[evlatests] How Fast Can We Step?

[Steven T. Myers]

Fair enough - pick your poison.

Would have to test this with CBE averaging (which is different 
than the intrinsic correlator BW).

On Mon, 9 Mar 2015, Rick Perley wrote:

>    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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>> > 
>>
>>  -------------------------------------------------------------------------
>> | : |  Steven T. Myers                      |:| Astronomer 
>> | : |  |:|
>> | : |  National Radio Astronomy Observatory |:|  Ph:  (575) 835-7294 
>> | : | 
>> | : |  P.O. Box O, Socorro, NM 87801        |:|  FAX: (575) 835-7027 
>> | : | 
>> | : |  http://www.aoc.nrao.edu/~smyers      |:| smyers at nrao.edu 
>> | : |  |:|
>>  -------------------------------------------------------------------------
>

-------------------------------------------------------------------------
|:| Steven T. Myers                      |:|  Astronomer               |:|
|:| National Radio Astronomy Observatory |:|  Ph:  (575) 835-7294      |:|
|:| P.O. Box O, Socorro, NM 87801        |:|  FAX: (575) 835-7027      |:|
|:| http://www.aoc.nrao.edu/~smyers      |:|  smyers at nrao.edu          |:|
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