[evla-sw-discuss] Synchronization of switched power & visibility data

[George Moellenbrock]

Michael-

> We use Pdiff & Psum to scale the visibilities and the weights, and as
> diagnostics (e.g., to flag the data without poring through all the
> visibilities).   I think we *do* want to label Pdiff & Psum with the
> correct times, but we do *not* need to synchronize those times with
> visibility dumps.

This is fine from the post-processing in CASA perspective.   Is it correct
to assume that Pdiff and Psum will share a common timestamp
per sample?  I.e., they are not Pon and Poff, which would have alternating
timestamps.  Delivering (time,Psum,Pdiff) triples per antenna, subband, etc.
would be very desirable from the offline perspective.

> - For scaling purposes this requires that the scale factors do not change
>  much from one integration to the next.

We are limited by the noise on the power measurements, no?  (At least
for narrower bandwidths.)

The more sensitive the process is to _precise_ synchronization of vis
and power data (cf application of smoothed and interpolated factors
a la offline calibration), the more important that it be handled ONline,
in my view.  Offline processing in CASA is prepared to handle straightforward
interpolation of the power information onto the visibility samples, including
simple smoothing over over-sampled gain info, if necessary.    Any
more intricate processing (e.g., assignment of specific power samples
to specific visibility samples) should be handled ONline, or made
implicit by coordinating timestamps in the data delivered to offline
processing.  (My impression is that we are not confronting such
sensitivity, though you sound a bit more guarded in the
current email....)

> The SDM does NOT require that calibration data and visibilities be on
> the same time grid.  I believe CASA & AIPS do not require this either, but
> maybe GEORGE and ERIC can comment.

Correct, in CASA, they will be interpolated in a conventional manner.

> 2- The measurements on the individual StB may not be made at the same time.
>  Even measurements at various filters on a single StB may not be made
>  simultaneously.  The SDM can handle this but it may be a pain to
>  deal with this approach in post-processing.  As suggested above we
>  could over-sample Pdiff & Psum and interpolate to a common grid, maybe in
>  the filler (or even in MCAF???), but that's pretty yucky.  Alternatively
>  we can ignore the disagreement.  Others (Ken?) will have to comment
>  on how big these time offsets might be.
>

CASA will be generating a cal table containing both gain factors and
conventional Tsys derived from the power measurements and Tcal.
At first, at least, these will be sampled onto a common time grid for
all antennas (per subband).   (For cal tables in CASA, we currently
assume all antennas share the same timestamps, per spw.)  The
resolution of the grid will be determined by the rate at which the power
data arrive, but could be user-revisable to something like the vis data
rate (or even slower), e.g., depending on desired/required SNR.
Resampling on a grid commensurate with the vis data would
optimize flag-on-apply properties, but would not be required.  At
least at first, it is this caltable that will be available for examination,
flagging, smoothing, etc.   Specialized processing of the _raw_
unresampled power info would take a bit more planning and
work in CASA (unlikely for OSRO, given other priorities).  We'll
concentrate on the shortest path to corrected data in the
near term.

-George




> Comments? Counter-proposals?  Rotten eggs?
>
>                Michael
>
>