[evlatests] WIDAR wobbles vs. time, contd.

[Vivek Dhawan]

At this morning's meeting it was though worthwhile to dig a bit more.

For my Rupee's worth (~2 cents) this is not the cause of the imaging
troubles, I think it averages out. But that's just a guess - it would
be nice to squash it.

-----------------------

Data is C band, July 09 UT 21-22:30, 3C273 (~30Jy say).

a. The wobbles are non-closing (not antenna based), details below.

b. There are a variety of periods, a few Hz, quite a bit faster then the
    fringe rate.

c. The periods do change over an hour, but I see no way to relate them to
    the corresponding fringe rate changes.

d. There is no differential in the wobble across the band = it is _not_
    a delay change = there is no delay jitter associated with the wobbles.
Plot 1: red= phase wobbles, averaged over band. blue= phase jump at
    delay change, differential between low/high half of band. time span =
    10 sec, 40ms steps.

e. The noise-diode switching at 9.6Hz is seen at ~few % in the autocor
    and is at about 0.1-1% of the cross-correlation amplitude on most
    baselines. Assuming 30K Tsys, 3K noise injection, that is ~30Jy, so
    the f_shift is suppressing the noise diode by a factor of >100 (1000
    on most baselines), else it would be comparable to 3C273.

f. The 9.6Hz is strongest on all baselines to ant 19, which is an offender
    at other wobble frequencies. Otherwise, I see no relation between the
    strength of the 9.6 and the other modulations. i.e. no clear sign of
    cross talk between antennas. I dont see how to rule out beat notes.

g. I see no way with astronomical or even radiated cross-talk, to make
    the amplitude and phase signatures we see. Not sure of this argument,
    but I think all these have to be a small vector rotating at a smooth
    and Earth-related rate, perturbing a large signal vector. This gives
    equal and orthogonal real/imaginary wobbles, not what we see.

Plot 2. = Example of modulation = power spectrum of visibility
    variation. baselines are 2-18, 2-19, 18-19 (top left, Right, bottom)
    Peak is off the top at ~300 units.

h. Details of which baselines have what: (most have 9.6Hz at 0.2% or so).
    I think this says that the antennas with even slot IDs are worse -
    right Michael?

1-*   clean
2-18 ~1.9Hz 1% of cross corr peak on 3C273
2-19  3.7Hz 5%
2-28 ~1Hz   2%
3-*   clean
9-*   5Hz   weak ~0.2%
9-18  6.5Hz 1%
18-19 6.5Hz 2%
19-*  9.6Hz 1-2% noise diode
19-24 2Hz   2%
19-28 3Hz   8%
23-*  clean (23-3, 23-19, 23-25 have 9.6Hz 1%)

----------------

Vivek Dhawan wrote:
> Michael inflicted upon me 80 minutes of data taken at 40msec
> integrations, 20GB, to look for changes in wobbles vs time, etc.
> 
> 1. load data and index it in short scans - it makes subsequent
>    tasks start quicker.
> 
> 2. FRING on 10sec of data, zero rates, then apply to all data.
> 
> 3. BPASS on 1min, apply to all.
> 
> 4. Average up across band, plot phase/amp vs time.
> 
> 5. Wobbles seen mainly between ant 2, 18, 19, 28; regardless of
>    whether the delay and BPass are applied or not, so their
>    details are not crucial.
> 
> 6. FRPLT (= Fringe rate plot, Fourier transform of phase vs. time)
>    of 1min of data was done every 10 min to see changes. The period
>    of the wobbles does change in come cases by a factor 2 over 80min.
> 
> 7. I need to relate the wobble period to the natural fringe rate,
>    not yet done.  Anyone have a handy program?
> 
> 8. I did find a more curious thing: The amplitude wobbles may be
>    different from the phase wobbles on the same baseline! e.g.
>    Amp 8%, phase 5deg (all pk-pk) on ant 19-28
>    amp 1%, phase 8deg on 2-19.
> 
>    Even more curious, the period of the amp wobbles (0.4 sec) is
>    half that of the phase wobbles on the same baseline (19-28).
>    In most of the cases seen, phase and amp have the same period.
> 
> Heard enough? I have - see y'all tomorrow
> 
> Vivek.
> 
> 
> 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: aaa.pdf
Type: application/pdf
Size: 30607 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20090715/09a99bb3/attachment.pdf>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: bbb.pdf
Type: application/pdf
Size: 32326 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20090715/09a99bb3/attachment-0001.pdf>