[evla-sw-discuss] pulsar data

[Brent Carlson]

Well, I thought I would wade into this discussion...

1. There is indeed one pulsar timer per baseband pair, meaning 4 timers 
in total.  Each of these can be set for a different pulsar ephemeris.  
The restriction is that all of the correlations happening within one 
sub-band correlator *must* use the same ephemeris.  With 16 sub-band 
correlators, this leaves plenty of flexibility to track 4 pulsars 
simultaneously.

2. Pulsar gating and pulsar phase binning are indeed two different 
modes, and operate as Michael describes.  Given the pulsar phasing 
capability, I can't imagine why one would want to use gating...but it is 
there with the ability to generate different gates on different 
sub-bands to track dispersion.

3. Officially, the correlator does not support "bunching" of phase bins 
at some particular phase of the pulsar.  However, the signaling to 
generate phase bins is controlled by software, and so this is entirely 
possible...but you should decide if this is indeed an important mode 
reasonably soon so the real-time s/w people can program in the capability.

4. With my limited experience, having to get the pulse phase correct to 
track the pulsar seems like an nightmare to me.  I think a better long 
term solution, if bunching of phase bins is required and/or pulsar 
gating is required, is a feedback mechanism whereby for the first few 
minutes (or however long it takes to get a detection), phase binning is 
used with even distribution of bins across the period to find the epoch, 
and then that epoch is immediately subsequently applied for phase bin 
bunching or gating.

5. The 65536 phase bins mode is possible, however due to CBE and output 
bandwidth limitations the time resolution would be on the order of 100 
msec (for all spectral channels...decreasing with fewer).

Brent

Michael Rupen wrote:

>Hi Walter,
>   nice writeup!  A few comments/questions...
>
>  
>
>>This message describes my best guess as to the astronomers interface to
>>the EVLA pulsar modes with the WIDAR correlator.  At the moment it is
>>mainly for the benefit of David Harland who is working on the obs prep
>>tool.  Any comments are welcome.
>>
>>The pulsar gating hardware:  (EVLA project book 8.2.14)
>>~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>>WIDAR will support one pulsar timer per 2 GHz baseband (4 total).  Each
>>timer is driven by a polynomial which returns pulse phase as a function of
>>the current time (UTC).  The pulse phase is in the range [0,1).  Each of
>>the 16 subbands per IF can operate with a different fixed delay relative
>>to the IF timer.  Up to 1000 time bins can be accumulated.  Each time bin
>>has a start time and a stop time.  Start and stop times are specified by a
>>"pulse phase" ranging between 0 and 1.  I believe that just as for
>>frequency channels the number of pulsar bins can be different for each
>>subband.  Note: pulsars' spin periods range from 1.5 ms to 8.5 sec.
>>    
>>
>
>I thought we had one timer per each of *eight* 2 GHz basebands (four BB pairs, 
>2 pol'ns each).
>
>I would really like to stick with "baseband" rather than "IF", or at least not 
>mix the two.  The VLA's IF modes confuse every observer I've ever met.
>
>I'm confused as to gating vs. time bins -- I thought these were separate
>beasties.  Gating just means "correlate only at these times, and store one 
>output stream"; time binning means more output streams.  I'm not clear on the 
>"beating" between the gating and the phase bins.
>
>A couple additions:
>* WIDAR can produce 2000 time bins if you don't mind some dead time while
>   downloading.  The CBE purports to be able to provide up to 65.536
>   bins per baseline, through software accumulation.
>
>* You can dump as many spectral channels as you want (apart from the
>   data output rate), at a cost in the narrowness of the time bins.  If you
>   dump all channels, you get ~200microsec; if you dump only
>   64/sub-band/baseline, you get ~15 microsec.
>
>I will check on the number of bins vs. sub-band question.  It's all
>in DUMPTRIG so I believe you can do this, but I'll check to be sure.
>
>
>  
>
>>Common use cases:
>>~~~~~~~~~~~~~~~~~
>>    
>>
>
>Am I right in thinking pulsar monitoring requires phasing up the array for 
>maximum sensitivity?  WIDAR will be delivered with 8 digitally phased 
>sub-bands for a total BW of 1 GHz.  WIDAR can phase up all 18 sub-bands of 
>every baseband, but we won't initially have that hardware.  Antennas may be 
>assigned to multiple phased sub-arrays; phased sub-arrays may be defined 
>differently on different sub-bands.  This implies for instance that one has to 
>have accurate positions which may be different for the different sub-arrays, 
>and for the different sub-bands.
>
>  
>
>>1. Monitoring a single pulsar
>>
>>The same polynomial will drive all pulsar timers.  A different delay for
>>each subband will be added to account for pulsar dispersion delay (a
>>frequency dependence in pulse arrival time due to propagation through
>>ionized intersteller medium).  This delay is parameterized by a single
>>parameter called the dispersion measure.
>>    
>>
>
>  
>
>>The user will choose a number of phase bins <= 1000 and will specify start
>>and stop times for each.  In the case of > 100 channels, it is likely that
>>the bins will be uniformly distributed across [0,1) as would be used in
>>"phase-ignorant" observations where the observer does not know the pulse
>>phase prior to observation.
>>
>>If the pulse phase is known in advance the astronomer may place a far
>>smaller number of phase bins at pulse phases of interest to him/her.
>>
>>2. Monitoring up to 4 pulsars
>>
>>Same as above, but in cases where multiple pulsars are in the same primary
>>beam of the telescope it will be possible to observe up to 4 pulsars
>>simuntaneously, each with its own polynomial.
>>    
>>
>
>
>Am I right in thinking this limit of 4 stems from the maximum of 4 phased
>sub-arrays per sub-band?  Does this increase if one is willing to spend
>fewer than the full number of sub-bands on each pulsar?
>
>  
>
>>"Monitoring" can mean either "imaging" or "timing", two distinct science
>>goals.  From the perspective of the correlator these two actions are the
>>same.  Typically timing will favor denser phase bins and imaging will
>>favor shorter integration times.
>>    
>>
>
>Any thoughts on pulsar searches?
>
>Anything special for higher frequency or astrometric observations? Though
>the latter may be boring without EVLA-II.
>
>
>  
>
>>Specifying pulsar information:
>>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>>The exact mechanics of this are still to be discussed, but I imagine the
>>following is nearly accurate:
>>
>>1. Specify "Pulsar Observing Mode" which will require the following
>>   information:
>>  A. For each 2 GHz IF:
>>      1. pulsar ephemeris (polynomial table) specifier (see below)
>>      2. pulse dispersion measure (DM) (note -- the purpose here is only
>>         to warn the user if the dispersion smearing within one channels
>>         is detrimental.  The DM used in driving the phase bins will come
>>         from the ephemeris.
>>      3. pulse period (for same purpose as DM above)
>>    
>>
>
>For David: there will be eight 2 GHz basebands (actually four BB pairs).
>
>  
>
>>  B. For each sub-band :
>>      1. gate enable (default enabled)
>>      2. peculiar gate delay (default zero.  This is in addition to
>>	 dispersion delay)
>>      3. number of phase bins required (default: 64???)
>>      4. table of start/stop phases  OR  one or two start/stop times for
>>	 the requested bins to be evenly divided into. (default: [0,1) )
>>
>>2. Before the observation begins (but perhaps well after the scheduling
>>blocks have been submitted) the astronomer must provide the ephemeris
>>which contains a series of polynomials each valid for about 1 or 2 hours
>>and a dispersion measure used to calculate dispersive delays.  Each
>>submitted ephemeris should be labeled with a specifier that the observe
>>file references.  The pulsar ephemeris would be loaded into a database as
>>is done for the VLBA correlator.  The reason that the astronomer might
>>want to wait until the last moment to submit a polynomial is that many
>>pulsars are "unstable rotators" which have phases that can drift by 10s of
>>degrees relative to an ephemeris on 10s of days timescales.  Along similar
>>lines newly discovered pulsars often have very poorly known spin
>>parameters.  The use of the VLA to determine a position can be very useful
>>in refining the these.
>>    
>>
>
>Are these likely to be refined during the observation? or is that too painful 
>to contemplate? I'm wondering whether one might start with an even 
>distribution of bins, then zoom in on the phases of interest.
>
>  
>
>>Note to the non-specialists:  Dispersion measure (DM) is the column
>>density of electrons along the line of sight to the pulsar.  Astronomers
>>use units of pc/cm^3 (parsecs per cubic centimeter) which is dimensionally
>>an inverse area.  In these units typical values range between 2 and 2000.
>>This number is never negative.  The delay incurred is frequency dependent
>>and is roughly equal to
>>	dt ~= 4150 * DM / freq^2
>>with dt in seconds, DM in pc/cm^3 and freq in MHz.
>>    
>>
>
>Is the ionospheric dispersion likely to be important at the lower frequencies?
>
>  
>
>>At observe time if no ephemeris has been loaded into the database the
>>pulsar mode should be disabled and "standard interferometry" mode should
>>be used.  An alert should be triggered.
>>
>>
>>Open questions:
>>~~~~~~~~~~~~~~~
>>
>>1. Is fast-switching inconsistent with pulsar mode where a fairly
>>   substantial correlator mode change is required?
>>    
>>
>
>I don't believe so -- we'd asked about this earlier.  Again I'll check to be 
>sure.  Assuming one can reconfigure the correlator on these timescales,
>we will want observe a calibrator with a wide-open bandwidth, then derive
>and apply appropriate phases/delays for each of multiple sub-bands and 
>phasing locations.
>
>Alternatively, given the initial hardware limitations, one could imagine
>simultaneously observing the pulsars in phased-array mode using 1 GHz, and an 
>in-beam calibrator with a broad bandwidth.  This calibrator might even be (a 
>gated version of?) the pulsar itself.
>
>  
>
>>2. How does pulsar mode interact with sub-arrays?
>>    
>>
>
>See above for some aspects of this.  A related question:
>
>* Can one trade phased antennas for phased bandwidth?  I think not, but
>   this should be checked.
>
>
>Cheers,
>
>              Michael
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>  
>

-- 
Brent R. Carlson
Brent.Carlson at nrc-cnrc.gc.ca
Tel/Tél: (250) 493-2277 (ext. 346) |  Fax: (250) 493-7767
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