*(3000/2)*(25/36400) = 1.03*beta
>
> so at S-band you would want to keep the phase center within 1/3 of a 
> PB of a
> pointing center (~5'). Note at higher frequencies this is more 
> lenient, and you
> will be dominated by the error losses by the PB smearing over a single 
> integration (regardless of how you set phase centers).
>
> Example: 15GHz with 2MHz chans, A-config so d/PB_FWHM < 5.1*beta so 
> you can step
> phase centers every beam or so without a problem.
>
> Note - the main argument for stepping phase center each integration or 
> two is to
> allow imaging using the approximation that FieldCenter = 
> PointingCenter w/o creating and using the POINTING table. But I still 
> say that if you step phase centers at 0.33 beams and integrations 
> every 0.1 beams you are ok using nstep=3
> in the OPT for S-band and L-band, and longer above this.
>
> Barry and Rick - am I missing an effect here?
>
> -Steve
>
> On Mon, 9 Mar 2015, Rick Perley wrote:
>
>>    With the current system, phase/delay stepping is limited to about 
>> 0.7 Hz (a step every 1.5 seconds). This is much to slow for sky 
>> surveys done at high frequencies, where the primary beam crossing 
>> time can be less than 2 seconds.
>>
>>    The dominant limitation is in the station board.  Bruce made a 
>> change to permit faster stepping.
>>
>>    I ran a short test this morning, using 3C48, and an antenna motion 
>> of 1 arcminute/second (4 X 'sidereal').  Dump time was 0.1 second.  
>> The phase/delay center was moved at four different rates: 1.5 
>> seconds, 1.0 seconds, 0.75 seconds, 0.5 seconds.  The scans were in 
>> RA, and utilized the 'back-and-forth' method (for future analysis), 
>> going from first null to first null.
>>
>>    Results:
>>
>>     1) The three slowest rates are all good.  The system easily keeps 
>> up.
>>     2) The fastest rate (2 Hz) gave a mixed result:
>>           a)  For the W to E motion, the system could not quite keep 
>> up -- many of the initial delay/phase models were incorrect.
>>           b)  For the E to W motion, all was fine.  No incorrect 
>> calculations.
>>
>>    Bruce says he can speed it up a little more.  Shall we try?
>>
>>    Rick
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>
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> |:| Steven T. Myers                      |:| Astronomer               |:|
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