[mmaimcal]Re: ACA Phase Calibration]

[Ryohei Kawabe]

Dear all,


>  > Hi Mark,
>  >
>  >     I fully agree with the conclusions, but someone should work on
>  > proper use of a WVR for the ACA.
>
> Perhaps that is a job for a Japanese collaboration with Richard Hills
> (here I am giving work to everyone but me!)


We have just reorganized ALMA-J Science team, and we would like to work on
the
job which perhaps needs collabolation with Richard Hills.

Ryohei Kawabe

ALMA-J project office
NAOJ






>  > There are two reasons for that:
>  >      - Phase calibrating the ACA will too frequently would INCREASE the
>  > phase noise on these short baselines.
>  >     (essentially you add up the phase fluctuation on the calibrator to
>  > the ones on the source, and they are uncorrelated
>  >      on short baselines and short timescales)
>
> Ah, yes!  Thank you for this point.
>
>  >     - Yet, with the WVR, the maximum time between calibration is given
>  > by the WVR stability. One should look
>  >     whether there is any conflict with the above minimum time between
>  > calibrations...
>  >
>  > More generally for ALMA, improper use of fast switching will increase
>  > phase noise on short baseline. Can we solve for phases on different
>
> I think this is not correct -- when doing fast switching, the
> short baseline's phase errors follow the structure function,
> and the longer baselines have a phase error equal to the structure
> function at the effective calibration baseline = (vt/2 + d).
> This is an experimental result (ie, MMA Memo 126, which seems
> to be unavailable from the web;  MMA Memo 262, figure 7 --
> oops, that is for self-calibration, though.)  --  anyway, it is
> possible that it would make things worse by sqrt(2), which might not
> show up so clearly on the typical log-log plots as in Memo 126.
>
>
>  > timescales for short and longer baselines ? More precisely, can we do
>  > that without induly breaking phase closure ? There is a some research
to
>  > be done in an optimal calibration technique from this point of view...
>  >
>  > A related point: I did not get how you would increase the S/N on the
ACA
>  > antenna gains by not solving for the 12-m antenna gains when they are
>  > cross-correlated together. How do you get the gain of these 12-m
>
> For example, lets say you are doing a pointing calibration, you could
> hold the 4 12m dishes fixed and just solve for the gains on the
> ACA antennas -- if you don't need to solve for the 12m pointing,
> you win big.  It is an unproven algorithm, and there may be some tricky
> details involved, but I think it could sometimes speed things up.
>
>     -Mark
>
>  > antennas then ? And how does the error on this gain (which necessarily
>  > exists) reflects on the error on the ACA antenna gains ? Doesn't that
>  > cancels to some extent the improvement obtained in S/N ?  I believe you
>  > should detail your calculation there.
>  >
>  >         Stephane
>  >
>  >
>
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