[evlatests] Flagging at scan beginnings

[Jim Ulvestad]

Hi all,

Many of you are not VLBA afficionados, but this is a problem
that is similar to one we've had with the VLBA for many years.
In the case of the VLBA, it is caused by the fact that
not all antennas start moving off source at the
same instant, and so some records for some antennas
get assigned to the wrong source.  In the end, it was not
possible to deal with this in the monitor data without a lot
of unproductive work, so the "TAIL" option in QUACK was
invented.  Do you still get this error on EVLA when you
have a very long slew between scans, or is it only for
slews that last less than an integration time?  (Maybe
Gustaaf tested this, but I don't recall.)

What would happen if the executor started slewing the
antennas a few milliseconds before the end of the
scan?  Would we just move a similar error to the
previous scan?  Or would we have a visibility error
of magnitude (few msec/integration time) < 1% that
nobody would ever notice?  Or is even such a small
error enough to still cause problems with very high
dynamic range imaging?

Best,

Jim
>
> Message: 2
> Date: Wed, 17 Sep 2008 08:50:35 -0600
> From: Gustaaf Van Moorsel <gvanmoor at nrao.edu>
> Subject: [evlatests] unflagged records at the beginning of scans
> To: EVLA tests <evlatests at nrao.edu>
> Message-ID: <48D1193B.9070907 at aoc.nrao.edu>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> The tests
> ---------
>
> This message describes further testing of the problem that under
> some conditions a number of records at the end of a scan are not
> being flagged after the antennas have started moving.  These rec-
> ords are then assigned to the *following* scan.
>
> Last week I demonstrated that the problem is *not* specific to
> fast switching: it showed up both in true fast switching and in
> the fast switching emulating mode.
>
> All those tests were done with 0.4 sec integration time.  New
> tests expand this to 0.4, 1 2/3, and 3 1/3 seconds integration time.
>
>
> Results
> -------
>
> At 0.4 sec integration time, the problem occurs at *every* source
> change.   It can involve one or two integrations, never more than
> two, never less than one.  At 1 2/3 and 3 1/3 seconds integration
> time the problem was *never* seen in 28 source changes (14 fast
> switching, 14 emulated fast switching).
>
>
> Conclusion
> ----------
>
> The current system appears to correctly flag all data after the
> antennas have started moving for all integration times of 1 2/3
> seconds and longer.  Of course this is no proof that the problem
> is gone completely, and we should remain on the lookout for any
> renewed occurrence.
>
> Gustaaf
>
>
>
>
>
>
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>
> End of evlatests Digest, Vol 40, Issue 10
> *****************************************
>